Louisiana State University LSU Digital Commons LSU Historical Dissertations and Theses Graduate School 1994 The Structural Basis for Kinetic and Allosteric Differences Between Two Bacterial Phosphofructokinases. Walton Malcolm Byrnes Louisiana State University and Agricultural & Mechanical College Follow this and additional works at: https://digitalcommons.lsu.edu/gradschool_disstheses This Dissertation is brought to you for free and open access by the Graduate School at LSU Digital Commons. It has been accepted for inclusion in LSU Historical Dissertations and Theses by an authorized administrator of LSU Digital Commons. For more information, please contact gradetd@lsu.edu. Recommended Citation Byrnes, Walton Malcolm, "The Structural Basis for Kinetic and Allosteric Differences Between Two Bacterial Phosphofructokinases." (1994). LSU Historical Dissertations and Theses. 5859. https://digitalcommons.lsu.edu/gradschool_disstheses/5859 INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely affect reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. Each original is also photographed in one exposure and is included in reduced form at the back of the book. Photographs included in the original manuscript have been reproduced xerographically in this copy. Higher quality 6" x 9" black and white photographic prints are available for any photographs or illustrations appearing in this copy for an additional charge. Contact UMI directly to order. A Bell & Howell Information Company 300 North Zeeb Road. Ann Arbor. Ml 48106-1346 USA 313/761-4700 800/521-

t h e s t r u c t u r a l b a s i s f o r k in e t ic a n d a l l o s t e r i c d i f f e r e n c e s BETWEEN TW O BACTERIAL PHOSPHOFRUCTOKINASES A Dissertation Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment o f the requirements for the degree of Doctor of Philosophy in The Department of Biochemistry by Walton Malcolm Byrnes B. S., Xavier University of Louisiana, 1981 December 1994 UMI N u m b er: 9 5 2 4 4 3 8 UMI Microform Edition 9524438 Copyright 1995, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 Acknowledgments There are many people without whom this dissertation would not have come to pass. To these people I owe a debt of sincere gratitude. I thank Dr. Simon H. Chang, my advisor, for his professional and financial support. I thank also the members of my advisory committee, Dr. Eric Achberger, Dr. Sue Bartlett, Dr. Ding Shih, and Dr. Ezzat Younathan, for their guidance and advice. Two people deserve special thanks: Sr. Mary Carl Malmstrom, S.B.S., my friend and mentor at Xavier who never spoke a discouraging word despite my wanderings since graduating from Xavier in 1981, and my Uncle Alva B. O'Brien, whose encouragement and kind help led to my returning to graduate school. I am also grateful to my parents, V. Malcolm and Christine F. Byrnes, and my family, who have supported and encouraged me during the last six years. Finally, my deepest gratitude and appreciation are saved for my wife, Noni, whose patience, love, understanding and sacrifice (also money) have carried me through these last few months of preparation. Preface This dissertation is divided into three major parts: a general introduction or literature review (chapter 1), a presentation of the results and a discussion of the results (chapters 2, 3, and 4), and some concluding statements and suggestions for future studies (chapter 5). Chapters 2, 3, and 4 form the core of the dissertation. Each of these three chapters is presented in the form of a manuscript consisting of Introduction, Materials and Methods, Results, and Discussion sections. Chapter 2 has been published (Byrnes, M„ Zhu, X., Younathan, E. S., & Chang, S. H. (1994) Biochemistry 33, 34243431 ), chapter 3 has been accepted for publication in the Journal o f Biological Chemistry, and chapter 4 is currently being submitted to Biochemistry. Some of the kinetic results presented in Tables 4.2, 4.3, and 4.4- those for the E. coli PFK mutants- were obtained by Dr. Isabelle Auzat, who came from the Laboratoire d'Enzymologie-CNRS in France to visit the Chang laboratory in March, 1994. There may be some overlap in the introductory sections of the three core chapters. Chapter 1 is designed to be an "umbrella" introduction that will hopefully provide a comprehensive review of the bacterial PFK literature, including information not included in the core chapter introductions. Finally, chapter 5 sums up the whole body o f work and proposes some specific future experiments. Hopefully, one or more of the suggested future experiments in chapter 5 will ignite someone's interest in pursuing further studies of bacterial PFK. Table of Contents ACKNOW LEDGEM ENTS.........................................................................................................ii PR EFA C E ...................................................................................................................................... iii LIST OF T A B L E S ..........................................................................................................................v LIST OF FIG U R E S................................................................................................... vi LIST OF ABBREVIATIONS AND SYMBOLS.................................................................. viii A B STR A C T....................................................................................................................................x CHAPTER ONE: ........................................................................................................................ 1 General Introduction CHAPTER TWO: ........................................................................................................................39 Kinetic Characteristics of Phosphofructokinase from Bacillus stearothennophilus: MgATP Non-allosterically Inhibits the Enzyme CHAPTER T H R E E :....................................................................................................................71 A Chimeric Bacterial Phosphofructokinase Exhibits Cooperativity in the Absence of Heterotropic Regulation CHAPTER FOUR: ....................................................................................................................107 The Role of Residue 161 in the Allosteric Transitions of Two Bacterial Phosphofructokinases CHAPTER F IV E :...................................................... 133 Conclusions and Future Studies LITERATURE C IT E D ........................................................................................................... 140 APPENDIX: Permission L e tte r...............................................................................................145 VITA...............................................................................................................................................148 List of Tables 1.1 Active Site Residues...........................................................................................................6 1.2 Effector Site Residues.........................................................................................................7 1.3 Residues Important for C atalysis...................................................................................15 1.4 Kinetic and Allosteric Parameters for E. coli P F K ....................................................27 1.5 Residues Important for Homotropic Cooperativity of E. coli P F K ....................... 34 2.1 Product Inhibition Patterns for the Reverse R eaction............................................... 49 2.2 Alternative Nucleoside Triphosphate (NTP) Substrates ............................. 59 2.3 Product and Dead-end Inhibition Patterns for the Forward R eaction .................... 61 3.1 Kinetic Parameters for ChiPFK and the Native E nzym es........................................85 3.2 Dissociation Constants and Maximum Fluorescence Changes for Fru-6P and AMPPNP Binding to E. coli P F K ................................................... 98 3.3 Changes in Intrinsic Fluorescence Induced by Ligand Binding to E. coli PFK and the Chimeric P F K ......................................................................... 99 4.1 Mutagenic O ligonucleotides..................................................................................... 114 4.2 Steady-state Kinetic Parameters for Wild-type and Mutant P F K s....................... 119 4.3 PEP Inhibition of Wild-type and Mutant P F K s........................................................121 4.4 Activation of EcPFK and Its Mutants by G D P .........................................................124 4.5 Activation of BsPFK and Its Mutants by GDP and F ru-6P ....................................128 v List of Figures 1.1 The Glycolytic Pathw ay.................................................................................................. 3 1.2 Sequences of the PFKs from E. coli and B. stearothermophilus..............................5 1.3 The Bacterial PFK T etram er............................................................................................9 1.4 The Proposed Transition State of E. coli P F K ........................................................... 13 1.5 The Open-to-Closed Conformational C hange............................................................18 1.6 The Proposed Reaction Cycle of E. coli P F K .............................................................19 1.7 The Effector Site of E. coli P F K ...................................................................................22 1.8 Co-ordinated Movement of the 8H and 6F L oops.....................................................32 1.9 Reorganization of the 6F L o o p ..................................................................................... 33 2.1 Initial Velocity Patterns for the Reverse R eaction................................................... 47 2.2 Saturation Curves and Initial Velocity Plots with Respect to F ru -6P ...................51 2.3 Dependence of the BsPFK-catalyzed Reaction Rate on MgATP Concentration............................................................................................. 56 2.4 MgATP Inhibition of the Mutant GV212 E nzym e................................................... 58 2.5 AMPPNP Inhibition P atterns .........................................................................................63 3.1 Molecular Graphics Image of the Chimeric Subunit................................................ 80 3.2 Structural Properties o f the Chimeric P F K ................................................................. 82 3.3 Dependence of PFK Activity on Fru-6P Concentration...........................................86 3.4 Dependence of PFK Activity on MgATP Concentration when Fru-6P Concentration is Low (50 p M )........................................................... 88 3.5 Effect of PEP and GDP on PFK A ctiv ity ................................................................... 91 3.6 Protection of (A) Native E. coli PFK and (B) Chimeric PFK Against Thermal Inactivation...................................................................................... 93 3.7 Effect of AMPPNP on the Fru-6P-Dependent Cooperative Behavior of Chimeric P F K ...........................................................................................96 3.8 Combined Effects of Fru-6P and AMPPNP on Intrinsic Fluorescence of the Chimeric P F K ............................................................................100 vi 4.1 Reorganization of the 6F Loop (156-162) of B. stearothermophilus PFK . . . .110 4.2 The Amino Acid Sequences of the 6F Loops of B. stearothermophilus PFK, E. coli PFK, and the Five Mutant Enzymes . . . . I l l 4.3 Fructose 6-phosphate Saturation Curves for B. stearothermophilus PFK (O) and E. coli PFK ( • ) .................................... 117 4.4 PEP Inhibition of B. stearothermophilus PFK (O) and E. coli PFK ( • ) ...............120 4.5 GDP Activation of E. coli P F K ................................................................................... 123 4.6 Activation of PEP-Inhibited B. stearothermophilus P F K .................................... 126 vii List of Abbreviations and Symbols PFK = phosphofructokinase (ATP: (3,D-fructose 6-phosphate 1-phosphotransferase) BsPFK = phosphofructokinase from Bacillus stearothermophilus EcPFK = phosphofructokinase from Escherichia coli ChiPFK = the chimeric phosphofructokinase Bs EQ161 = mutant B. stearothermophilus PFK having a glutamine at position 161 Bs EA161 = mutant B. stearothermophilus PFK having an alanine at position 161 Ec QE161 = mutant E. coli PFK having a glutamate at position 161 Ec QA161 = mutant E. coli PFK having an alanine at position 161 Ec QR161 = mutant E. coli PFK having an arginine at position 161 Fru-6P = fructose 6-phosphate PEP = phosphoenolpyruvate GDP = guanosine 5'-diphosphate Fru-1,6BP = fructose 1,6-bisphosphate Ara-5P = Arabinose 5-phosphate AMPPCP = P,y-methyleneadenosine 5'-triphosphate AMPPNP = adenylimidodiphosphate K;s and Kji = equilibrium constants for dissociation of an inhibitor from its inhibitory complex with the enzyme, as determined from slope and intercept replots, respectively KactGDP = the equilibrium constant for dissociation of GDP from the effector site, as determined from the GDP activation profile 11/2 = the half-saturation concentration for an inhibitor DTT = dithiothreitol SDS-PAGE = sodium dodecylsulfate polyacrylamide gel electrophoresis X-gal = 5-bromo-4-chloro-3-indoyl-(3,D-galactoside IPTG = isopropyl-(3,D-thiogalactoside MWC = model of allosteric behavior proposed by Monod, Wyman, and Changeux ix Abstract The fructose 6-phosphate (Fru-6P) saturation curve for phosphofructokinase (PFK) from E. coli is sigmoidal in the presence of saturating MgATP levels, while the corresponding curve for B. stearothermophilus PFK is essentially hyperbolic. Sigmoidality can be due to apparent cooperativity arising from the kinetic mechanism of an enzyme. We have determined the kinetic mechanism of B. stearothermophilus PFK (BsPFK). BsPFK was found to obey a non rapid-equilibrium random mechanism similar to the one E. coli PFK (EcPFK) follows. Substrate inhibition by MgATP was observed. We propose that substrates bind to BsPFK through two alternative pathways, one of which is slower. The substrate inhibition arises in part from reaction flux through the slower pathway. Although EcPFK and BsPFK obey similar kinetic mechanisms, they are inhibited differently by MgATP: EcPFK is profoundly inhibited, BsPFK only weakly. The structural basis for this difference could be closure of the active site via a conformational transition that occurs in EcPFK, but not BsPFK. To investigate the importance of this transition for MgATP inhibition of EcPFK, we have constructed a chimeric enzyme (ChiPFK) that contains the "rigid" ATP-binding domain of BsPFK grafted onto the remainder of the EcPFK subunit. Our results indicate that ChiPFK is locked in an "open" structure resembling the activated form of EcPFK. It is insensitive to heterotropic regulation. Nevertheless, ChiPFK exhibits residual cooperativity. Possible explanations for the cooperativity are discussed. The 6F loop is proposed to be important in PFK allosteric behavior. Residues along the loop are largely conserved between BsPFK and EcPFK, except for 161, which is a glutamate in BsPFK, and a glutamine in EcPFK. Using site-directed mutagenesis, Glu 161 of BsPFK has been changed to glutamine and alanine. Similarly, Gin 161 of x EcPFK has been changed to glutamate, arginine and alanine. Of the five mutants, one, QA161, was particularly interesting. Though activated normally by GDP, it was completely insensitive to PEP inhibition. This indicates that the hydrogen-bonding ability of residue 161 is critical for PEP inhibition of EcPFK, and that GDP activation and PEP inhibition follow different structural pathways. xi Chapter 1 General Introduction 1 2 Phosphofructokinase (PFK, ATP: (3-D-fructose 6-phosphate 1phosphotransferase , EC 2.7.1.11) is a key regulatory enzyme o f glycolysis. Positioned near the top o f the glycolytic pathway (Fig. 1.1), PFK catalyzes the first committed step of the pathway, the MgATP-dependent phosphorylation o f fructose 6-phosphate (Fru6P ) to produce MgADP and fructose 1,6-bisphosphate (Fru-1,6BP): MgATP + Fru-6P > MgADP + Fru-1,6BP. A divalent Mg++ ion, as well as a monovalent NH4 + or K+ ion, is required for the catalysis. The enzymatic activity o f PFK, hence flux through the glycolytic pathway, is regulated by a number o f metabolites. In mammalian cells, the regulation is complex, with ATP and citrate serving as inhibitors o f the enzyme, and Fru-6P, ADP, AMP and Pi as well as fructose 2,6-bisphosphate, serving as activators (see Uyeda, 1979 for a review). In bacteria, however, regulation o f PFK is simpler, involving the activator ADP (or GDP) and the inhibitor phosphoenolpyruvate (PEP). Exceptions to this are the PFKs from the bacteria Lactococcus bulgaricus and Spiroplasma citri, which are regulated by PEP but not ADP (Auzat et al., 1994). ATP also inhibits bacterial PFK, but the potency of the inhibition varies from one bacterial species to another. For example, ATP inhibits Escherichia coli PFK strongly at low Fru-6P concentration, but Bacillus stearothermophilus PFK only weakly. The genes for both E. coli PFK and B. stearothermophilus PFK have been cloned (Hellinga & Evans, 1985; French & Chang, 1987), and the three-dimensional structures o f the two enzymes have been determined from X-ray diffraction studies (Shirakihara & Evans, 1988; Evans et al., 1981). The cloning o f the genes has made possible not only a deduction of the primary structures (amino acid sequences) of the enzymes, but also high-level expression o f the enzymes in PFK-deficient E. coli cells (Hellinga & Evans, 1985; French et al., 1987), and site-directed mutagenesis for the H exokinase Glucose ATPV ^ADP Glucose 6-phosphate P hosp h o g lu co se isom erase Fructose 6-phosphate ' A T P Phosphofruc to kinase ^ADP Fructose 1,6-bisphosphate A ldofase D ihydroxyacetone phosphate \ Triose ph o sp h a te isom erase . , ,. G lyceraldenyde 3-phosphate G lyceraldehyde 3 -p hosphate d ehyd rogenase ^NAD+ + Pj ^ NADH + H+ 1,3-B isphosphoglycerate ÂDP S»ATP Phosphoglycera te kinase 3-Phosphoglycerate Phosphoglycerom utase 2-Phosphoglycerate Enolase Phosphoenolpyruvate L-ADP Pyruvate kinase ÂTP Pyruvate FIGURE 1.1. The Glycolytic Pathway. (Taken with permission from Shyer (1988) Biochemistry . 3rd. ed„ W. H. Freeman and Co., New York. Copyright 1988 W. H. Freeman and Co.). purpose o f structure-function studies. High-level expression has resulted in purer, more homogeneous crystals o f B. stearothermophilus PFK, which has in turn allowed a higher-resolution X-ray structure (Schirmer & Evans, 1990). The availability o f highresolution structures o f the proteins has in turn spawned structure-function studies using site-directed mutagenesis and recombinant DNA, which confirm or deny predictions made from the crystal structures. Thus, recombinant DNA technology and site-directed mutagenesis on one hand, and X-ray crystallography on the other, have been used together to unravel the intricacies o f PFK catalysis and regulation. Furthermore, the combination o f site-directed mutagenesis, recombinant DNA methods, and X-ray crystallography with classical enzymology and ligand binding studies allows one to undertake a very comprehensive study of the structure and function o f PFK, and indeed any regulatory enzyme. Bacterial PFK Structure The primary structures o f E. coli PFK (EcPFK) and B. stearothermophilus PFK (BsPFK) are shown in Fig. 1.2. These structures were deduced from the nucleotide sequences o f their genes (Hellinga & Evans, 1985; French & Chang, 1987), but a partial amino acid sequence o f BsPFK had been determined some time earlier (Kolb et al., 1980) from an analysis o f cyanogen bromide and other peptic fragments. Examination of the two sequences in Fig. 1.2 reveals that EcPFK and BsPFK share 55% amino acid identity. Moreover, a comparison among the amino acids that bind substrates in the active site (Table 1.1) and effectors in the regulatory site (Table 1.2) o f the two enzymes shows that most of the residues are either identical or conserved between the two. This close homology between EcPFK and BsPFK is in contrast to the complete lack of homology observed between EcPFK and the non-allosteric PFK from E. coli (the two are designated PFK 1 and PFK 2, respectively), whose gene has also been cloned and sequenced (Daldal, 1984). (PFK 2 accounts for about 10% of the total PFK activity in wild-type E. coli cells). The PFK 2 enzyme has been purified and studied in terms of 5 1 % 10 20 30 40 E. coli MIKKIGVLTSGGDAPGHNAAIRGVVRSALTEGLEVMGIYDGYLGLYED RM B. St MKRIGVLTSGG-DSPGMNAAIRSWRKAIYHGVEVYGVYHGYAGLIAG N I P-A a-1 P-B a -2 50 60 70 80 90 VQLDRYSVSDMINRGGTFLGSARFPEFRDENIRAVAIENLKKRGIDALW KKLEVGDVGDI1HRGGTILYTARCPEFKT EEGOKKGIEGLKKHGI E G L W p-c 3 io -4 A a -4 p-D 100 110 120 130 140 IGGDGSYMGAMRLTEMGFPCIGLPGTIDNDIKGTDYTIGFFTALSTWEA I G GDGSYOGAKKLTEHGF PCVGVP G T ID N D IP G T D F T IGFDTALNTVIDA a -5 p-E a 6 150 160 170 180 190 IDRLRDTSSSHQRISWEVMGRYCGDLTLAAAIAGGCEFVWPEVEFSRE IDKIRDTATSHERTYVIEVMGRHAGD I ALWSGI.AGGABT I L IPEADYDMN p-F a -7 p-G 200 210 220 230 240 DLVNEIKAGIAKGKKHAIVAITEHMCDVDELAHFIEKETGRETRATVLGH DVIARLKRGHERGKKHS I H V A EGVGSGVDFGRQIOEATGFETRVTVL GH a -8 p-H a -9 p-I 250 2 60 270 280 2 90 IQRGGSPVPYDRILASRMGAYAIDLLLAGYGGRCVGIQNEQLVHHD I IDA VORGGSPTAFDRVLAgRLGARAVELLLEGiCGGRCVGIONNOIjVDHD IAEA 3 io 1 0 a 1 1 p-J p-K a -1 2 300 310 320 IENMKRPFKGDWLDCAKKLY L A -NKHTIDORMYALSKELEI a 13 FIGURE 1.2. Sequences of the PFKs from E. coli and B. stearothermophilus. The residues o f EcPFK are numbered from 0 to 319, while those o f BsPFK are from 1 to 320. Residues identical between the two are bolded. A gap at position 302 o f BsPFK allows alignment o f the C-termini. Positions a-helices and p-strands are indicated by horizontal lines. Corrections o f the BsPFK sequence due to French & Chang (1987) are in italics. TABLE 1.1 Active Site Residues A. Fru-1,6BP Binding. Residue No. Ec Bs Interactions 125 Thr Thr H bond to 1-phosphate 127 Asp Asp Catalytic residue, H bond to 0(3) 129 Asp Asp Binds water attached to Mg++ 162* Arg Arg Binds 6-phosphate 169 Met Met Hydrophobic contact to 0(2), 0(3) 170 Gly Gly Main-chain H bond to 0(3) 171 Arg Arg Near 1-phosphate 222 Glu Glu H bond to 0(4) 243* Arg Arg Binds 6-phosphate 249 His His Binds 6-phosphate 252 Arg Arg H bond to 6-phosphate and 0(2) B. ADP Binding. Residue No. Ec Bs Interactions 11 Gly Gly Main-chain amide H bond to fl-P 41 Tyr Tyr Contact with ribose 72 Arg Arg Bridges ADP oP and Fru-1,6BP P's 73 Phe Cvs Main-chain bonds to ribose OH 77 Arg Lvs Hydrophobic contact with adenine 103 Asp Asp Binds to Mg++ 104 Gly Gly Main-chain amide bond to (3-P 105 Ser Ser H bonds to Jl-P from OH and NH 107 Met Gin Hydrophobic contact with adenine 108 Gly Gly Contacts adenine Ec, E. coli PFK; Bs, B. stearothermophilus PFK. Residues indicated are those that interact with products because the crystal structure was determined with products in the active site. *, from another subunit. Underlined residues are different between Ec and Bs. (From Shirakihara, Y., & Evans, P. R. (1988) J. Mol. Biol. 204, 973-994). 7 TABLE 1.2 Effector Site Residues ADP Binding. Residue No. Ec Bs Interactions 21* Arg Arg H bond to p-P and M g4-*via water 25* Arg Arg H bond to p-phosphate 54* Arg m H bind to a-P and C terminus 55* Tyr Gly Stacks against adenine ring 58* Ser GIv H bond to 1-P and Arg 25 59* Asp Asp Main chain to p-P, bond to 0(3) 154 Arg Arg Two H bonds to p-P 185 Gly Gly Main chain carbonyl bond to Mg++ 187 Glu Glu Carboxyl binds to Mg++ 211 Lys Arg Alternative to Arg 54 213 Lys Lys H bond to C-terminus and Glu 187 214 Lys Lys Main chain amide H bond to 0(4) 215 His His Bond to Mg++ via water C terminus Tyr Ik H bond to Arg 54 in E. coli Ec, E. coli; Bs, B. stearothermophilus. The effector site is formed between the two subunits o f the rigid PFK dimer. The asterisks indicate residues from one o f the two subunits. Note that Arg 54 o f E. coli PFK is replaced by Arg 211 in B. stearothermophilus PFK. (From Shirakihara, Y., & Evans, P. R. (1988) J. Mol. Biol. 204, 973-994). its enzymology (Babul, 1978), its regulation by ATP (Kotlarz & Buc, 1981; Guixe & Buc, 1985), its dimer-tetramer equilibrium (Kotlarz & Buc, 1989), its kinetic mechanism (Campos et al, 1984), and its possible physiological function (Daldal et al., 1982; Daldal & Fraenkel, 1983; Torres & Babul, 1991). Because o f its apparent unrelatedness to EcPFK and BsPFK, however, PFK 2 will not be discussed further. Both BsPFK and EcPFK are tetramers o f four identical subunits, each composed of 319 amino acids (320 for BsPFK). The subunit molecular mass is 35,000 ± 1,000 daltons. The subunit structures o f the two enzymes are remarkably similar; they share the same secondary structural elements, and their a-carbon traces are nearly superimposable. Fig. 1.3 is a schematic representation o f the bacterial PFK tetramer. Each subunit has two domains, a large one and a small one. The catalytic site is located within the cleft formed between the two domains: it binds the substrates Fru-6P and ATP, as well as the essential Mg++ ion. (The binding site o f the NH4 + ion has not been located yet). Movement of the two domains relative to each other in EcPFK allows the active site to open and close, a process that is important for catalysis (Evans, 1992) and may be important for allosteric regulation as well. Three of the residues important for catalysis (see Table 1.1) lie on a polypeptide loop (residues 125 to 129) connecting the two domains. All of the residues that bind ATP are from the large domain, while those that bind Fru-6P are from the small domain, but include two residues, Arg 162 and Arg 243, from across the dimer-dimer inteiface. As will be shown below, Arg 162 is directly involved in the allosteric transition. The bacterial PFK tetramer can be thought o f as a dimer o f rigid dimers A:B and C:D (Fig. 1.3). The A-B and C-D interfaces (there are two in the tetramer) are called regulatory interfaces because they form the four effector sites. The A:B-C:D dimerdimer interface is called the active interface because it forms part of the four Fru-6P sites. The regulatory interface has more contact area than the active interface (about twice as much), so it is considerably more stable. Subunit A has no contact with subunit 9 Fru 6PActive site Fru 6P ATP ATP /rw W x * - ;. ».<j| Effector site FIGURE 1.3. The Bacterial PFK Tetramer. The A, B, C, and D subunits are indicated, as are the locations o f the ATP and Fru-6P sites, and the effector sites, r, p, and q are the axes o f symmetry. The directions o f rotation in the T- > R and R- > T transitions are indicated. (Taken with permission from Evans, P. R. (1992) Proc. Robert A. Welch Found. Conf. on Chem. Res. XXXVI. Regulation o f Proteins by Ligands, pp. 39-54). C; likewise B has no contact with D. J.-R. Garel and co-workers have studied the dissociation (Le Bras et al., 1989; Deville-Bonne et al., 1989) and re-association (Martel & Garel, 1984; Teschner & Garel, 1989) o f EcPFK in the chaotropic agents potassium thiocyanate, guanidinium hydrochloride, and urea. The protein is found to dissociate in an ordered, stepwise, and reversible manner according to the following scheme N - > 2 D - > 4 M - > 4 U where N represents native EcPFK, D a dimeric species, M a monomeric species, and U the unfolded monomer. Reassociation proceeds by the reverse scheme slow 4 M - > 2 D ---------- > N involving two bimolecular events, the second of which is the slower, rate-limiting step. The species "D" was shown to be the regulatory dimer (A:B or C:D above) since its disappearance or appearance was correlated with the enhancement or quenching, respectively, o f the intrinsic fluorescence o f a unique tryptophan (Trp 311) that is situated along the regulatory interface. Saturating levels o f Fru-6P were found to retard dissociation o f the native tetramer into dimers, presumably because interactions o f Fru6P with Arg 162 and Arg 243 across the active interface serve to "cross-link" the dimers together. The strength o f the interaction is so strong, in fact, that the pathway o f ureainduced dissociation is modified, causing the regulatory interface to become distorted first. The weakened tetramer then falls apart into monomers in an all-or-none process (Teschner et al., 1990). Fru-6P also prevents inactivation and dissociation o f a proteolyzed EcPFK tetramer which is not stable in its absence (Le Bras & Garel, 1985). It is interesting that several features o f bacterial PFK geometry, i.e., that the tetramer is a dimer o f dimers having two kinds o f interfaces (regulatory and active) and that Fru-6P interacts with residues across the active interface, have been predicted by these dissociation/reassociation studies independent of other structural data. 11 The tetramer o f the PFK from rabbit muscle is about twice the size o f the bacterial one (subunit molecular masses are 82,000 versus 35,000 daltons), and its bl- and C-halves show clear homology to BsPFK and to each other (Poorman et al., 1984; Lee et al., 1987). Because o f this, Poorman et al. (1984) have proposed that eucaryotic PFK arose from a procaryotic progenitor by series gene duplication and divergence. Furthermore, because the N-half o f rabbit muscle PFK has a higher homology to BsPFK than does the C-half, and because it has key catalytic residues that the C-half does not have, Poorman et al. (1984) have suggested that the N-half contains the catalytic site, while the C-half contains the regulatory site(s). Alignment o f amino acid sequences based on the Poorman model has been used as a guide for site-directed mutagenesis of rabbit muscle PFK cDNA (Li et al., 1993), which has been cloned and sequenced (Li et al., 1990). The Active Site and Catalysis X-ray diffraction studies have identified the amino acid residues that bind ATP and Fru-6P in the active sites o f EcPFK and BsPFK (Evans et al., 1981; Shirakihara & Evans, 1988). These are listed in Table 1.1. As can be seen from the table, all o f the residues that bind Fru-6P are the same between EcPFK and BsPFK, while those that bind A IT are almost all the same. Those that are different are Phe 73 (Cys in BsPFK), Arg 77 (Lys), and Met 107 (Gin). These differences are probably not significant since residue 73 interacts with the ribose hydroxyl group via its main chain amide and carbonyl groups, Lys is similar to Arg, and residue 107 interacts with the adenine ring of ATP by hydrophobic contact. The strict conservation o f active site residues between EcPFK and BsPFK is surprising given their very different responses to Fru-6P when the MgATP level is high: sigmoidal for EcPFK versus hyperbolic for BsPFK. Two different kinds o f studies have been done to determine the reaction mechanism of PFK, i.e., the manner in which the phosphoryl group is transferred from ATP to Fru-6P in the PFK-catalyzed reaction. First, 31P-NMR has been used to 12 determine the stereochemical course o f phosphoryl transfer in both the BsPFKand rabbit muscle PFK-catalyzed reactions. It was found that the transfer involves an inversion o f configuration at the phosphorous atom of the transferred phosphoryl group, which is indicative of an in-line nucleophilic attack o f the 1-hydroxyl group o f Fru-6P on the y-phosphate of ATP (Jarvest et a t, 1981). Second, site-directed mutagenesis has identified Asp 127, which is conserved among all known PFKs, as the key catalytic residue (Hellinga & Evans, 1987). When Asp 127 was mutated to Ser in EcPFK, the catalytic rate constant kcat dropped 18,000-fold for the forward reaction, and 3,100-fold for the reverse reaction, indicating that Asp 127 is critical for catalysis. From these results, and from the proximity o f Asp 127 to the attacking 1-hydroxyl of Fru-6P, Hellinga & Evans (1987) have proposed that it acts as the general base to increase the nucleophilicity of the Fru-6P 1-hydroxyl group. Studies of the pH-dependence of EcPFK activity have confirmed the important role of Asp 127 in catalysis, and have suggested a secondary role for Asp 129 ( Auzat & Garel, 1992; Laine et al., 1992). Site-directed mutagenesis in conjunction with X-ray crystallography has identified the residues important for stabilizing the EcPFK transition state and coordinating the Mg++ ion in the active site. Fig. 1.4 depicts the pentacoordinate transition state complex proposed by Berger & Evans (1992). Residues important for stabilizing the transition state include Arg 72, Thr 125, and possibly Gly 11. Arg 72 is proposed to help neutralize the negative charge o f the transferring phosphate. Mutation o f Arg 72 to His changes the pH-dependence of enzyme activity (Zheng & Kemp, 1994) and lowers the kcat 100-fold. This result indicates that Arg 72 plays a role in catalysis; it also suggests that phosphoryl transfer is associative rather than dissociative since an associative mechanism would be helped by a positive charge in the vicinity, while a dissociative mechanism would be hindered. An interaction with Thr 125 is also proposed to help stabilize the transition state since mutation of Thr 125 to Ala decreases kcat 900-fold in the forward reaction, and 3,200-fold in the reverse reaction (Berger & A r g 1 6 2 hn HjN - 'S / NH, + NH2 . i2i * V ! / + / H,N " n A r g 2 4 3 MH" Arg72 G ly 11 - NH' •NH NH, I O' O His249 NH V -v ' O . Arg 171 0X F r u 6 P o- ' '*oĥ ^ ^ V ^ ohV oh. ° v o f 0. / ° i , . . 2 / S * ' " o A 6 3 m2 2 2■o-X NH, N . -. n I ~ N „ A T P o S .\ / \ u % N H O O' | O H ' * O' _ _ / \ f > 2 HD O- Gly 104 nh o \ Th'l-lgs ^v*Si>12// r OH, A s p l g g FIGURE 1.4. The Proposed Transition State o f E. coli PFK. Residues Arg 162 and Arg 243 are from an adjacent subunit. (Taken with permission from Berger, S. A., and Evans, P. R. (1992) Biochemistry 3 1 ,9237-9242. Copyright 1992 American Chemical Society). 14 Evans, 1992). Thr 125 also interacts with the neutral oxygen o f the y-phosphate o f ATP before transfer (Auzat et al., 1994a). Gly 11, which is proposed to hydrogen-bond to an oxygen o f the transferred phosphoryl group via its main chain amide group (Fig. 1.4), has not yet been mutated and studied. Finally, site-directed mutagenesis has identified residues Asp 103 and Asp 129 as important for catalysis; they are involved in coordination o f the essential Mg++ ion (Berger & Evans, 1992). A list o f the residues involved in catalysis and their roles is presented in Table 1.3. The Kinetic M echanism The kinetic mechanism o f an enzyme is a description o f the way that substrates bind to and products are released from the active site before and after catalysis. The kinetic mechanism o f several PFKs including rabbit muscle PFK (Bar-Tana & Cleland, 1974a,b), Ascaris suum PFK (Rao et al., 1987), E. coli PFK (Deville-Bonne et al., 1991b), and a non-allosteric PFK from Lactobacillus plantarum (Simon & Hofer, 1978) as well as others, e.g., PFKs from heart and liver, the PFK from Dictyostelium discoideum, and E. coli PFK 2, have been determined. Generally, the methodology used to determine the kinetic mechanism of an enzyme is the one formulated by W. W. Cleland (1963, 1979). The method involves constructing double-reciprocal plots for initial velocity studies, as well as product and dead-end (non-reactive substrate analog) inhibition studies. The patterns o f lines obtained identify the kinetic mechanism, and fitting the data to the appropriate initial velocity equations generates a set o f parameters that quantitatively describe the kinetic mechanism. Initial velocity plots are double-reciprocal plots o f data obtained by varying the concentration of one substrate while keeping the concentration o f the other constant at different fixed levels near the Km value. Two families o f double-reciprocal lines are obtained for a bi-substrate reaction: one family for each substrate. Convergence o f both families o f lines indicates that the kinetic mechanism is sequential, meaning that both substrates must bind to the enzyme before catalysis can occur. If one family o f lines TABLE 1.3 Residues Important fo r Catalysis Residue Mutation Fold Drop in kcat Function Asp 127 - >Ser 18,000 general base Asp 129 - >Ser 960 binds Mg++ Thr 125 - >Ala 890 binds y-phosphate Arg 72 - >His 100 binds y-phosphate Asp 103 - >Ala 28 binds Mg++ Arg 171 - >Ser 3 binds Fru-1,6BP 1-P 16 does not converge, but the lines are parallel, then the mechanism is ping-pong, meaning that one substrate binds and is released before the second binds . Actually, PFK must necessarily follow a sequential rather than a ping-pong kinetic mechanism because the reaction mechanism o f PFK involves in-line nucleophilic attack, not a phosphoenzyme intermediate that would be formed if a ping-pong mechanism were involved. It comes as no surprise, then, that the kinetic mechanisms o f all the PFKs mentioned above are proposed to be sequential. However, differences are present among the PFKs listed above in terms of whether the kinetic mechanism is ordered or random, whether rapid-equilibrium or steady-state. The patterns o f lines obtained from product and dead-end inhibition plots generally establish whether the mechanism is random or ordered: if random, the plots will converge; if ordered, one of the inhibition plots with respect to the substrate that binds first will contain parallel lines. If the substrates do not bind in rapid-equilibrium, i.e., if their binding and/or release is slow relative to catalysis, then initial velocity and inhibition plots will have curved lines. However, the degree o f curvature may be small; under these conditions, the curvature may not be evident and lines that would normally converge may appear to be parallel. This was the case for rabbit muscle PFK (Bai -Tana & Cleland, 1974a,b). By application of the methodology outlined above, Bar-Tana & Cleland (1974a,b) established that the kinetic mechanism of rabbit muscle PFK involves sequential, random, non-rapid equilibrium binding o f substrates Fru-6P and ATP. Rao et a/.(1987) found that the mechanism of A. suum PFK is predominantly steady-state ordered with MgATP binding first to the enzyme, but there exists some degree of randomness, i.e., Fru-6P can bind first to a small extent. The kinetic mechanism o f E. coli PFK, like that of rabbit muscle PFK, is steady-state random (Deville-Bonne et al., 1991b; Johnson & Reinhart, 1992; Zheng & Kemp, 1992), while the kinetic mechanism o f the PFK from L. plantarum is proposed to be ordered with Fru-6P binding first, but a 17 random mechanism is not ruled out (Simon & Hofer, 1978). All but the last o f these four PFKs are allosteric. Therefore, determination o f their kinetic mechanisms is complicated by their cooperative behavior. It is o f interest to unequivocally determine the kinetic mechanism o f PFK. As stated by Uyeda (1978): "The final determination of the [kinetic] mechanism may come from the use o f non-allosteric PFK." B. stearothermophilus PFK is non-allosteric in the absence o f inhibitor PEP. Thus, it is a good candidate for finally laying to rest the kinetic mechanism of PFK. The Open-to-Closed Conformational Change o f E. coli PFK The crystal structure of the EcPFK tetramer complexed with its reaction products MgADP and Fru1,6BP (Shirakihara & Evans, 1988) was found to contain an asymmetrical unit composed o f two subunits: one "open", the other "closed." Superimposition o f the a-carbon traces o f the open and closed subunits reveals that they have substantially different conformations in the outer helical layer o f their large domains, around the ATP-binding site (Fig. 1.5). The regions that are different include residues 41 to 49 (helix 2), 71 to 95 (helices 4a and 4), and 101 to 118 (helix 5) o f the large domain. In addition, residues 303 to 309 near the C-terminal end appear to be in different positions in the open versus closed subunit structures. Presumably, an opento -closed conformational change can occur within the EcPFK subunit. One effect o f the conformational change is closure o f the active site, which brings the two substrates Fru6P and MgATP, which are too far apart in the open structure, closer together so that reaction can occur (Evans, 1992). The closing and opening of the active site is postulated to be an integral part of the catalytic cycle, as shown in Fig. 1.6. Substrates bind to the open subunit, it closes, phosphoryl transfer takes place, products are formed, the subunit opens, and products are released. Evans (1992) does not attribute any allosteric significance to the open-to-closed conformational change. Nevertheless, it may be important for the mechanism by which ATP "allosterically" inhibits EcPFK, and Fru-6P overcomes that inhibition (discussed in 4-0 - 2 - ------129 - 10 3 -5 CC an: r fr RflCRFR 3 -0 n 41 49 3 0 3 30995 101 118 CJCJ co "Tn S 2 • 0 oo «o 0 5 5 00 100 150 200 2 5 0 3 0 0 (a) FIGURE 1.5. The Open-to-Closed Conformational Change. A plot of the magnitude o f the differences in coordinates of a-carbons between the open and closed subunit forms o f E. coli PFK versus amino acid number. (Taken with permission from Shirakihara, Y., & Evans, P. R. (1988) J. Mol. Biol. 2 0 4 ,973-994. Copyright 1988 Academic Press). oo 19 ATP F6P \ / Substrates 15? ?§£■ ADP FBP OpenOpen Products N ^ C Iosed / \ c l o s e d y FIGURE 1.6. T he Proposed Reaction Cycle o f E. coli PFK. (Taken with permission from Evans, P. R. (1992) Proc. Robert A. Welch Found. Conf. on Chem. Res. XXXVI. Regulation o f Proteins by Ligands, pp. 39-54). 20 more detail below). It may also be involved in GDP activation o f the ATP-inhibited enzyme. Garel and co-workers have identified a region near the C-terminus o f EcPFK that acts as a "built-in" effector; its removal by proteolysis makes the enzyme dependent upon Fru-6P for stability, and destroys the sensitivity o f the enzyme to allosteric effectors PEP and GDP (Le Bras & Garel, 1982; Le Bras & Garel, 1986). Removal of this C-terminal region also causes the cooperativity o f the enzyme with respect to Fru6P to drop two-fold (the Hill number falls from 4.0 to 2.0). The position o f the proposed built-in effector has been localized to a stretch o f 30 amino acid residues (between 280 and 310) near the C-terminal end (Serre & Garel, 1990). An examination o f Fig. 1.5 shows that the residues at the C-terminus that move during the open-toclosed transition are squarely within this 30-amino acid region. Furthermore, the loop between p-strands J and K (the J-K loop) which is found to move between the unliganded (Rypniewski & Evans, 1989) and open liganded (Shirakihara & Evans, 1988) forms of the EcPFK subunit, is also within the region identified by Serre & Garel (1990). Rypniewski & Evans (1989) have shown that the J-K loop o f EcPFK moves toward the catalytic loop composed o f residues 125 to 138 as the subunits go from unliganded to substrate-liganded structures. Specific hydrogen bonds are formed between residues in the J-K loop and those in the catalytic loop, and others hydrogen bonds are broken, during this process. The importance o f the C-terminal "effector" region (residues 280 to 310) for EcPFK allosteric behavior can be explored using sitedirected mutagenesis, and also by constructing chimeric Bs/Ec PFKs (see chapter five). X-ray crystallography shows that, unlike EcPFK, the large (ATP-binding) domain o f BsPFK remains rigid during the allosteric transition o f the enzyme (Schirmer & Evans, 1990). There is no evidence that the BsPFK subunit undergoes on open-toclosed conformational change. The lack of this conformational change in BsPFK, along with the corresponding lack of allosteric inhibition o f BsPFK by MgATP suggests that the two phenomena- the conformational change and inhibition- may be associated in EcPFK. The Effector Site The effector sites o f EcPFK and BsPFK lie in deep clefts between subunits related by the molecular p axis, i.e., between subunits o f the rigid dimer (Fig. 1.3). The effector site residues that bind the activator MgADP and the inhibitor PEP have been identified by X-ray crystallography (Evans et al., 1981; Shirakihara & Evans, 1988). As can be seen from Table 1.2, half o f the residues in the site are contributed by one subunit o f the rigid dimer, the other half by the other subunit. Schematic diagrams of the effector site o f EcPFK with MgADP bound, as well as with PEP bound, are shown in Figs. 1.7A and 1.7B, respectively. Using site-directed mutagenesis, several effector site mutants have been created, and analyzed for their ability to bind GDP and PEP. Truncation o f each o f the positively-charged residues Arg 21, Arg 25, Arg 54, Arg 154, and Lys 213 to Ala in EcPFK weakens the binding o f both MgGDP and PEP by 2 to 3 kcal/mole, a result consistent with the disruption o f charged hydrogen bonds important for effector binding (Lau & Fersht, 1989). Mutation of Tyr 55 to Gly has no effect on the Kt(PEP) and only a small effect on Kr(GDP), indicating that the proposed hydrophobic interaction between residue 55 and the purine ring o f ADP or GDP is not critical (Lau et al., 1987). Thus, the key interactions for effector binding appear to be with the phosphates o f GDP (or ADP), or the phosphate and carbonyl o f PEP, as indicated in Fig 1.7. In BsPFK, Arg 211 replaces Arg 54, and residue 54 is a valine in BsPFK (Table 1.2). Valdez et al. (1988) have mutated Arg 25, Asp 59 and Arg 211 in BsPFK. As expected, both arginine residues were found to be important for effector binding. Asp 59, on the other hand, was found not to be essential, probably because its interaction with effector is via its main-chain amide and carbonyl groups. Interestingly, the Arg 25 22 A Subunit A Glu-187 Lys-213 T y r-3 1 9 ■NHi A r g 154 M g'NH HDt x T yr-55- O- p NH OH CH,NH;Arg-21 NH Subunit BNH A rg-54 Arg-25 Asp-59 B Subunit A G lu-187 L y s -2 1 3 T y r-3 1 9 NH, NH; NH2 O H j N - ( N H / ; A rg-21 H2N ,n h 2 Tyr-55 Subunit B NHNH A rg-54A rg-25 A sp -5 9 FIGURE 1.7. The Effector Site o f E. coli PFK. Proposed structure o f the effector site with (A) ADP, and (B) PEP bound. Subunits A and B are the subunits o f the rigid dimer. (Taken with permission from Lau, F. T., & Fersht, A. R. (1989) Biochemistry 28, 6841-6847. Copyright 1989 American Chemical Society). 23 - > Ala mutant had a two-fold higher S i/2Fru'6P value, and exhibited sigmoidality (n=2.0). Thus, a mutation at the effector site can affect the active site. Glu 187 in another residue at the effector site whose mutation affects the active site. Glu 187 is proposed to help coordinate the Mg++ ion essential for GDP (or ADP) binding. Its mutation to Ala causes PEP to activate the enzyme rather than inhibit it (Lau & Fersht, 1987). The mutation also affects the homotropic interactions o f Fru-6P binding. At low Fru-6P concentration, the mutant enzyme is more than 100-fold more active than wild-type in the presence of 10 mM PEP, and the cooperativity with respect to Fru-6P drops from 3.7 to 1.1. Auzat et al. (1994b) have found that the mutation Glu 187 - > Asp destroys PEP inhibition, but not GDP activation, at pH 8.2. However, mutation o f Glu 187 to Leu destroys both inhibition and activation. Thus, the function of Glu 187 is to discriminate between PEP and GDP binding in the effector site. PFK and the Allosteric Model of Monod, Wyman and Changeux Bacterial PFK is allosterically inhibited by PEP and activated by ADP (or GDP). In addition, the substrate MgATP apparently "allosterically" inhibits E. coli PFK when Fru-6P concentrations are low, causing the Fru-6P saturation curve to be highly sigmoidal in the presence of a saturating concentration o f MgATP. (MgATP does not inhibit B. stearothermophilus PFK by the same mechanism. The Fru-6P saturation kinetics o f BsPFK are essentially hyperbolic). The word "allosteric" refers to an effect on the binding of a ligand that results from the binding o f the same or another ligand at a distinct, specific site (Monod et al., 1965). An allosteric interaction can be either homotropic or heterotropic. It is homotropic when between identical ligands, heterotropic when between different ligands. According to these definitions, the positive cooperativity observed in the Fru-6P saturation curve of EcPFK would be the result o f homotropic allosteric interactions between Fru-6P molecules binding to distinct sites on the protein. On the other hand, the effects o f PEP and GDP in decreasing and 24 increasing, respectively, the cooperativity o f Fru-6P binding would be due to heterotropic allosteric interactions between the effectors and Fru-6P. E. coli PFK was one o f the first enzymes shown to obey the concerted model of Monod, Wyman, and Changeux (the MWC model; 1965). Many of the regulatory and structural properties o f the enzyme have since been interpreted in terms o f the model. Therefore, it is useful to present its basic tenets and show how it has been applied to EcPFK (Blangy et al., 1968). The MWC model is simple and elegant; it assumes that a symmetry o f allosteric effects due to ligand binding is correlated to protein molecular symmetry. A number of regulatory enzymes besides PFK, including E. coli aspartate transcarbamylase (Kantrowitz & Lipscomb, 1988) and rabbit muscle glycogen phosphorylase (Barford & Johnson, 1989), obey the MWC model. Paraphrasing Monod eta l.( 1965), the MWC model makes the following statements: (1) the protomers (subunits) o f oligomeric allosteric proteins are symmetrically arranged relative to an axis o f symmetry, (2) the distinct ligand binding sites (one per protomer) are also symmetrically arranged, (3) the conformation o f one protomer is constrained by its association with the others, i.e., protomers are allosterically-linked, (4) the allosteric oligomer can reversibly exist in two (at least) conformational states, (5) because of this, the affinity of the binding site for its ligand is altered when a transition occurs from one state to the other, and (6) molecular symmetry is conserved in the allosteric transition. Suppose F is a ligand with differential affinity toward two states o f the oligomer. In the absence of F, the oligomer is in equilibrium between the two states R0 and T0; the equilibrium constant for the R0 <- > T0 transition is L, the "allosteric constant" (L= To/Ro)We define Kr and Kp as the microscopic dissociation constants for ligand F from a specific binding site in the R and T states, respectively. After writing equilibrium equations for the binding of F to the R and T states, the following 25 expression, which describes the fraction o f sites on the oligomer actually occupied by ligand F, i.e., the "saturation function" Yp , can be written: Lea (1 + ca)11'1 + a(l + a)n l Y F = - ----------------- (eqn. 1) L(1 + c a)n + (1 + a)n where L= Tq/Ro is the allosteric constant; c= K r/K j is the non-exclusive binding constant, a measure o f the affinity o f ligand F for the R state versus the T state; a = F/Kr is the normalized concentration o f ligand F; and n is the number o f homologous binding sites on the protein for F. Equation 1 describes homotropic interactions among ligands F. The cooperativity o f the response o f the oligomer to F depends on the values o f L and c. Cooperativity is pronounced when L is large, i.e., when T0 is strongly favored in the R0 <- > T0 equilibrium, and when c is small. When c is very small, as is proposed to be the case for EcPFK (Blangy et al., 1968), the saturation function reduces to a(l + a)"'1 Yp = (eqn. 2) L + (1 + a)n When the ligand F has no preference for either the T or the R state, i.e., when c= 1, and L is small, the equation reduces to Y f = F/(Kr + F),which is the Michaelis-Menten equation. The MWC model also describes heterotropic interactions between different ligands. Assume that a substrate S has significant affinity for its binding site in only one state, e.g., R, and that o f two other allosteric ligands I and A, I has affinity only for the T-state while A has affinity only for the R-state. The effects o f ligands I and A will be to displace the equilibrium between R and T states, so that L becomes L', a new apparent allosteric constant. L' = L[(l + P)n / (1 +y)n], where P= l/K \, and y= A /K a • Ki 26 and Ka are the microscopic dissociation constants for I and A from the T and R states, respectively. Finally, the saturation function for S, Y s, is given by: oc(l + a)""1 Y s = ------------- (eqn. 3) r (1 + P )n „ ^L ------ -- + (1 + ex) (1 + Y) Using the MWC model, we can distinguish two kinds o f allosteric systems involving a substrate S and an effector F: (1) a K system. Both F and S have differential affinities toward the T and R states, i.e., both are allosteric, and (2) a V system. S has the same affinity for both states. In this case, F does not affect binding o f S, and vice-versa. But, F can affect the catalytic rate, either as an activator (positive V system) or as an inhibitor (negative V system). Blangy, Buc and Monod (1968) have shown that E. coli PFK obeys the MWC model, at least in the presence of an ATP concentration close to its Km. From a thorough analysis of steady-state kinetic data in terms o f the model, the kinetic and allosteric parameters in Table 1.4 were obtained. Based on these parameters, one can conclude that (i) unliganded EcPFK exists overwhelmingly in the T state (only one molecule in 4 million will be in the R state since L= 4 x 106), (ii) the oligomer contains 4 protomers, (iii) ATP has no preference for either T or R and thus is not an allosteric ligand (K r = K j), (iv) Fru-6P prefers the R state by a ratio o f 2,000 to 1( c= 5 x 10^), (v) GDP binds almost exclusively to the R-state, and (vi) PEP binds almost exclusively to the T-state. Several of the afore-mentioned predictions about EcPFK based on the MWC model have been shown to be untrue over the course o f time. First, X-ray diffraction studies o f unliganded EcPFK (Rypniewski & Evans, 1989) have shown that the unliganded enzyme resembles the form that has a high affinity for Fru-6P, i.e., the 27 TABLE 1.4 Kinetic and Allosteric Parameters fo r E. coli PFK Ligand Kr (mM) KT (mM) ATP 0.06 0.06 Fru-6P 0.0125 25 ADP 0.025 1.3 GDP 0.04 >40 PEP >750 0.75 n = 4 L = 4 x 106 c = 5 x 10-4 28 R-state form. Using equilibrium dialysis, Deville-Bonne et al. (1992) have shown that L=2. Second, ATP is now known to be involved in the allosteric response o f EcPFK. It profoundly inhibits the enzyme when Fru-6P concentration is low (Kundrot & Evans, 1991). Fluorescence studies show that the ATP analog AMPPCP causes the Fru-6P binding profile to become sigmoidal (Deville-Bonne et al., 1992). This result suggest that antagonistic interactions between MgATP and Fru-6P are involved in EcPFK homotropic cooperativity. ATP has been termed an "allosteric" inhibitor o f EcPFK (Evans, 1992). Third, GDP can super-activate EcPFK that is already saturated by Fru6P , increasing kcat 10 to 20% (Deville-Bonne et al., 1991a). Thus, GDP affects not only the binding o f Fru-6P, but also the catalytic rate, and indicates (at least) that EcPFK is not a perfect K-system enzyme. Finally, the observations that (i) both kcat and the Hill number for Fru-6P saturation are pH-dependent and (ii) Hill numbers can be as high as 6.0, suggests that the cooperativity of EcPFK has a kinetic origin. Hill numbers higher than the number o f binding sites (4 for Fru-6P binding to EcPFK) are not predicted by the MWC model; they arise when the enzyme does not operate at equilibrium. Such non-equilibrium conditions, which can result in kinetic cooperativity, are present when catalysis is fast relative to either substrate binding and/or release, or an essential structural transition. The resulting kinetic cooperativity can then add to the binding cooperativity, causing the Hill number to increase to values beyond the number of binding sites. Blangy et al. (1968) were not the first to investigate the regulation o f E. coli PFK. Atkinson & Walton (1965) studied the mutual effects o f Fru-6P, ATP, AMP, and M g++ on the rate o f the EcPFK-catalyzed reaction. Their results show clearly that ATP negatively regulates the enzyme when its concentration is saturating (above 1.0 mM). The complex patterns of curved lines that Atkinson and Walton obtained stand in stark contrast to the graceful patterns obtained by Blangy et al. (1968), which fit the MWC 29 model so perfectiy, albeit under a very limited set o f experimental conditions (0.1 mM ATP). One must wonder if Blangy et al. (1968) read the 1965 paper, which states: ". . . the whole kinetic picture [for PFK] depends on the initial assay conditions. Thus, if a relatively high concentration o f ATP were used in determining rate as a function of Fru-6P and Mg++ concentrations, and if just saturating levels of Fru-6P and Mg++ were used in turn to study the rate versus ATP concentration . . . a self-consistent set o f [kinetic parameters] would be obtained. If, however, the same procedure were repeated with a lower initial concentration o f ATP, a completely different but equally self-consistent set o f values would result. More intensive kinetic analysis o f either set o f results could obviously not yield generally applicable parameters." In conclusion to this section, the MWC model does partially describe the regulation o f EcPFK (and BsPFK). However, it is inadequate and too simple to account completely for PFK cooperative behavior. Other models, such as those involving kinetic cooperativity arising from a slow transition (Frieden, 1970; Ainslie et al., 1972) or from the kinetic mechanism (Ferdinand, 1966; Sweeny & Fisher, 1968) may prove to be important. The Hill Equation Because the concerted model is not valid for PFK under many conditions, its equation (equation 2) was not used to fit the sigmoidal substrate saturation data obtained for EcPFK or BsPFK. Rather, the Hill equation (Hill, 1910), which assumes no particular model and is more empirical, was used. The general form of the Hill equation that is used to describe saturation o f a protein by a ligand is: Y log T~y = h log "log K (eqn-4) where Y is the degree o f saturation o f the protein by ligand, [A] is the concentration of free ligand, and K is a constant. A plot of log (Y /(l-Y )) versus log [A] gives a straight line, the slope o f which is the Hill number h (or nn)The Hill number describes the cooperativity o f the response o f the protein to ligand. The ligand concentration at 30 half-saturation, [A] 1/2 , is equal to K1/". The form of the Hill equation generally used in studying the sigmoidality o f saturation curves obtained from kinetic data is: VTM* [A]h v = ------------ -------- - (eqn. 5) [A]h + A1/2h In this form, the binding term Y /(l-Y ) has been replaced by the kinetic term v/(Vmax-v). A Structural Model for the PFK Allosteric Transition Although the MWC model is now known to be too simple to explain the regulation o f bacterial PFK, it has been used extensively to make sense o f differences in protein conformation found to exist in X-ray structures obtained under different conditions (Evans & Hudson, 1979; Evans et al., 1981; Evans et al., 1986; Shirakihara & Evans, 1988; Schirmer & Evans, 1990). Schirmer & Evans (1990) have proposed a structural model for the allosteric transition o f B. stearothermophilus PFK based on Xray diffraction data obtained from crystals under two different conditions: (a) in the presence o f the inhibitor 2-phosphoglycolate (an analog o f PEP), when the enzyme is presumably in its T-state, and (b) in the presence o f the substrate Fru-6P and activator MgADP, when BsPFK is in the R-state. A comparison of the T-and R-state structures of the tetramer reveals that the structural changes accompanying an ostensible T-to-R transition can be described as three separate events that occur simultaneously: (1) rotation o f the two rigid dimers A:B and C:D by 7° relative to each other, (2) a passive change along the "active" dimer-dimer interface that allows an ordered layer o f water molecules to come in between the two dimers, and (3) the coordinated back-and-forth movement of a pair o f loops, the 8H and 6F loops, that are situated between the effector and active sites. This last change involves the switching o f two residues, Glu 161 and Arg 162, into and out o f the active site. It is described in more detail below. 31 The 8H loop (residues 211 to 216) lines one edge o f the effector cleft (Fig. 1.8). In the absence o f effector, the cleft is open. Binding o f inhibitor PEP pulls the 8H loop inward, closing the cleft and placing the enzyme in the T-state. Binding o f ADP (or GDP), on the other hand, pushes the 8H loop away from the effector site and places BPFK in the R-state. Movement o f the 8H loop is coordinated with a reorganization o f the 6F loop (residues Thr 156 to Arg 162) along the dimer-dimer interface. In the T-toR transition, Glu 161, which points into the active site across the dimer-dimer interface in the T-state, rotates away. At the same time, Arg 162 swings around into the active site, occupying the position formerly occupied by Glu 161 (Fig. 1.9). The positivelycharged guanidinium groups of Arg 162 and Arg 243 favorably interact with the negative charges o f the phosphate of the incoming Fru-6P, thereby increasing its binding affinity. The R-to-T transition involves the reverse sequence o f events, with Glu 161 swinging back into the active site and Arg 162 rotating away. In this way, the binding o f GDP and PEP at the effector site trigger the heterotropic allosteric transition and alter the affinity o f the active site for Fru-6P. An interaction important for stabilizing the T-state structure of BsPFK is the salt bridge between Arg 72 and Glu 241. This salt bridge is absent in the R-state but present in the T-state (Fig. 1.9). Thus, Arg 72, which is critical for catalysis, is sequestered away by Glu 241 when the enzyme undergoes the R-to-T transition. The Active Site, Cooperativity, and the Role o f ATP Site-directed mutagenesis has identified a number o f residues in the active site of EcPFK (and one in BsPFK) important for the enzyme's cooperative response to Fru-6P. These residues are listed in Table 1.5. As mentioned above, Arg 162 and Arg 243 bind the phosphate group o f Fru-6P from across the dimer-dimer interface (Shirakihara & Evans, 1988). When these two residues are changed to Ser, the cooperativity drops significantly (Berger & Evans, 1990) The same happens when Arg 72 is changed to a FIGURE 1.8. Co-ordinated M ovement o f the 8H and 6F Loops. The region between the effector site and active site that undergoes large structural change between the Tstate (solid lines) and R-state (open lines). 8H loop: residues 211-216; 6F loop: residues 156-162. **, residues from the same dimer. *, residues from the other dimer. PGC, the inhibitor 2-phosphoglycolate, which binds to the T-state. ADP and Fru-6P bind to the R-state. R-state residues are primed. (Taken with permission from Schirmer, T., & Evans, P. R. (1990) Nature 3 4 3 ,140-145. Copyright 1990 Macmillan Magazines, Limited). subunit A 7 0 Asp12 -.6 6 / 0 . 0 NH ' r , n „ j* r * S e n 59 ? Arg252 V / \ n h / + NHz \ o ' G l u 161 x0 + y ^ nh Arg243 subunit D 155Arg72 Alai 57 s1 60 J L :Glu241 > A r g 1 6 2 subunit A A d e n o s in e \ ° ADP ° 7 \0 0 \ A s p 1 2 , ' " iH A -,n\ His160 />■: A 9 0 o 'NH2 + NH2 1 «, \ - ''- nh H;o Glu241 Ar9 252 K il2/ fi° s v N? / - . + ; ^ nHj _ Arg 162 r / - ' Ser159 G l u 1 61 Arg243 subunit D FIGURE 1.9. Reorganization o f the 6F Loop, a, the T-state structure, showing Glu 161 at the active site, b, the R-state structure, showing Arg 162 and Arg 243 interacting with the phosphate group o f Fru-6P. The wavy line represents the interface between dimers. (Taken with permission from Schirmer, T., & Evans, P. R. (1990) Nature 3 4 3 ,141-145. Copyright 1990 Macmillan Magazines, Limited). 34 TABLE 1.5 Residues Important fo r Homotropic Cooperativity ofE . coli PFK Mutation Fold Increase in S i/2 Fru'6P Hill Number* R 162- >S 7 2 . 1 R 243- >S 30 2.7 R 72- >S 1 2 . 2 S 159- >N > 1 0 0 0 ( 1 ) T 156->(GS) > 1 0 0 ( 1 ) D 129- >S 4 2.7 T 125- >A 1 . 6 1 D 127- >Y 1.5 1 R 252- >A** 1500 N. A. *, the Hill numbers in parentheses are only estimates since saturation with respect to Fru-6P was not achieved. **, from B. stearothermophilus PFK. N. A., not applicable. Ser (Table 1.5). Threonine 156 and Ser 159 are within the 6 F loop o f EcPFK (as is Arg 162), which becomes reorganized during the allosteric T-to-R transition. Mutation of either o f these residues severely affects Fru-6 P binding, and the mutant enzymes appear to be locked in the T-state since their catalytic rate constants are very low (about 100fold lower than wild-type) and their Fru-6 P saturation profiles are hyperbolic (Kundrot & Evans, 1991). Arg 252 is proposed to bind to the ribose hydroxyl o f Fru-6 P. Mutation of Arg 252 to Ala in BsPFK severely lowers the affinity o f the enzyme for Fru-6 P and also lowers the cooperativity o f its response to Fru-6 P when inhibited by PEP (Valdez et al., 1989). Finally, Thr 125 binds the y-phosphate o f ATP and helps stabilize the transition state, v/hile Asp 129 helps coordinate the Mg++ ion. Mutation o f Thr 125 and Asp 129 to Ala and Ser, respectively, both cause a 100-fold drop in kcat and a moderate increase in S i/2 Fru"6P, but have somewhat different effects on cooperativity. The mutation Thr 125- >Ala completely abolishes cooperativity (Hill number is 1.0), while the Asp 129- >Ser mutation reduces it (nn drops from 4 to 2.7). Thus, Thr 125 appears to be critical for cooperativity, but Asp 129 is less important. Auzat et al. (1994a) have further shown that an interaction between Thr 125 and the yphosphate o f ATP is involved not only in the cooperative response o f EcPFK to Fru-6 P, but also in its response to PEP. The observation that mutation o f a residue that interacts directly with the y-phosphate of ATP, e.g., Thr 125, abolishes cooperativity, while mutation of other active site residues, e.g., Asp 129, simply lowers cooperativity, suggests that the y-phosphate itself is essential for the cooperative response. A growing body of experimental evidence indicates that antagonistic interactions between ATP and Fru-6 P in the active site o f EcPFK trigger the conformational change associated with the homotropic allosteric transition. DevilleBonne et al. (1991b) have found that MgATP antagonizes the binding o f Fru-6 P in the active site of EcPFK, and vice-versa. Fluorescence studies support this finding. Whereas titration of the intrinsic fluorescence of EcPFK with Fru-6 P gives a hyperbolic 36 binding curve, titration with Fru-6 P in the presence o f AMPPCP (Deville-Bonne et al., 1992) gives a sigmoidal curve (nn is above 2). A Lys 213- > Ala mutant EcPFK enzyme that is insensitive to heterotropic regulation but retains homotropic cooperativity has also been used to study the effect o f ATP on Fru-6 P binding (Berger & Evans, 1991). As with the wild-type enzyme, ATP induces positive cooperativity in the Fru-6 P binding curve of the mutant. Thus, ATP (or AMPPCP) can induce positive cooperativity in the Fru-6 P binding curve of EcPFK. Interestingly, the converse is also true: Fru-6 P induces positive cooperativity (nH is 2.0) in the AMPPCP binding curve (Deville-Bonne et al., 1992). Zheng & Kemp (1992) propose that antagonistic interactions between ATP (via its y-phosphate group) and Fru-6 P are important for "allosteric" ATP inhibition and cooperativity, but suggest a purely kinetic origin for the cooperativity. In contrast, Johnson & Reinhart (1992) maintain that an inter-active site interaction mediated through protein conformational change is partly responsible for the antagonism between Fru-6 P and MgATP. Interestingly, Johnson & Reinhart (1994) found that MgADP in the effector site requires the presence of MgATP in the active site to activate the enzyme; in its absence, MgADP actually inhibits Fru-6 P binding. Thus, whatever the mechanism of ATP inhibition, it appears that Fru-6 P cannot act as a cooperative substrate by itself, but requires the presence of MgATP beside it in the active site in order to fulfill its role as a "cooperative" ligand. The mechanism by which MgATP inhibits EcPFK has been investigated (Zheng & Kemp, 1992) but has not been conclusively determined. An intriguing possibility for the mechanism is that ATP inhibition is mediated by closure o f the active site. If subsequent Fru-6 P binding, i.e., Fru-6 P binding after MgATP, causes re-opening o f the active site via a transition that is slow relative to catalysis, the result could be sigmoidality in the Fru-6 P saturation curve. 37 Mutation of Leucine 178 to Tryptophan Leu 178 is close to neither the active site nor the effector site, yet it appears to be involved in the heterotropic allosteric response o f E. coll PFK. Its mutation to Trp causes specific suppression o f heterotropic cooperativity but allows retention of homotropic cooperativity. In other words, the mutant enzyme cannot be inhibited by PEP or activated by GDP, but its Fru-6 P saturation curve is highly sigmoidal (Serre et al., 1990). This result is important because it shows that heterotropic and homotropic effects can be uncoupled: thus, they probably involve different conformational changes. Such a result is not predicted by the MWC model, which assumes a common conformational change for both homotropic and heterotropic allosteric responses. Presumably, Leu 178 lies along the structural pathway for transmission o f heterotropic, but not homotropic, signals. Fluorescence Studies Both BsPFK and EcPFK have a unique tryptophan residue. It is at position 179 in BsPFK and position 311 in EcPFK. The intrinsic fluorescence o f Trp 179 o f BsPFK is largely insensitive to the binding o f substrates or allosteric effectors (Kim e t al., 1993). Thus, it is not possible to use intrinsic fluorescence to probe conformational changes associated with allostery in BsPFK. It may be possible, however, to use sitedirected mutagenesis to strategically place a tryptophan residue in the right place in BsPFK, allowing the fluorescence to be sensitive to conformational changes without disrupting protein structure This would open up a whole new set o f binding studies for BsPFK (discussed in chapter five). Unlike Trp 179 o f BsPFK, Trp 311 of EcPFK is responsive to conformational changes induced in the enzyme by the binding o f substrates and effectors. Saturation of the enzyme with Fru-6 P causes a 20% decrease in intrinsic fluorescence. ADP causes a similar 20% decrease, but ATP (or AMPPCP) causes a 10% increase and PEP binding results in a 5 to 10% increase (Berger & Evans, 1991; Deville-Bonne et al., 1992). 38 Thus, Fru-6 P and ADP appear to place the enzyme in a low-fluorescence state, while ATP (or AMPPCP) and PEP place the enzyme in a high-fluorescence state. A number of fluorescence studies have been performed on wild-type and mutant EcPFK enzymes (Berger & Evans, 1991; Deville-Bonne e ta l., 1992; Johnson & Reinhart, 1992; Johnson & Reinhart, 1994; Auzat et al., 1994a). These studies have contributed significantly to our knowledge o f the regulation o f EcPFK, especially in regard to the role that ATP plays (see discussion above). Objectives of Dissertation Research The objectives are threefold: (1) to determine the kinetic mechanism of B. stearothermophilus PFK using initial velocity and inhibition studies, (2) to investigate the role of the open-to-closed conformational transition in allosteric regulation o f E. coli PFK by constructing a chimeric PFK that contains the "rigid" large (ATP-binding) domain o f B. stearothermophilus PFK grafted onto the remainder o f the E. coli PFK subunit, and (3) to uncover the importance o f residue 161 in the allosteric transitions of both B. stearothermophilus PFK and E. coli PFK using site-directed mutagenesis. Chapter 2 Kinetic Characteristics of Phosphofructokinase from Bacillus stearothermophilus'. MgATP Non-allosterically Inhibits the Enzyme (Reprinted with permission from Byrnes, M., Zhu, X., Younathan, E. S., and Chang, S. H. (1994) Biochemistry 33 ,3424-3431. Copyright 1994 American Chemical Society.) 3 9 4 0 Phosphofructokinase (PFK, EC 2.7.1.11) catalyzes the first committed step of glycolysis, the transfer of a terminal phosphate from ATP to (3 J>-fructose 6 -phosphate (Fru-6 P) to form ADP and (3-fructose 1,6-bisphosphate (Fru-1,6BP): MgATP + Fru-6 P > MgADP + Fru-1,6BP. The allosteric PFK from the thermophilic bacterium B. stearothermophilus , like its counterpart from E. coli, is a tetramer o f identical subunits, each composed o f 319 amino acids (320 for E. coli PFK). Fifty-five percent of the amino acids are identical between the two bacterial enzymes (French and Chang, 1987). Both B. stearothermophilus PFK (BsPFK) and E. coli PFK (EcPFK) are allosterically inhibited by phospho(enol)pyruvate (PEP). This inhibition is reversed by the activators adenosine diphosphate (ADP) and guanosine diphosphate (GDP) (Valdez e ta l., 1989; Blangy et al., 1968). A comparison between the crystal structures o f BsPFK and EcPFK (Evans et al., 1981; Shirakihara and Evans, 1988) reveals striking structural similarity. The two enzymes have all the same secondary structural elements, and their subunit a-carbon traces are nearly superimposable. In both enzymes, the subunit is divided into a large and a small domain, with the active site located within the subunit in a cleft between the two domains. The active site residues that bind substrates are either the same or similar between the two enzymes. Despite the high degree of structural similarity between BsPFK and EcPFK, there is a significant kinetic difference between them. Steady-state initial velocity studies reveal that whereas saturation o f EcPFK by Fru-6 P is highly cooperative in the presence of saturating MgATP (Blangy eta l., 1968), saturation o f BsPFK by Fru-6 P shows little or no cooperativity under the same conditions (Valdez et al., 1989). Fructose 6 -phosphate saturation o f BsPFK is cooperative, however, in the presence of 41 PEP (Valdez et al., 1989). Thus, BsPFK resembles the PFK from the extreme thermophilic bacterium Flavobacterium thermophilum (Yoshida, 1972), which likewise displays a hyperbolic Fru-6 P saturation profile that becomes sigmoidal in the presence of PEP. Schirmer and Evans (1990) have proposed a structural model for the allosteric transition of BsPFK based on X-ray diffraction data. Although the structure o f BsPFK has been extensively investigated, its kinetic characteristics have remained largely unstudied. For example, although the amino acid residues involved in binding the substrates MgATP and Fru-6 P have been known for some time (Evans et al., 1981), the kinetic mechanism has not been studied. Several observations suggest that there are differences between the active site regions o f BsPFK and EcPFK: (i) the active site structure o f BsPFK is more open than that o f EcPFK (Shirakihara and Evans, 1988), (ii) significant binding o f GDP occurs in the active site of BsPFK (Valdez e t al., 1989), but not in the active site o f EcPFK (Blangy et al., 1968), and (iii) the large (ATP-binding) domain of EcPFK moves as the enzyme changes conformation from a "closed" to an "open" subunit structure (Shirakihara and Evans, 1988), whereas the large domain o f BsPFK remains essentially rigid during the allosteric transition o f the enzyme (Schirmer and Evans, 1990). These observations have stimulated our interest in studying the kinetic characteristics o f the BsPFK-catalyzed reaction in both the forward and reverse directions. Initial velocity, product inhibition, and mixed alternate substrate studies o f the reverse reaction indicate that the kinetic mechanism is sequential random. Product and dead-end inhibition studies of the forward reaction corroborate this result. However, initial velocity studies o f the forward reaction indicate that MgATP is a nonallosteric substrate inhibitor, and the binding of MgATP is a non rapid-equilibrium process. Based on these results, it is proposed that substrate binding proceeds via two alternative steady-state pathways, with one pathway kinetically favored over the other. Substrate inhibition by MgATP is proposed to result from both abortive binding o f MgATP in the Fru-6 P site and reaction flux through the disfavored pathway. Materials and Methods Enzymes and Chemicals-NADH, NADP+, Fru-6 P, ATP, Fru-1,6BP, ADP, PEP, Ara-5P, AMPPCP, and the auxiliary enzymes for the coupled PFK activity assays were all obtained from Sigma Chemical Co. (St. Louis, MO). The Cibacron Blue 3GAagarose (type 3000-CL-L) resin was also obtained from Sigma. Restriction endonucleases used for cloning were obtained from either New England Biolabs (Beverly, MA) or United States Biochemical (Cleveland, OH). Expression and Purification o f PFK-BsPFK was expressed in PFK-deficient E. coli cells (DF1020 cells) from a recombinant plasmid constructed by cloning the bspfk gene (French and Chang, 1987) into the EcoRI/Hindlll sites o f pUC18. The enzyme was subsequently purified by a three-step procedure (Valdez et al., 1989) involving (1) sonication, (2) heat-treatment at 70°C, and (3) chromatography on a Cibacron blue 3GA affinity column. The purified enzyme preparation had a specific activity o f 160 units/mg. Its purity was demonstrated by the presence o f a single band on an SDSpolyacrylamide gel stained with Coomassie Brilliant Blue dye. The purified enzyme solution was dialyzed at 4°C in 100 mM Tris-Cl, pH 7.4, containing 1 mM dithiothreitol and 50% glycerol, and stored at -20°C. Protein concentration was determined using the Bio-Rad (Richmond, CA) protein assay. Site-directed Mutagenesis, and Expression and Purification of Mutant BsPFK- The site-directed mutagenesis procedure to mutate the bspfk gene, as well as the procedures for expression and purification o f the mutant enzyme, are to be described separately (Zhu et al., manuscript in preparation). Kinetic Assays-The initial velocity of the PFK-catalyzed reaction in the forward direction was measured at 30°C in 100 mM Tris-Cl, pH 8.2, containing 10 mM MgCU and 5 mM NH4 CI by coupling the production o f either Fru-1,6BP or ADP to the 43 oxidation o f NADH (0.20 mM.) (The saturation curves with respect to substrates Fru6 P and MgATP obtained at 30°C were similar to those obtained at 60°C, except that the kcat was lower at 30°C. Fru-6 P saturation was non-cooperative at both temperatures, and substrate inhibition was apparent with high MgATP at both temperatures). The Fru-1,6 BP-coupled assay utilized the auxiliary enzymes aldolase (20 pg/ml), triosephosphate isomerase ( 1 0 (ig/ml) and a-glycerophosphate dehydrogenase ( 1 0 |ig/ml), while the ADP-coupled assay utilized pyruvate kinase (10 pg/ml), phospho(enol)pyruvate (PEP, 200 (iM) and lactate dehydrogenase (10 pg/ml.) The ADP-coupled assay system could not be used for Fru-6 P saturation studies since PEP is an allosteric inhibitor with respect to Fru-6 P; however, PEP at this concentration (200 |iM ) does not alter MgATP saturation behavior. The ATP-regenerating system that utilizes creatine kinase and creatine phosphate, which is required for the EcPFK activity assay, was found to be unnecessary since ADP has little or no activating effect on BsPFK (in the absence o f PEP). In all forward reaction assays, the free Mg++ concentration was kept 5 to 10 mM in excess o f the ATP concentration to avoid inhibition by free ATP. Typically, assays were initiated by the addition o f 0.10 ug of PFK. Since the BsPFK assay generally involved an initial nonlinear phase, a 1or 2minute lag period was allowed prior to recording the Abs3 4o change over time, which was done for at least one minute. A thermostatted Hitachi UV-2000 spectrophotometer was used for these measurements. Duplicate assays were run for each data point. Initial velocities are expressed in units o f umole o f product formed per minute. Initial velocity measurements in the reverse direction were similar to those in the forward direction, except that the production o f either Fru-6 P or ATP was coupled to the reduction of NADP+ (0.20 mM). Assays were run at pH 8.2, and Mg++ concentration was kept 9 to 10 mM in excess of the ADP concentration. For the Fru-6 P-coupled assay, the auxiliary enzymes phosphoglucoisomerase ( 1 0 fig/ml) and glucose 6 phosphate dehydrogenase (10 pg/ml) were used, whereas for the ATP-coupled assay, 44 hexokinase (20 |ig/ml), glucose (3 mM) and glucose 6 -phosphate dehydrogenase (10 p,g/ml) were used. Typically, assays were initiated with the addition o f 2.0 ug o f PFK. Treatment o f Kinetic Data-Initial velocity data for the forward reaction were fit to either the Michaelis-Menten equation (eqn. 6 ), the Hill equation (eqn. 7), the initial velocity equation for a rapid-equilibrium sequential mechanism (eqn. 8 ; Cleland, 1963), or the initial velocity equation for a sequential random mechanism assuming steadystate (non rapid-equilibrium) conditions (eqn. 9; Ferdinand, 1966). In equations 6-12 below, [A] and [B] represent the concentrations o f substrates A and B, and Vmax is the maximum velocity. In equation (6 ), Km is the Michaelis constant, whereas in eqn. (7), [A] 1 /2 is the concentration o f substrate at half-saturation and n is the Hill coefficient. In equation (8 ), Kja is the equilibrium constant for dissociation o f substrate A from the binary complex EA (E represents enzyme). There is an analogous constant, Kfc, for dissociation o f B from EB. Ka and Kb are equilibrium constants for dissociation o f A and B, respectively, from the ternary complex EAB. In equation (9), the terms i, j, k, 1, and m are complex functions of [B] and the rate constants for the various steps in Scheme I (see Discussion section). As such, i-m have no physical significance. v V max [A ] [A] + K m V (eqn. 6) v (eqn. 7) [A ] 1 / 2 + [A ]n V max [A] [B] (eqn. 8)v K |a K b + K b [A ] + K a [B ] + [A ] [B ] i [A ] 2 + j [A] (eqn. 9)v k + 1 [A ] 2 + m [A ] 45 Initial velocity data for product inhibition, dead-end inhibition and alternate substrate studies were first plotted graphically as double-reciprocal plots. On the basis of these primary plots, the inhibition patterns were identified, and each data set was fit to the equation for either linear competitive (eqn. 1 0 ), linear non-competitive (eqn. 1 1 ), or linear uncompetitive (eqn. 12) inhibition (Cleland, 1979). In equations 10-12, the constant K is the equilibrium constant for dissociation of varied substrate A from the ternary complex EAB, whereas Kjs and Kjj are equilibrium constants for dissociation of inhibitor I from its inhibitory complex, as determined from slope and intercept replots, respectively. ^Biax f A] V = K (1 + [I] /Kis) + [A] ( e q n * 10)'■isx v max [A] v = K (1 + [I] /Kis) + [A] (1 + [I]/K„) V mas [A ] (eqn. 11) v = K + [A] (1 + [I]/K,j) (eqn. 12) All curve-fitting to equations 6-12 was performed by non-linear regression analysis using the program INPLOT (GraphPad, Inc., San Diego, CA). Results The Reverse Reaction: Initial Velocity, Product Inhibition and Mixed Alternate Substrate Studies-Because the kinetic behavior o f BsPFK in the forward direction is more complex than in the reverse direction, studies on the latter are presented first. The turnover number kcat of the reverse reaction (3.2 s-1) was about 35-fold lower than that of the forward reaction ( 1 12 s 1). This compares well to the 40-fold lower value for the reverse reaction o f EcPFK relative to its forward reaction (Hellinga and Evans, 1987). 46 Initial velocity studies were performed on the reverse reaction by varying Fru1 ,6BP concentration while keeping MgADP concentration constant at different fixed levels, and vice-versa. Double-reciprocal plots were constructed for both sets o f data. The families of lines obtained for both (Figs. 2.1 A & 2. IB) intersect to the left o f the ordinate, a result which denotes a sequential kinetic mechanism. The initial velocity data were fit to the equation for a rapid-equilibrium random or ordered bireactant mechanism (eqn. 8 ), assuming that A is MgADP and B is Fru-1,6BP. The following parameters were obtained by performing nonlinear regression analysis o f the initial velocity data (Cleland, 1979) using eqn. (8 ): kcat= 3.2 ± 0.2 s_1, Ka= 0 .11± 0.01 mM, Kb= 0.44 ± 0.05 mM, Kia= 0.33 ± 0.04 mM, and Kib= 0.19 ± 0.08 mM. Product inhibition studies were also performed on the reverse reaction to further distinguish whether its mechanism is random or ordered. For these studies, the concentration o f one substrate (Fru-1,6BP or MgADP) was varied while the other was kept constant at a fixed level in the absence or presence of different amounts o f one of the two products (Fru-6 P or MgATP). Four sets o f product inhibition data were obtained. The data from each inhibition study were fit to the equation for either linear competitive (eqn. 1 0 ), linear noncompetitive (eqn. 1 1 ), or linear uncompetitive (eqn. 1 2 ) inhibition. Table 2.1 lists the pertinent kinetic parameters obtained, and the patterns of lines observed in double-reciprocal plots. The results are consistent with a rapidequilibrium random mechanism in the reverse direction. The noncompetitive product inhibition by Fru-6 P with respect to MgADP indicates the formation o f a dead-end EMgADP -Fru-6 P complex. However, the equations for both competitive and noncompetitive inhibition fit equally well to the data for inhibition by MgATP with respect to Fru-1,6BP, a result which indicates that the dead-end E-MgATP-Fru-l,6 BP complex probably does not form readily, if it forms at all. Additional evidence that the mechanism in the reverse direction is random was obtained from studies using GDP, which can function as an alternate substrate for the 47 A 2.0 o 1.5 X 5! ±i V C ▼■H ID 1.0 CD D 0.5 * 0.0i - '- i- «- i- i- i- i i . i 0 1 2 3 4 5 6 l/[Fru-l,6BP] (mM-1) FIGURE 2.1. Initial Velocity Patterns for the Reverse Reaction. (A) Plot o f the reciprocal o f initial velocity versus the reciprocal o f Fru-1,6BP concentration at different fixed levels o f MgADP: (A) 0.05 mM, (■ ) 0.10 mM, (O) 0.20 mM, and (®) 0.5 mM MgADP. (B) Plot o f the reciprocal o f initial velocity versus the reciprocal of MgADP concentration at different fixed levels o f Fru-1,6BP: (O) 0.20 mM, (A) 0.50 mM, (□ ) 1.0 mM, and (®) 2.0 mM. The lines were drawn using linear equations containing parameters generated from direct curve-fitting to equation (8 ). Initial velocities (units/ug) were measured using the Fru-6 P-coupled assay. 1/velocity (u g /u n it s x 10 w 3 ) CJ I j I I "" " " ' I "" ' " I ......... l 4 Co TABLE 2.1 Product Inhibition Patterns fo r the Reverse Reaction [Fru-1,6BP] (mM) [MgADP] (mM) inhibitor pattern3 Kis (\iM ) Kii ()tM) varied 0.91 MgATpb NC (or C) 5 2 + 4 676 ± 4 7 0.50 varied MgATP C 1 4 + 1 N.A. 0.50 varied Fru-6 PC NC 7 ± 2 55 ± 11 varied 0.91 Fru-6 P C 6 ± 0 .5 N.A. aNC= noncompetitive; C=competitive inhibition. ^When MgATP was the product inhibitor, the Fru-6 P-coupled assay was used. cWhen Fru-6 P was the product inhibitor, the ATP-coupled assay was used. N.A., not applicable. 50 reverse reaction. When GDP is added to a PFK assay in which NADP+ reduction is coupled to ATP via hexokinase (see Methods section), which will not accept GTP as a substrate, the GTP produced in the reverse PFK reaction is not detected and "inhibition" results. The curvature or linearity of double-reciprocal plots for initial velocity studies in the presence o f GDP can provide evidence as to whether the mechanism is ordered with ADP (GDP) binding first (curved plots), or random (linear plots). As expected, the plots obtained showed that GDP is a competitive "inhibitor" with respect to MgADP and a noncompetitive "inhibitor" with respect to Fru-1,6BP, indicating that ADP (or GDP) can bind first to the enzyme in either an ordered or a random mechanism. More significantly, the plots were linear. This result is consistent with a random mechanism, since nonlinear inhibition would have been observed had the mechanism been ordered. A similar result has been reported for rabbit muscle PFK (Hanson e t al., 1973). Altogether, the results o f both the product inhibition and the mixed alternate substrate studies in the reverse direction are consistent with a rapid-equilibrium random mechanism that includes the formation o f an abortive E-MgADP-Fru-6 P complex. The Forward Reaction: Fructose 6-phosphate Saturation o f BsPFK-Valdez and coworkers (1989) showed that Fru-6 P saturation o f BsPFK is hyperbolic. Our findings confirm this result, and further indicate that simple Michaelis-Menten kinetics cannot fully explain the saturation behavior of BsPFK. As shown in Fig. 2.2A (dashed curve), in the presence of varied Fru-6 P concentrations that are less than about one-third the fixed MgATP concentration, the Fru-6 P saturation data fit the Michaelis-Menten equation well (R2 =0.994). A Km(Fru-6 P) o f 0.03 mM is obtained under these conditions. However, when the [Fru-6 P]/[MgATP] ratio is greater than about one-third (Fig. 2.2A, upper curve), a flattening is observed. Thus, when analyzed over the entire 0.01 to 1.0 mM range, the saturation data fit the Michaelis-Menten equation less well (R2=0.988). 51 A .25 .20 / ' N O) D Ui 1C- ±! -1 -* c D 60 SO 3^ +J .10 3 0a o (U > 20 .05 20 40 80 100 120 0.0 0.2 0.4 0.6 0.8 1.0 1.2 [ F r u 6 P ] (mM) FIGURE 2.2. Saturation Curves and Initial Velocity Plots with Respect to Fru-6 P. (A) Fru-6 P Saturation Curves. Dependence o f the velocity o f the BsPFK-catalyzed reaction on Fru-6 P concentration in the presence o f ( • ) 0.5 mM or (O) 10.0 mM MgATP. The upper curve was generated by fitting the data to eqn. (9), whereas the lower curve was generated by curve-fitting to eqn. (6 ). The dashed line was generated by fitting the data between 0.01 and 0.2 mM Fru-6 P in the upper curve to eqn. (6 ). (inset) Double-reciprocal plots o f the same data. The lines were drawn using linear equations containing parameters generated from fitting the data between 0 . 0 1 and 0 . 2 mM Fru-6 P to eqn (6 ). Initial velocities (units/ug) were measured using the Fru-1,6BPcoupled assay. (B) Initial velocity plot. Reciprocal o f initial velocity versus reciprocal o f Fru-6 P concentration between 0.01 and 0.20 mM Fru-6 P in the presence o f (A) 0.05, (A)0. 10, and (O) 0.20 mM MgATP. The lines were drawn using linear equations containing parameters generated by direct fitting o f a selected range o f data points to eqn. (8 ). (C) Initial velocity p lot Reciprocal o f initial velocity versus reciprocal of Fru-6 P concentration between 0.033 and 5.0 mM Fru-6 P in the presence o f different fixed low levels o f MgATP: (□ ) 0.04, (■*-) 0.05, (O) 0.067, and ( • ) 0.10 mM. Lines were generated by linear regression analysis. c Icr» T) ii-* 1/velocity (ug/units) o roo oo o *-* o o I-kro o cn NJ o c Icr> XI roo coo 1/velocity (ug/units) o roo coo ■ 1 1 ( 1 c.1 ■ * () * < O □ < 0 * □ i I 0 *■ Cl - • O * 4 i 0 1 * 1 o uio c/i OJ 54 In the presence o f high levels o f MgATP (greater than 30 x Km) Fru-6 P saturation curves do not flatten within the normal 0.01 to 1.0 mM Fru-6 P concentration range. Rather, saturation follows Michaelis-Menten kinetics (Fig. 2.2A, lower curve) throughout the entire range. Inhibition by MgATP is apparent, and this inhibition appears competitive with respect to Fru-6 P (Fig. 2.2A, inset). Indeed, when Fru-6 P saturation data collected in the presence o f fixed levels o f MgATP equal to 0.5 mM, 5.0 mM and 10.0 mM are analyzed according to the method of Cleland (1979), the data fit the equation for linear competitive inhibition (eqn. 1 0 ) well, yielding a linear slope replot and a Kis o f 5.2 ± 0.5 mM. A Hill coefficient o f 1.05 ± 0.05 is obtained for each of the three saturation curves. Thus, MgATP inhibition is not associated with cooperative Fru-6 P binding as it is for EcPFK, where Hill coefficients increase from 1.4 to 3.6 in the presence o f fixed MgATP concentrations ranging from very low levels to above-saturating levels (Johnson and Reinhart, 1992). The flattening effect observed in the Fru-6 P saturation curve when the [Fru6 P]/[MgATP] ratio is greater than about one-third is seen in double-reciprocal plots as a leveling-off o f the slope near the 1/v axis. This leveling-off effect is evident in Fig. 2.2B, which shows initial velocity plots with respect to variable Fru-6 P concentration at different fixed levels o f MgATP near its Km value. These results indicate that MgATP concentration is rate-limiting when the [MgATP]/[Fru-6 P] ratio is less than two or three. The effect is more apparent in Fig. 2.2C, which shows that the reaction rate remains steady and independent o f Fru-6 P concentration between 0.033 and 5.0 mM in the presence o f fixed MgATP concentrations at or below its Km value (0.1 mM). Overall, these studies suggest that MgATP binds to BsPFK under non rapid-equilibrium conditions, and that the rate o f catalysis is faster than the rate o f MgATP binding. [The deviation from linearity at low Fru-6 P concentration in the presence of 0.1 and 0.2 mM MgATP in Fig. 2.2B is most likely the result o f substrate binding via a kineticallydisfavored pathway (see Discussion section)]. 55 The linear regions of the plots in Fig. 2.2B can be fit to the initial velocity equation for a sequential bireactant mechanism in rapid-equilibrium (eqn. 8 ), assuming A is MgATP and B is Fru-6 P. When this is done, the following kinetic parameters are obtained: kcat= 112 ± 7 s 1, Ka= 40 ± 27 (iM, Kb= 2± 18 |lM, and Kib= 6 8 ± 43 pM. The double-reciprocal lines generated from curve-fitting to eqn. (8 ) intersect to the left of the ordinate and on the abscissa, a result consistent with a sequential mechanism in the forward direction. MgATP Saturation and Substrate Inhibition-The Michaelis-Menten equation fits the MgATP saturation data well (R2 =0.996) in the presence o f saturating Fru-6 P as long as the varied MgATP concentration is below ten-fold the fixed Fru-6 P concentration (Fig. 2.3, thick dashed line). A Km(MgATP) o f 0.1 mM is obtained under these conditions. However, inhibition becomes apparent at high MgATP concentrations (Fig. 2.3), and this inhibition is more pronounced at the lower Fru-6 P concentration (lower curve). Furthermore, as the fixed level o f Fru-6 P is decreased, the inhibition becomes apparent at a lower MgATP concentration. The saturation curves tend to rise to a maximum, then decrease to a plateau. The inhibition by high levels of MgATP results in non-linear double-reciprocal plots with respect to variable MgATP concentration. A s shown in Figure 2.3 (inset), the double-reciprocal plots pass through a minimum, then bend upward as they approach the 1/v axis. This initial velocity pattern is indicative o f substrate inhibition (Segel, 1975a) by high levels o f MgATP. Because o f their upward curvature, the lines do not intersect to the left of the ordinate. Thus, the initial velocity data with respect to MgATP do not fit the equation for a sequential bireactant mechanism in rapid equilibrium (eqn. 8 ). MgATP Saturation o f the GV212 BsPFK Mutant-It is conceivable that the inhibition exhibited by MgATP results from the binding o f MgATP in a site other than the active site, e.g., the effector site. In order to address this possibility, the ability of .25 .20/-N O) D X in + j 'c Dv_' .15 +j o jO 0 > .10 .05 0 2 4 6 8 10 [MgATP] (mM) FIGURE 2.3. Dependence of the BsPFK-catalyzed Reaction Rate on MgATP concentration. Fru-6 P concentration was kept fixed at ( • ) 0.05 mM or (O) 0.2 mM. Both curves were generated by fitting the data to eqn. (9). The thick dashed line was generated by fitting the data between 0.05 and 1.0 mM in the upper curve to eqn. (6 ). (inset) Double-reciprocal plots of the same data. 57 MgATP to inhibit a mutant BsPFK enzyme that has the glycine at position 212 replaced with a valine (GV212) was investigated. Glycine 212 is located at the hinge o f the 8 H loop, which has been proposed to be important in the BsPFK allosteric transition (Schirmer and Evans, 1990). Residue 212 is situated along the border o f the effector binding cleft near the site where the purine ring o f a bound ADP or GDP molecule would be located. We have found that PEP inhibition o f the GV212 mutant cannot be reversed by GDP or ADP, and that neither ADP nor GDP bind well to the effector site (Zhu et al., manuscript in preparation). Presumably, binding o f MgATP in the effector site would likewise be disrupted. Figure 2.4 shows that the ability o f MgATP to inhibit the enzyme is unaffected by the mutation (the curves appear nearly identical). This observation suggests that the inhibition is not due to the binding of MgATP in the effector site. Alternative Nucleoside Triphosphate (NTP) Substrates-In addition to ATP, the nucleoside triphosphates GTP, UTP and CTP can all serve as phosphoryl donors in the BsPFK-catalyzed reaction. All four NTPs display saturation profiles similar to those in Fig. 2.3, which tend to rise to a maximum, then drop to a plateau at high MgNTP concentrations. Surprisingly, although the four NTPs have different Michaelis constants (Table 2.2), they all give the same relative velocity (v/Vmax) at a concentration o f 10 mM (not shown). Thus, they inhibit BsPFK to essentially the same extent when present at high (10 mM) concentration. Product and Dead-end Inhibition Studies o f the Forward Reaction-Product inhibition studies as well as dead-end inhibition studies using the non-reactive ATP analogs P/y-methyleneadenosine 5 'triphosphate (AMPPCP) and 5' adenylimidodiphosphate (AMPPNP), and the non-reactive Fru-6 P analog arabinose 5phosphate (Ara-5P), were performed on the forward reaction in order to further delineate the kinetic mechanism. As with the reverse reaction, primary 1.0 / A x <5 0.8 E > > 0.6 4-> O o « 0.4 <L)> JS 0.2 0 L_ 0.0 0 2 4 6 [M gA TP] (mM) FIGURE 2 .4 . MgATP Inhibition of the Mutant GV212 BsPFK Enzyme. Initial velocity was measured versus MgATP concentration for ( • ) mutant and (O) wild-type BsPFK in the presence of 0.05 mM Fru-6 P. Both curves were generated by curve-fitting to eqn. (9). TABLE 2.2 Alternative Nucleoside Triphosphate (NTP) Substrates NTP Km(NTP)a (mM) Percent of k cat (ATP) Km(Fru-6 P)*> (M-M) ATP 0.07 ± 0 .01 1 0 0 33 ± 5 GTP 0.18 ± 0 . 0 1 73 28 ± 4 CTP 2 . 6 ± 0 . 2 98 146 ± 1 9 UTP 2 . 8 ± 0 . 2 93 6 8 ± 7 aFru-6 P concentration was kept fixed at 1.0 mM (saturating) for these determinations. bNTP concentration was kept fixed at 5 x Km(NTP). In each assay, Mg++ was present in excess of the NTP concentration. double-reciprocal plots were constructed to determine the nature of the inhibition, and the saturation data were fit directly to equations for linear competitive, linear noncompetitive, and linear uncompetitive inhibition. The patterns obtained in the plots and the parameters obtained from curve-fitting are shown in Table 2.3. The product and dead-end inhibition results for the forward reaction are somewhat more difficult to interpret than are the product inhibition results for the reverse reaction. The double-reciprocal plots for inhibition by both Fru-1,6BP and Ara5P with respect to MgATP (up to 1 mM) yield nearly parallel yet divergent lines (Table 2.3). In both o f these inhibition studies, when the MgATP concentration is extended to 10 mM, the plots bend upward as they approach the 1/v axis, indicating substrate inhibition at high (>2 mM) MgATP concentration. Because o f the curvature, it is not possible from these plots alone to establish whether the inhibition is uncompetitive (parallel lines) or noncompetitive (convergent lines). An uncompetitive inhibition pattern would be expected in these studies if the kinetic mechanism were ordered with MgATP binding first, whereas a noncompetitive pattern would be expected if the mechanism were random. However, the ordered mechanism under consideration here is incompatible with the competitive nature o f the inhibition by MgATP with respect to Fru-6 P seen in Fig. 2.2A. Competitive substrate inhibition by MgATP would be possible for an ordered mechanism only when Fru-6 P is the first substrate to bind to the enzyme; it is not possible for an ordered mechanism in which MgATP binds first to the enzyme (Segel, 1975a). On the other hand, competitive substrate inhibition by MgATP with respect to Fru-6 P is consistent with a random mechanism. In this case, inhibition can occur in several ways, including the two being proposed here: ( 1 ) abortive binding of MgATP in the Fru-6 P site, and (2) reaction flux through a kinetically-disfavored substrate binding pathway (see below). Thus, taking the substrate inhibition into account, the most likely interpretation o f the product and dead-end inhibition results is that Fru-6 P and MgATP bind to the enzyme in random order. TABLE 2.3 Product and Dead-end Inhibition Patterns fo r the Forward Reaction A. Product Inhibition Patterns. [Fru-6 P] [MgATP] product Kis Kii (mM) (mM) inhibitor pattern3 (mM) (mM) 0 . 2 varied MgADP C 0.50 ±0 .01 N.A. varied 0.87 MgADPb NC 2 2 ± 2 3 .9 2 1 0 .0 5 1 . 0 varied Fru-1,6BP° UC (NC)d N.A. 1 4 1 2 B. Dead-end Inhibition Patterns.^ [Fru-6 P] [MgATP] dead-end Kis Kii (mM) (mM) inhibitor pattern (mM) (mM) 0 . 2 0 varied AMPPCP C 1 .1 2 1 0 .0 4 N.A. varied 0.87 AMPPCP NC 1 0 .2 1 0 .9 9 .8 1 0 .8 0 . 2 0 varied AMPPNP C (or NC) 0 .0 5 1 0 .0 2 1 .5610 .81 varied 0.87 AMPPNP NC 0 .2 9 1 0 .0 9 0 .4 0 1 0 .1 2 varied 0.87 Ara-5P C 1 .2 1 0 . 2 N.A. 0 . 2 0 varied Ara-5P UC (NC)d N.A. 6 .2 1 0 .5 aNC=noncompetitive; C=competitive; and UC=uncompetitive. ^The Frul , 6 BP-coupled assay was used for these studies. cWhen Fru-1,6BP was the product inhibitor, the ADP-coupled assay system was used. Fru-1,6BP inhibition with respect to Fru-6 P could not be determined since the phospho(enol)pyruvate required for the assay is an allosteric inhibitor o f BsPFK at low Fru-6 P concentration. ^Parallel lines were observed in the double-reciprocal plots. See the text for further discussion. The pattern in parentheses is that expected in the absence o f substrate inhibition. N.A., not applicable. 62 An examination o f the AMPPCP and AMPPNP inhibition patterns (Table 2.3) sheds light on the mechanism by which ATP inhibits BsPFK. Although AMPPCP inhibition with respect to ATP is purely competitive, AMPPNP inhibition with respect to ATP, though mostly competitive, nevertheless has some noncompetitive character. In the latter case, the lines in the double-reciprocal plot (Fig. 2.5A) clearly converge just to the left o f the 1/v axis. The simplest interpretation o f this result is that AMPPNP (and presumably ATP) binds abortively in the Fru-6 P site, forming a dead-end EMgATP -AMPPNP [or E-(MgATP)2 ] complex. Further evidence for the formation of this complex is seen in the double-reciprocal plot for AMPPNP inhibition with respect to Fru-6 P (Fig. 2.5B), which gives an essentially noncompetitive (mixed-type) pattern composed o f lines that intersect progressively closer to the 1/v axis as AMPPNP concentration is increased. This effect is not seen for AMPPCP. AMPPCP apparently does not bind abortively as does AMPPNP, most likely because the orientation o f its yphosphate group is quite different from that of AMPPNP, which is similar to that of ATP (Yount et al„ 1971; Larsen et al., 1969). These results suggest that ATP binds in the Fru-6 P site via its y-phosphate group. Thus, abortive binding at least partially explains the inhibition seen at high MgATP concentration. Discussion The intersecting patterns o f lines obtained in the double-reciprocal initial velocity plots for the reverse reaction indicate that the kinetic mechanism is sequential rather than ping-pong. Further, the patterns o f lines obtained in the reverse reaction product inhibition (Table 2.1) and mixed alternate substrate (GDP) studies indicate that the kinetic mechanism of the reverse reaction is rapid equilibrium random. The results of product inhibition and dead-end inhibition studies of the forward reaction (Table 2.3) are also consistent with a random mechanism, after taking into account the effect of substrate inhibition by MgATP. The binding of MgATP to the enzyme was found to be rate-limiting when its concentration is low. Thus, the kinetic mechanism o f BsPFK in 63 A 30 25 20 in 15 § 20 O) D 05 0.0 [AMPPNP] (nsM)U O 0> -6 -4 -2 0 2 4 6 8 10 12 l/[M gATP] (mM-1 ) FIGURE 2.5. AM PPNP Inhibition Patterns. (A) Plot o f the reciprocal o f initial velocity versus the reciprocal o f MgATP concentration in the presence o f 0.2 mM Fru6 P and ( • ) 0 mM, (O) 0.1 mM, (□ ) 0.2 mM, or (A ) 0.3 mM AMPPNP. (inset) Slope replot. (B) Plot o f the reciprocal o f initial velocity versus the reciprocal o f Fru-6 P concentration in the presence of 0.87 mM MgATP and ( • ) 0 mM, (□ ) 0.2 mM, (O) 0.5 mM, or (A) 0.81 mM AMPPNP. (inset) Intercept replot. Initial velocities (units/ug) were measured using the Fru-l,6 BP-coupled assay. 1/velocity (ug/units) y-trrtercept w o & 65 the forward direction can best be described as sequential random with binding o f MgATP being rate-limiting, i.e., the rate o f its binding is lower than the catalytic rate. h i g h [A] l o w [B] h i g h [B] l o w [A] SCHEME I The kinetic mechanism in the forward direction is depicted in Scheme I, where E is free enzyme, A and B are substrates MgATP and Fru-6 P, respectively, E-A and E-B are their binary complexes, and E-A-B is the reactive, ternary complex. The constants ki through kio are rate constants for the steps indicated. It is assumed that the binding of MgATP is slow relative to the catalytic step, i.e., k i, k6 « k9 . The curved arrows in Scheme I indicate that different substrate binding pathways are followed in the presence of different relative amounts o f the substrates, assuming non rapid-equilibrium conditions. The MgATP saturation results for the mutant GV212 enzyme (Fig. 2.4) indicate that the substrate inhibition observed is not due to MgATP binding in the effector site. Thus, inhibition by MgATP occurs as a result o f its binding in the active site. Within 66 this context, there are several possible mechanisms by which MgATP inhibition can occur: (1) binding o f MgATP to the binary product complex E-Fru-1,6BP to form a dead-end ternary complex E-Fru-1,6 BP-MgATP, from which Fru-1,6BP dissociates more slowly than it does from the normal product ternary complex. [Inhibition o f liver alcohol dehydrogenase by ethanol occurs via a similar mechanism (Dalziel and Dickinson, 1966)]. (2) abortive binding o f MgATP in the Fru-6 P site (linear substrate inhibition; Cleland, 1979), or (3) the existence o f two alternative pathways to the reactive ternary complex, with one pathway kinetically-favored (Dalziel, 1957; Ferdinand, 1966). In this last mechanism, inhibition by MgATP would result from the binding o f substrates via the disfavored pathway. A mechanism for MgATP inhibition that involves formation o f a dead-end complex E-Fru-l,6 BP-MgATP is shown to be unlikely from the results o f product inhibition studies. Specifically, inhibition by MgATP with respect to Fru-1,6BP in the reverse direction (Table 2.1), indicates that the complex E-Fru1,6BP-MgATP does not form readily. The results o f the dead-end inhibition studies in the forward direction using AMPPNP (Table 2.3 and Fig. 2.5) give evidence for abortive binding o f AMPPNP, and presumably ATP, in the Fru-6 P site via their 7 -phosphate groups. Such abortive binding at least partially explains the observed inhibition by MgATP. However, the experimental evidence suggests that this is not the only mechanism involved. Dalziel (1957) has shown that substrate inhibition (and activation) are inherent in an altemative-pathways mechanism when the binding or release o f substrates is ratelimiting and the rate constants for the steps involved are such that one o f the two pathways is kinetically-disfavored. Ferdinand (1966) presented the theoretical basis (eqn. 9) for this mechanism, which can give rise to a variety o f non-hyperbolic initial velocity curves (Segel, 1975b), the shapes o f which depend on the relative magnitudes of the rate constants for the steps o f the mechanism. There is evidence that BsPFK 67 obeys such a mechanism: (1) The binding o f MgATP is clearly rate-limiting when its concentration is low, i.e., when it is less than about three times the Fru-6P concentration. This indicates that catalysis is fast relative to the rate o f MgATP binding. (2) The observed substrate inhibition by MgATP when its concentration is greater than 10-fold the Fru-6P concentration (Fig. 2.2B and Fig. 2.3) is consistent with the binding o f substrates through a kinetically-disfavored pathway when MgATP concentration is relatively high. Thus, the upper binding pathway in Scheme I, with MgATP binding first and Fru-6P binding second, is the disfavored pathway for BsPFK. (3) The initial velocity equation for the altemative-pathways mechanism (eqn. 2.4) fits the Fru-6P saturation data well (R2 is 1.000 for the upper curve in Fig. 2.2A) with the assumption that im=jl and kicmj, and it fits the MgATP saturation data well (Revalues are 0.99S and 0.984 for the curves in Fig. 2.3) with the assumption that imcjl and kicmj. Thus, a steady-state altemative-pathways mechanism is consistent with not only the observed substrate inhibition by MgATP, but also with other aspects o f BsPFK kinetic behavior in the forward direction. Substrate activation can be observed in a steady-state altemative-pathways mechanism. Typically, this activation is evident as sigmoidicity in saturation curves with respect to the first substrate to bind the enzyme in the kinetically-favored pathway. However, whether or not activation (hence, sigmoidicity) is observed depends on the various rate constants involved, and the extent to which one pathway is kineticallyfavored (or disfavored). Thus, the reason sigmoidicity is not observed in the Fru-6P saturation curve for BsPFK is probably because the extent to which the lower pathway in Scheme I is favored (or the upper pathway is disfavored) is not great. Altogether, the results discussed above provide evidence that BsPFK in the reverse direction obeys a rapid-equilibrium random mechanism. In the forward direction, it obeys an altemative-pathways kinetic mechanism that involves (1) non rapid-equilibrium binding o f MgATP (k^ k6 « k 9 in Scheme I), evident when MgATP 68 concentration is low, and (2 ) substrate binding through a kinetically-disfavored pathway (the upper pathway in Scheme I) evident when MgATP concentration is high. In addition, abortive binding o f the y-phosphate o f ATP in the Fru-6 P site contributes to some extent to the inhibition seen at high MgATP concentration. As such, the kinetic mechanism of BsPFK shares features in common with the kinetic mechanisms o f other PFKs, including those from rabbit muscle, Ascaris suum, and E. coli. Using initial velocity, product inhibition and dead-end inhibition studies, Bar-Tana and Cleland (1974a,b) have shown that the kinetic mechanism of rabbit muscle PFK is sequential random, being rapid-equilibrium in the reverse direction but not in the forward direction, where the rate constants for the release o f substrates are lower than the catalytic constant. Although the kinetic mechanism o f Ascaris suum PFK in the forward direction is predominantly steady-state ordered (Rao et al., 1987), it nevertheless involves some randomness in the order o f substrate binding. In the reverse direction, the mechanism is essentially rapid-equilibrium random. Studies by Deville-Bonne et al. (1991a) have indicated that the kinetic mechanism of EcPFK is sequential random, and that the binding o f one substrate antagonizes the binding of the other. Johnson and Reinhart (1992) have further studied the interactions between MgATP and Fru-6 P in the active site o f EcPFK by thermodynamic linked-function analysis. Their results indicate that all the observed features o f substrate interaction can be explained by two independent couplings: an antagonistic MgATP/Fru-6 P coupling extending between active sites, and a MgATPinduced Fru-6 P/Fru-6 P coupling. Zheng and Kemp (1992) have recently proposed that ATP inhibition o f EcPFK involves substrate antagonism coupled with a steady-state random mechanism in which the rate o f catalysis is too high to permit rapid equilibration o f substrates. It is interesting to note that the kinetic behavior o f 3-deoxyD -araZ?mo-heptulosonate 7-phosphate synthetase from Rhodomicrobium vannielii, 69 which is proposed to follow an altemative-pathways mechanism (Jensen and Trentini, 1970), closely resembles that o f EcPFK (Zheng and Kemp, 1992). Despite similarities between the kinetic mechanisms o f EcPFK and BsPFK, the mechanisms by which MgATP inhibits the two enzymes are clearly different. Specifically, MgATP inhibition appears to be allosteric for EcPFK (Berger and Evans, 1991) but not for BsPFK. The reason for this difference has only recently begun to be understood. Steady-state fluorescence studies show that MgATP and Fru-6 P each bind to EcPFK non-cooperatively in the absence o f the other (Deville-Bonne and Garel, 1992). In these studies, cooperative Fru-6 P binding was induced by the presence of AMPPCP (and presumably ATP), although the Hill coefficient for the cooperative binding was not as high as that observed in steady-state kinetic measurements (2 . 0 versus 3.8-4.0). Based on studies showing that both kcat and n (the Hill coefficient) vary with pH, Deville-Bonne et al. (1991b) have suggested that catalysis and cooperativity are linked in EcPFK. Thus, it is the superimposition o f (1) "kinetic cooperativity" resulting from non-equilibrium conditions and (2 ) concerted binding cooperativity (Monod et al., 1965) that fully explains the allosteric behavior o f EcPFK. The lack o f allosteric regulation o f BsPFK by MgATP suggests that, unlike EcPFK, catalysis and cooperative Fru-6 P binding are not linked. Cooperative binding of Fru-6 P does occur, but only in the presence o f PEP. The data presented in this paper indicate that MgATP inhibition o f BsPFK occurs entirely within the active site by means o f (1) abortive MgATP binding and (2) reaction flux through the kineticallydisfavored substrate binding pathway. Thus, inhibition o f BsPFK by MgATP is a process distinct from allosteric inhibition by PEP. Finally, the nature o f the regulation by MgATP and PEP of BsPFK is compatible with the thermostability o f the enzyme. Large conformational changes, which could lead to structural destabilization, are apparently not part o f the response o f the enzyme to inhibitors. The allosteric transition (Schirmer and Evans, 1990) involves minimal movement: a rotation by 7 ° o f one pair o f rigid dimers relative to the other, along with a coordinated back-and-forth movement of a pair o f loops (the 8 H and 6 F loops) across the dimer-dimer interface. Likewise, inhibition by ATP apparently involves little if any conformational change. Chapter 3 A Chimeric Bacterial Phosphofructokinase Exhibits Cooperativity in the Absence of Heterotropic Regulation 71 72 Phosphofructokinase (PFK, EC 2.7.1.11) catalyzes the transfer o f the yphosphate from MgATP to fructose-6 -phosphate (Fru-6 P) in the first committed step of glycolysis. In bacteria, PFK is regulated heterotropically by the activator ADP (or GDP) and the inhibitor phosphoenolpyruvate (PEP). The PFKs from the bacteria Escherichia coli and Bacillus stearothermophilus are remarkably similar in structure (Evans et al., 1981; Shirakihara & Evans, 1988). The enzymes are both tetramers of identical subunits, their subunit a-carbon traces are nearly superimposable, and they share 55% amino acid identity. They can be viewed as dimers o f rigid dimers. In each enzyme, the subunit is divided into a large and a small domain, with the active site located in a cleft between the two domains. There are thus four active sites. There are also four effector sites into which PEP and ADP (or GDP) bind; these are located in deep clefts between subunits o f the rigid dimer. Within the active site, the amino acid residues that bind ATP are almost entirely from the large domain, while those that bind Fru-6 P are mostly from the small domain, but include two arginines (arginines 162 and 243) from across the dimer-dimer interface. Coordination o f the Mg++ ion as well as transfer o f the y-phosphate o f ATP involves residues from both domains. Despite the remarkable structural similarity between E. coli PFK (EcPFK) and B. stearothermophilus PFK (BsPFK), there is a significant kinetic and allosteric difference between them. Whereas Fru-6 P saturation of BsPFK is hyperbolic in the presence o f saturating MgATP levels (Valdez et al., 1989), Fru-6 P saturation o f EcPFK is highly cooperative (Hill number around 4.0) under the same conditions (Blangy et al., 1968). ATP has been termed an "allosteric" inhibitor o f EcPFK (Evans, 1992) because of its ability to profoundly inhibit the enzyme at low Fru-6 P concentration (Kundrot & Evans, 1990). On the other hand, ATP only slightly inhibits BsPFK, and the inhibition is clearly non-allosteric. Blangy et al. (1968) have shown that EcPFK obeys the Monod-Wyman-Changeux (MWC) model o f allosteric behavior (Monod et al., 1965), in which the protein exists in an equilibrium involving two conformational states, a 73 low-activity T-state and a high-activity R-state. The allosteric behavior o f EcPFK has been explained in terms o f this model. Recent studies, however, suggest that the MWC model cannot fully account for the cooperativity o f EcPFK (Deville-Bonne et al., 1991a; Berger & Evans, 1992). X-ray diffraction studies indicate that two different subunit structures are present in the EcPFK tetramer: "open" and "closed" (Shirakihara & Evans, 1988). Therefore, an open-to-closed transtition may be taking place within the EcPFK subunit. A comparison between the a-carbon traces o f the open and closed subunit structures reveals that most o f the movement occurs within a region o f the large (ATP-binding) domain. The transition apparently does not occur within the large domain o f BsPFK (Schirmer & Evans, 1990). We have constructed and studied a chimeric PFK, composed o f parts o f BsPFK and EcPFK, in order to investigate the structural basis for their different regulation by ATP. The chimeric enzyme (ChiPFK) contains a portion o f the "rigid" large domain of BsPFK (the ATP-binding domain) grafted in-frame onto the remainder o f the EcPFK subunit. The active site of ChiPFK is thus composite: residues that bind ATP are from BsPFK while those that bind Fru-6 P are from EcPFK. Steady-state kinetics and fluorescence studies were performed on the chimeric PFK and the two native enzymes. The results indicate that active site o f ChiPFK is locked in an "open" conformation that resembles that of the activated form of EcPFK. Nevertheless, the enzyme displays sigmoidal Fru-6 P saturation kinetics (Hill number 1.7 ± 0.2). The absence o f regulation by PEP despite its ability to bind in the effector site indicates that the structural pathways of allosteric PEP inhibition are different between the two native enzymes. In many respects, ChiPFK resembles a proteolyzed derivative o f EcPFK (Le Bras & Garel, 1982) that is cooperative yet insensitive to allosteric effectors. The possible origins of ChiPFK sigmoidal saturation kinetics are discussed. 74 Materials and Methods Enzymes, Chemicals and Oligonucleotides-Restriction endonucleases, T4 DNA ligase and T4 polynucleotide kinase were either from New England Biolabs, Inc. (Beverly, MA) or U. S. Biochemical (Cleveland, OH). NADH, NAD+, Fru-6 P, ATP (and other NTPs), GDP, PEP, AMPPNP, and the auxiliary enzymes aldolase, triosephosphate isomerase and a-glycerophosphate dehydrogenase were all from Sigma Chemical Co. (St. Louis, MO), as was the Cibacron Blue 3GA-agarose resin (type 3000-CL-L). Oligonucleotides were either purchased from DNA International, Inc. (Lake Oswego, OR) or synthesized on an Applied Biosystems 380A DNA Synthesizer. Construction o f the Chimeric Gene: PCR, Cloning and Sequencing-The chimeric gene was constructed by ligating together two polymerase chain reaction (PCR) amplification products: one from bspfk, the other from ecpfk. The two PFK genes (French & Chang, 1987; Hellinga & Evans, 1985) had previously been cloned into pUC18 plasmids, and were oriented in opposite directions. Three oligonucleotide primers were used in the PCR amplification: limner 1 5'a g g a a a c a g c ta tg a c c a tg a tta c3' 2 5'CGTCCCCGGGGCCCCGACGCACG3' 3 5'cgtgcatcggggcccc ggg cactatc3' Primer 1 was designed to hybridize to pUC18 itself just beyond the Eco RI cloning site, which is located upstream of the bspfk promoter in recombinant bspfkjp\JC\%, and located downstream of the ecpfk termination sequence in recombinant ecpfk/p\5C\%. In both cases, primer 1 points into the pfk insert. Primer 2 was designed to hybridize to the bspfk coding strand with its 3' end located at the codon for Pro 118, pointing upstream. Primer 2 contains an internal Apa I restriction site (underlined above) that also causes the mutation Val 122 - > Ala. (It was reasoned that this mutation would not significantly alter the properties o f the chimeric enzyme since ( 1 ) the amino acid change 75 is not a drastic one, and (2 ) there is already a degree o f variation between the native enzymes at this position, i.e., valine versus leucine). Primer 3 was designed to hybridize to the ecpfk complementary strand with its 3' end located at the codon for lie 126, pointing downstream. Like primer 2, it contains an internal Apa I site. PCR using primers 1 and 2 amplifies a region o f bspfk stretching from beyond its promoter to codon 125. PCR using primers 1 and 3 results amplifies a region o f ecpfk stretching from codon 118 to beyond its termination sequence. PCR was performed using a Perkin Elmer-Cetus (Norwalk, CT) DNA thermal cycler, a GeneAmpTM PCR kit, and Am pliTaqTM DNA polymerase. The procedure suggested in the kit was followed. The chimeric gene was created by digesting the two PCR products with Apa I, then ligating the fragments together with T4 DNA ligase. In this way, the BsPFK gene up to codon 122 was grafted in-frame onto the EcPFK gene beginning at codon 123. The chimeric gene was cloned into pUC18 via its Hind III and Eco RI restriction sites. The integrity o f the chimeric gene was verified by directly sequencing the entire coding region using a SequenaseTM kit (U. S. Biochemical, Inc., Cleveland, OH). No unintentional mutations were found. Expression and Purification ofPFKs-Both the chimeric PFK and the two native PFKs were expressed in PFK-deficient E. coli cells (DF1020 cells) that had been transformed with recombinant pUC18 plasmids containing the PFK genes. BsPFK was purified as described by Valdez et al. (1989), and EcPFK was purified as described by Kundrot & Evans (1991), but on a 40-fold larger scale. The enzymes were shown to be pure by electrophoresis on a 12% SDS-polyacrylamide gel (see Fig. 3.2A) using the method o f Laemmli (1970). Expression o f ChiPFK was performed as follows: recombinant chipfk/pUC18 plasmid DNA was transformed into competent DF1020 cells. The cells were plated onto Luria broth (LB) agar containing 50 jig/ml ampicillin, and the plates were incubated overnight at 30°C. Six transformants were picked, directly inoculated into 76 3-ml volumes o f LB-ampicillin (50 (ig/ml) medium, and grown at 30°C with agitation. The cultures reached stationary phase after about 60 hours. A 1-ml aliquot o f the culture having the highest PFK activity was inoculated into 500 ml of LB-ampicillin (50 |lg/ml). This large culture was grown at 30°C, reaching stationary phase after about 24 hrs. The cells were harvested by centrifugation at 4°C. Cell pellets were brought up in buffer A (50 mM Tris-Cl, pH 7.9, 1 mM EDTA and 8 mM DTT) containing 1 mM phenylmethyl sulfonyl fluoride (PMSF), frozen in liquid nitrogen, and stored at -20°C until ready for use. The frozen cell suspension was thawed, then sonicated and centrifuged. The resulting clear supernatant was loaded onto a Cibacron Blue 3GA-agarose column that had been equilibrated with buffer A. The loaded column was washed with 10 volumes of buffer A, and ChiPFK was then eluted with 2 mM ATP/10 mM MgCl2 in buffer A. (The column was not washed with salt prior to elution with ATP because NaCl concentrations as low as 100 mM caused elution o f ChiPFK). The most active fractions were pooled and dialyzed against buffer A to remove the ATP and Mg++. SDS-PAGE analysis o f the PFK preparation at this point revealed the presence o f several contaminating bands. The column was regenerated by washing it first with several volumes o f 1.5 M NaCl, then extensively with buffer A. The dialyzed pool containing PFK activity was re-loaded, the column was washed with 5 volumes o f buffer A, and ChiPFK was then eluted with 0.5 mM Fru-6 P in buffer A. PFK activity came off in a broad peak. Fractions having the highest activity were pooled. The enzyme was shown to be pure by electrophoresis on a 12% SDS-polyacrylamide gel (Fig. 3.2A) using the Laemmli method (1970). Half o f the pool was concentrated using an Amicon (Beverly, MA) pressure cell, then stored at 4°C as a 55% ammonium sulfate suspension. The other half was first dialyzed in buffer A to remove the Fru-6 P, then concentrated in 50% glycerol/50% buffer A containing 2 mM ATP/10 mM MgCl2 , and stored at -20°C. 77 Sedimentation Studies-Sucrose gradient sedimentation was performed according to the method o f Martin & Ames (1961). Each 5 to 20% sucrose gradient was buffered with 100 mM Tris-Cl, pH 8.2, containing 10 mM MgCl2 , 5 mM NH4 CI, 0.25 mM EDTA, 2.0 mM DTT, and 0.5 mM Fru-6 P (buffer B). An aliquot o f a stock solution o f PFK containing 2 to 5 pg was dialyzed at 4°C against buffer B containing 2 % sucrose, combined with a similarly-dialyzed solution o f yeast alcohol dehydrogenase (ADH; 10 pg), then layered between the gradient (about 4 ml) and a volume o f buffer B (about 1.4 ml) placed over the gradient just prior to sample layering. The ADH was used as an internal reference standard for determining the molecular weight o f the sedimenting PFK species. Sedimentation was performed at 4°C in a Beckman SW 50.1 rotor spun at 44,000 rpm for 13 hours. After sedimentation, fractions were collected by puncturing the bottom of the tube, then analyzed for PFK activity as described below. Fractions were analyzed for ADH activity by measuring the increase in absorbance at 340 nm in 1-ml assays that contained 100 mM Tris-Cl, pH 8 .6 ,1 mM NAD+, and 8.4 pmoles (50 ul) o f ethanol. Enzyme Activity Assays-Initial velocities were measured at 30°C in 100 mM Tris-Cl, pH 8.2, containing 10 mM MgCl2 and 5 mM NH4 CI by coupling the production o f fructose 1,6-bisphosphate to the oxidation o f NADH (0.2 mM). The coupled assay (Kotlarz & Buc, 1982) utilized the auxiliary enzymes aldolase (20 pg/ml), triosephosphate isomerase ( 1 0 pg/ml) and a-glycerophosphate dehydrogenase (10 pg/ml). Auxiliary enzymes were dialyzed at 4°C against 100 mM Tris-Cl, pH 8.2, prior to use. ADP produced in the EcPFK-catalyzed reaction was regenerated to ATP using creatine phosphate (1 mM) and creatine kinase (10 pg/ml). However, this regenerating system was found to be unnecessary for the BsPFKand ChiPFKcatalyzed reactions. In all assays, the free Mg++ concentration was kept 5 to 10 mM in excess o f the ATP concentration to avoid inhibition by free ATP. Assays were initiated by addition o f PFK. The change in absorbance at 340 nm was measured for at least one 78 minute following an initial nonlinear phase. A thermostatted Hitachi UV-2000 spectrophotometer was used for the measurements. Fluorescence Measurements-The binding o f various substrates or effectors can either enhance or quench the intrinsic fluorescence o f the single tryptophan, Trp 311, of EcPFK (Berger & Evans, 1991). Trp 311 is located near the C-terminus o f the EcPFK subunit within the large subunit-subunit interface o f the rigid dimer. ChiPFK also has a tryptophan at position 311. (The intrinsic fluorescence o f the single tryptophan of BsPFK, Trp 179, is largely insensitive to ligand binding (Kim et al., 1993)). Steady-state fluorescence measurements were made at 25°C using either a SPEX 1680 or an SLM 8000C fluorescence spectrometer. The excitation source was a Xenon arc lamp. Intrinsic fluorescence emission due to the single tryptophan, Trp 311, of EcPFK or ChiPFK was measured at 340 nm following excitation at 295 nm. Slit widths were set at 8 nm. Protein concentrations were 5 to 25 pg/ml (below 1 pM), which is low enough to avoid the inner-filter effect. The enzyme was buffered in 100 mM TrisCl , pH 8.2, containing 10 mM M gCh, 5 mM NH4 CI, 0.25 mM EDTA, and 2 mM DTT. Two types of experiments were performed: (1) addition o f a saturating amount of ligand, and (2 ) addition of increments of ligand to the enzyme solution. Corrections were made to compensate for volume change, enzyme dilution, and nonspecific quenching. This was done by performing parallel experiments in which phosphate o f the same concentration was added to the enzyme. Data Analysis-Parameters were obtained from steady-state kinetic or fluorescence studies by fitting the substrate saturation data to either the MichaelisMenten equation for hyperbolic kinetics or the Hill equation for sigmoidal kinetics. Kinetic data for MgATP saturation at low Fru-6 P concentration (Figure 3.4B) were fit to the initial velocity equation for a sequential random kinetic mechanism assuming non rapid-equilibrium conditions (Segel, 1975; Ferdinand, 1966). The GDP and AMPPNP inhibition data were analyzed as follows: first, the inhibition pattern was identified (competitive, noncompetitive, etc.). Based on this identification, the inhibition data were fit, using nonlinear regression analysis, to the equation for either competitive or noncompetitive inhibition (Cleland, 1979). The parameters generated were used to construct linear equations for the double-reciprocal plots. Finally, slopes and/or intercepts o f the inhibition lines were plotted against inhibitor concentration to determine the values for Kjs and/or Kjj. All inhibition data were corrected for loss of activity due to enzyme instability. Curve-fitting was performed using the program INPLOT (GraphPad, Inc., San Diego, CA). Results The Chimeric Subunit-X-ray crystallography has revealed that the amino acid residues that bind Fru-6 P are the same between the active sites o f the two native enzymes (Evans et al., 1981; Shirakihara & Evans, 1988). Three o f the residues that bind ATP are different between the two active sites. They are, for BsPFK versus EcPFK: Cys instead o f Phe 73, Lys instead o f Arg 77, and Gin instead o f Met 107. In the chimeric subunit (Fig. 3.1), the BsPFK/EcPFK junction is at residue 122. The residues that bind ATP are from the BsPFK portion of the subunit, while those that bind Fru-6 P are from the EcPFK portion. Since residues 118 to 130 (except residue 122) are conserved between BsPFK and EcPFK, Thr 125, Asp 127, and Asp 129 can be considered as being from either enzyme portion. The region o f the ATP-binding domain o f EcPFK that moves (residues 71-95 and 101-118) during the open-to-closed transition (Shirakihara & Evans, 1988) has been replaced with the corresponding "rigid" region o f BsPFK (Schirmer & Evans, 1990). The residues that comprise the effector sites o f the two native enzymes differ at several positions : Val instead of Arg 54, Gly instead o f Tyr 55, Gly instead o f Ser 58, Arg instead o f Lys 211, and lie 320 instead o f Tyr 319 for BsPFK versus EcPFK, respectively. Because o f these differences, the effector site o f the chimeric enzyme is different from that of either native enzyme. In EcPFK, there are five positively-charged FIGURE 3.1. Molecular Graphics Image o f the Chimeric Subunit. The portion o f the subunit in blue is from B. stearothermophilus PFK, while the portion in purple is from E. coli PFK. ATP and Fru-6 P molecules are in red. Amino acid residues that bind ATP are in yellow, while those that bind Fru-6 P are in green. The lone tryptophan, Trp 311, is in yelloworange . The image was generated fromB. stearothermophilus PFK atomic coordinates using SYBYL (Tripos Associates, St. Louis, MO) on an Evans and Sutherland ps390 graphics terminal. 81 residues that bind the phosphate groups of GDP and PEP: Args 21, 25, 54 ,154 and Lys 213 (Shirakihara & Evans, 1988; Lau & Fersht, 1989). In BsPFK, these residues are Arg's 2 1 ,2 5 ,2 1 1 ,1 5 4 and Lys 213 (Evans e ta l., 1981; Valdez e ta l., 1988). Thus, in BsPFK, Arg 211 substitutes for Arg 54. In ChiPFK, however, neither Arg 54 nor Arg 211 is present. The presence of only three arginines in the ChiPFK effector site may explain its lower affinity for GDP (see below). Structural Properties - It is important to know something about the structural properties o f the chimeric enzyme before investigating its kinetic and allosteric properties. SDS-PAGE analysis (Fig. 3.2A) indicates that the subunit molecular weights o f the three enzymes are similar, being 36,000 ± 1,000 daltons. The results o f a sucrose gradient sedimentation study (Fig. 3.2B) show that ChiPFK exists as a tetramer 137,000 daltons in size. Sedimentation experiments performed under the same conditions for BsPFK and EcPFK give similar molecular weights o f 139,000 and 136,000, respectively. Thus, the three enzymes exist as tetramers similar in size. Analysis o f the ChiPFK sucrose gradient fractions by SDS-PAGE followed by silver staining (not shown) reveals that no dimers or monomers were present within the gradient. About 75% of the total enzyme activity loaded was recovered from the fractions collected in the sedimentation study. Stability-The chimeric enzyme, once diluted to a concentration appropriate for activity assays, began to lose activity over time. Specifically, about 15% of its original activity was lost over a period o f 2 hours. The instability is an intrinsic property o f ChiPFK since the diluted enzyme was unstable under a variety o f buffer and temperature conditions. Instability was also observed in the fluorescence experiments in which Trp 311 fluorescence was titrated by incremental addition o f ligand solution. A saturable quenching o f fluorescence was observed in these experiments regardless of whether ligand or buffer was added. Even MgATP, which caused an increase in fluorescence when added in saturating amounts, caused a saturable decrease in 82 A M A B C M sizes 66.0 45.0 36.0 29.0 24.0 20.1 14.2 FIGURE 3.2. Structural Properties of the Chimeric PFK. (A) SDS-PAGE Analysis. M, molecular mass markers; A, native B. stearothermophilus PFK; B, the chimeric PFK; C, native E. coli PFK. Marker sizes are indicated in kilodaltons. (B) Sucrose Gradient Sedimentation. Plot o f Percent o f PFK Activity Loaded versus Fraction Number. The position o f the yeast ADH marker (144,000 daltons) is indicated with a downward arrow. The sedimentation origin was at fraction 29. Percent of Activity Loaded M O N3O OJ o >rtH 6 >n ST. 0 d zd 1 nn © o N3 o w © 00 U) 84 fluorescence in titration experiments. Thus, both activity and fluorescence measurements gave evidence that ChiPFK is somewhat unstable. Kinetic Parameters-The chimeric enzyme was studied using steady-state kinetics, and the kinetic parameters obtained were compared to those for the two native enzymes. The results (Table 3.1) indicate that the chimeric enzyme is two-fold less catalytically active than either o f the native enzymes. However, its affinity for ATP as measured by KmATP is similar. The larger value o f KmATP for EcPFK (103 pM) is most likely the result o f substrate antagonism between MgATP and Fru-6 P in the active site (Deville-Bonne et al., 1991b) since, when Fru-6 P concentration is 50 pM instead of 1.5 mM, KmATP drops to 70 |iM. The Fru-6 P saturation curves in the presence of saturating MgATP concentration vary in degree o f sigmoidicity among the three enzymes. Whereas Fru-6 P saturation o f EcPFK is highly sigmoidal (nn is 5.8), it is hyperbolic for BsPFK (nn is 1.1), and somewhat sigmoidal for ChiPFK (nn is 1.7). The S i/2 Fru'6p-value for ChiPFK is closer to the value for BsPFK than that for EcPFK. However, when EcPFK is activated by GDP (2 mM), the Fru-6 P saturation curve becomes hyperbolic and Si/2 Fru'6P falls to 50 pM, which is close to the value for either BsPFK or ChiPFK. These results show that, in terms o f its kinetic parameters, ChiPFK is remarkably similar to BsPFK and the activated form of EcPFK. Fru-6P Saturation Kinetics-Figure 3.3 displays more clearly the differences among the three enzymes in terms of their saturation by Fru-6 P in the presence o f 1.0 mM MgATP. Whereas saturation of EcPFK is highly sigmoidal, saturation o f ChiPFK is nearly hyperbolic. Thus, cooperativity has been largely destroyed in ChiPFK. Some remains, however (nn is 1.7; Fig. 3.3, inset). The residual cooperativity persists (nn is 1.8) even for Fru-6 P saturation o f ChiPFK in the presence of less-than-saturating MgATP concentration (50 pM; not shown). It is also present (nH is 1.5) when other nucleoside triphosphates (NTPs) such as GTP or CTP serve as the phosphate donor. Together, these results suggest that ChiPFK cooperative behavior is independent o f the TABLE 3.1 Kinetic Parameters fo r ChiPFK and the Native Enzymes Enzyme kcat (s-1) KmATP (ixM) S l/2Fru'6P (M-M) nn ChiPFK 60 ± 2 85 ± 2 34 ± 1 1.7 ± 0 .2 BsPFK 139 ± 2 4 70 ± 3 28 ± 7 1 . 1 ± 0 . 1 EcPFK 127 + 5 103 ± 2 445 ± 3 5.8 ± 0.2 The conditions for the assays are described under "Materials and Methods." kcat, the catalytic rate constant; KmATT>, the MgATP concentration at half-maximal velocity; S i/2 Fru'6P, the Fru-6 P concentration at half-maximal velocity; and nn, the Hill number. In determining KmATP, the Fru-6 P concentration was 0.3 mM for BsPFK and ChiPFK, and 1.5 mM for EcPFK. In determining S i/2 Fru-6P and nn, the MgATP concentration was 1.0 mM. 1.2 x a E 1.0 > \ > ' - / 0.8 L0 >% 4-» > 0.6 4J O > 4 -J a 0 0.2 O ' 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2I • [F r u 6 P ] (mM) FIGURE 3.3. Dependence of PFK Activity on Fru-6P Concentration. (O), native B. stearothermophilus PFK; (•) , native E. coli PFK; (■), chimeric PFK. The inset shows the chimeric PFK plot on an expanded x-axis. [MgATP] was saturating at 1.0 mM. & oo O' 87 absolute amount o f ATP present and the identity o f the NTP. What appears to be important is the amount o f MgATP (or other NTP) relative to the amount o f Fru-6 P present. When MgATP concentration was low (50 jiM), ChiPFK activity did not increase with Fru-6 P concentration beyond 50 pM. This result indicates that MgATP binding and/or release is not a rapid-equilibrium process for ChiPFK. Similar results showing non rapid-equilibrium binding and/or release o f MgATP were obtained for BsPFK and the activated form o f EcPFK. MgATP Saturation in the Presence o f Low Fru-6P Concentration- MgATP saturation curves for ChiPFK and the two native enzymes obtained in the presence o f saturating Fru-6 P concentration are hyperbolic and yield similar KmATP-values (Table 3.1). However, when Fru-6 P concentration is low (50 pM), differences become apparent between the MgATP saturation curves o f EcPFK and ChiPFK (Figs. 3.4A & B). Under these conditions, significant substrate inhibition o f EcPFK by MgATP is evident (Fig. 3.4A; Kundrot & Evans, 1991; Johnson & Reinhart, 1992). The left-most portion of the curve in Fig. 3.4A can be fit to the Michaelis-Menten equation (inset), yielding a KmATP o f 0.5 ± 0 .1 pM. As shown in Fig. 3.4B, MgATP also inhibits ChiPFK at low Fru-6 P concentration (open circles), but the inhibition is much less severe. Furthermore, the data for ChiPFK can be fit to the equation for a steady-state random Bi Bi kinetic mechanism (Segel, 1975b) but those for unactivated-EcPFK (Fig. 3.4A) cannot. The results suggest that the mechanisms o f ATP inhibition o f ChiPFK and unactivated-EcPFK are different. However, whereas MgATP saturation curves at low Fru-6 P concentration for ChiPFK and unactivated-EcPFK are dramatically different, those for ChiPFK and EcPFK activated by 2 mM GDP (Fig. 3.4B, closed circles) are similar. Their profiles, and the extent o f inhibition relative to V max at 10 mM MgATP- 25% for ChiPFK and 20% for activated-EcPFK- are similar. These results suggest that the ChiPFK subunit is locked in a conformation that resembles the activated EcPFK subunit. A tn d w •p•H d 2 u o :> •H-p •r-t d .05 .04 .03 ° .02 .01 0 .03 .04 .03 > IW .03 .01 \ _i L i -----------1 - - i----------1 i i » ■ \ i ■ 10 15 [MgATP] (jiM) 20 FIGURE 3.4. Dependence of PFK Activity on MgATP Concentration when Fru-6 P Concentration is Low (50 uM). (A) Initial Velocity (units/ug) versus MgATP concentration for native EcPFK in the absence o f activator GDP. The inset shows the data between 0.1 and 4 uM MgATP fit to the Michaelis-Menten equation. (B) Relative Activity (v/Vmax) versus MgATP concentration for ( • ) , native E. coli PFK fully activated with 2 mM GDP and (O), chimeric PFK (no GDP present). The curve for BsPFK (dashed line) is included for comparison. The data were fit to the equation for a steady-state random kinetic mechanism (Ferdinand, 1966). 00 00 [M gA TP] (m M ) Relative Activity (v/Vmax) o o |-* ou i o to CJ\ oo ►-k o 90 Heterotropic Regulation-The abilities o f PEP to allosterically inhibit, and GDP to allosterically activate ChiPFK were investigated using steady-state kinetics. The results obtained indicate that ChiPFK activity is insensitive to regulation by either effector. Fig. 3.5A shows that both native enzymes are inhibited by PEP, although the sensitivity and cooperativity o f their responses differ. The inhibition profile for EcPFK is highly sigmoidal with a Hill number o f 4.3 (I1 /2 is 1.17 ± 0.01 mM), while the profile for BsPFK is much less sigmoidal with a Hill number o f 1.6 (11 /2 is 0.28 ± 0.01). However, PEP has essentially no effect on the chimeric enzyme. ChiPFK likewise cannot be activated by GDP. As shown in Fig. 3.5B, GDP strongly activates EcPFK when MgATP concentration is saturating and the Fru-6 P concentration is equal to the S 1/2 -value (0.45 mM). The KactGDP for the activation, which is hyperbolic, is 13 ± 0.4 uM. On the other hand, GDP can either activate or inhibit BsPFK, depending on the concentration o f GDP. In the presence o f Fru-6 P at a concentration equal to the S 1/2 value (30 uM), as GDP concentration increases, it first activates BsPFK weakly to about 20% (KactGDP is 90 ± 10 uM), then begins to inhibit (Fig. 3.5B). Since the inhibition by GDP is competitive with respect to MgATP (Kis= 1.5 ± 0.2 mM; not shown), it is due to binding in the active site. The chimeric enzyme cannot be activated by GDP but, like BsPFK, is inhibited by it. However, the inhibition pattern is mixed-type noncompetitive with respect to MgATP (Kis= 4.3 ± 0.9 mM; Kii= 26 ± 6 mM) rather than competitive. The origin of the inhibition is not clear. Thermal Inactivation Studies-A possible explanation for the lack of heterotropic regulation o f ChiPFK by PEP and GDP is that these ligands cannot bind the effector site o f the enzyme. To address this possibility, thermal inactivation experiments were performed in which ChiPFK was heated in the absence or presence of ligand. The results show that both Fru-6 P and MgGDP protect native EcPFK against thermal inactivation at 60°C (Fig. 3.6A). PEP also protects EcPFK, but the protection is partial, with 6 8 % o f activity remaining after 1 hour. Similar experiments were 91 A 100 u< <u u <u a . [PEP] (mM) FIGURE 3.5. Effect o f PEP and GDP on PFK Activity. (O ), B. stearothermophilus PFK; ( • ) , E. coli PFK; (■ ), chimeric PFK. (A), Effect o f PEP. Percent Activity Remaining versus PEP Concentration. [MgATP] was saturating at 1.0 mM. [Fru-6 P] was 0.3 mM for BsPFK and ChiPFK, and 1.5 mM for EcPFK. (B), Effect o f GDP. Percent Activation versus GDP Concentration. [MgATP] was saturating at 1.0 mM. [Fru-6 P] was equal to the S 1/2 -value, which was 0.03 mM for ChiPFK and BsPFK, and 0.45 mM for EcPFK. P er ce nt A ct iv at io n (% ) 92 B 100 50 0 -50 i ------------1________ i_________i------------1_________i________ i________ i________ i_________i________ i_________i_________i_________ i_________i________ 0 2 4 6 [GDP] (mM) 93 A O ) c: u < c 0 ) u u <D (X Q -i-i i i-I i i i i I i i i i I i i i i I i i t i I i i i 0 10 20 30 40 50 60 Time (min.) FIGURE 3.6. Protection o f (A) Native E. coli PFK and (B) Chimeric PFK Against Thermal Inactivation. Enzymes were incubated in 100 mM Tris-Cl, pH 8.2, containing 10 mM MgCl2 and 2 mM DTT (at 60°C and 50°C for EcPFK and ChiPFK, respectively) in the presence o f ( • ) , no ligand; (O ), 5 mM Fru-6 P; (■ ), 10 mM PEP; or (A), 5 mM GDP for the indicated time period. An aliquot was then removed and assayed for PFK activity. MgATP and Fru-6 P concentrations were both 1.0 mM in the activity assays. Percent Activity Remaining (%) w v£> A 95 performed on ChiPFK. (50°C was used instead of 60°C because o f the lower stability o f ChiPFK). As shown in Fig. 3.6B, Fru-6 P at 5 mM protects ChiPFK against thermal inactivation. This result contrasts with the inability o f a 10-fold lower concentration o f Fru-6 P to stabilize ChiPFK diluted for activity assays (above). It suggests that ChiPFK dissociates along its "active" interface, and that Fru-6 P prevents dissociation by interacting with Arg 162 and Arg 243 across it (Teschner et al., 1990). PEP also protects ChiPFK to about the same extent it protects native EcPFK (64% versus 6 8 % for EcPFK). Thus, PEP binds as well to ChiPFK as it does to EcPFK. In contrast, MgGDP offers little protection. Only 15% o f activity remains after a 1-hour incubation, compared to 9% in the absence o f ligand (Fig. 3.6B). The simplest interpretation o f this result is that MgGDP binds poorly in the effector site o f ChiPFK. The structural basis for poor binding could be the presence o f three rather than four arginine residues in the effector site. AMPPNP Inhibition Studies-Inhibition o f both activated-EcPFK and ChiPFK by AMPPNP with respect to MgATP was competitive (Kjs= 24 + 2 and 20 ± 2 (iM, respectively), as was AMPPNP inhibition o f BsPFK with respect to MgATP (Kjs o f 50 ± 20 pM; Byrnes et al, 1994). Thus, as expected, AMPPNP competes with MgATP for binding in the active site. Interestingly, AMPPNP inhibition o f both activated-EcPFK and ChiPFK with respect to Fru-6 P gave similar competitive-like patterns o f lines that intersected just to the right o f the 1/velocity axis in double-reciprocal plots. (KiS-values of 7 ± 7 and 9 ± 6 pM were obtained for activated-EcPFK and ChiPFK, respectively.) In contrast, AMPPNP inhibition o f BsPFK with respect to Fru-6 P is noncompetitive (Kjs = 0.29 ± 0.09 mM and Kii = 0.40 ± 0.12 mM; Byrnes et al., 1994). Thus, AMPPNP binding to activated-EcPFK and ChiPFK is different from its binding to BsPFK. The competitive-like inhibition with respect to Fru-6 P could be due to antagonism of Fru-6 P binding by AMPPNP in the active site (ATP shows a similar effect that is less pronounced). In addition, as shown in Figure 3.7, the Hill number for 96 X 3 uV E 3 z 1.6 1.5 0.3 1.4 1.3 1.2 0.1 1.0 X I I0.9 x 0.0x -5 0 50 100 150 200 25U o 3 IS > CO [AMPPNP] (uM) FIGURE 3.7. Effect o f AM PPNP on the Fru-6 P-Dependent Cooperative Behavior o f Chimeric PFK. Plot o f Hill Number ( • ) or S 1/2 -value (O ) versus AMPPNP Concentration. The Hill numbers and S 1/2 -values were obtained by fitting Fru-6 P saturation data, collected in the presence o f 0.93 mM MgATP and the indicated [AMPPNP], to the Hill equation. Error bars represent standard errors. 97 the Fru-6 P saturation curve obtained in the presence o f AMPPNP decreases with increasing AMPPNP concentration until it reaches 1.0 at 200 pM AMPPNP. Thus, AMPPNP abolishes the sigmoidicity o f the Fru-6 P saturation curve o f ChiPFK. Effect o f Ligand Binding on Trp 311 Intrinsic Fluorescence-Two types of steady-state fluorescence experiments were performed. In the first type, the intrinsic fluorescence o f EcPFK was titrated by addition o f small volumes o f ligand to the enzyme solution. (These experiments could not be done on ChiPFK because o f its instability). The results are presented in Table 3.2. In the second type o f experiment, a saturating amount o f ligand was added to the enzyme solution, and the change in fluorescence intensity measured (Table 3.3). Titration o f EcPFK fluorescence with Fru6 P, with AMPPNP, and with AMPPNP after incubation in the presence o f Fru-6 P (Table 3.2) gave results similar to those previously reported (Deville-Bonne & Garel, 1992; Johnson & Reinhart, 1992). The results in Table 3.3 show that AMPPNP (or ATP) binding cause the same fluorescence increase (12%) in both ChiPFK and EcPFK, but the fluorescence decrease induced in ChiPFK upon Fru-6 P binding (8 %) was less than that induced in EcPFK (19%). ChiPFK can thus exist in two conformational states: a high fluorescence state and a low fluorescence state. Addition o f Fru-6 P to EcPFK incubated in the presence of AMPPNP at concentrations as low as 5 pM resulted in no fluorescence change (Table 3.3; AMPPNP concentrations below 2 p.M did allow Fru-6 P (500 jiM) to bind, however). This dramatic blockage o f Fru-6 P binding by AMPPNP, which may indicate closure o f the EcPFK active site upon AMPPNP binding, was not seen in ChiPFK (Table 3.3). Finally, in order to investigate the effect o f interactions between Fru-6 P and AMPPNP on the fluorescence of ChiPFK, experiments were done in which a saturating amount of one ligand was added to the enzyme in the presence of increasing amounts o f the other. Fig. 3.8A shows that Fru-6 P (500 p M ) cannot bring ChiPFK completely to the low-fluorescence state when AMPPNP is present, even at low levels, and Fru-6 P is increasingly less effective in bringing ChiPFK to this 98 TABLE 3.2 Dissociation Constants and Maximum Fluorescence Changes fo r Fru-6P and AMPPNP Binding to E. coli PFK Ligand K d(pM ) w< 1 Fru-6 P 9.6 ± 1.2 2 0 ± 1 1 .0 (h) AMPPNP 0 . 1 ± 0 . 0 1 +16.6 ± 0 .3 1 .0 (h) AMPPNP/Fru-6 P 0.20 ± 0 .03 +35.6 ± 1 .0 1.9 ± 0 .1 Small increments o f the indicated ligand were added to the enzyme solution, and the changes in fluorescence measured. After correction, the fluorescence change was plotted against ligand concentration. The resulting curves were fit to either the Michaelis-Menten equation or the Hill equation. Kd, equilibrium dissociation constant; AFmax, maximum fluorescence change; np, Hill number, h, hyperbolic. AMPPNP/Fru6 P indicates incremental addition o f AMPPNP to EcPFK in the presence o f Fru-6 P (100 pM). TABLE 3.3 Changes in Intrinsic Fluorescence Induced by Ligand Binding to E. coli PFK and the Chimeric PFK Ligand Added Enzyme Fru-6 P Fru-6 P after AMPPNPfl ATP AMPPNP AMPPNP after Fru-6 P^ ADP PEP (500 pM) (500 pM) (20 pM) (20 pM) (20 pM) (100 pM) (2 mM) EcPFK -18.8 ±0.5 no change +11.6 ±0.5 +12.2± 0.3 +35.5 ±4.8 -13.9 ±0.1 +3.6 ±0.2 ChiPFK -8.4 ±0.6 -9.5 ± 0.8 +11.5 ±2.5 +13.4 ±1.0 +6.3 ±0.5 no change no change The change in steady-state fluorescence was measured upon addition of the indicated ligand at the indicated concentration, as described under "Materials and Methods". Each value is the average o f two measurements. Based on the dissociation constants in Table 3.2, concentrations of 500 pM and 20 pM were assumed to be saturating for Fru-6 P and AMPPNP, respectively. flThe enzyme was incubated with AMPPNP (20 pM), then Fru-6 P (500 pM) was added. ^The enzyme was incubated with Fru-6 P (500 pM), then AMPPNP (20 pM) was added. 100 A h ig h f l u o r e s c e n c e s ta te +14% +14% +14% + 12% 11 u n l i g a n d e d e n z y m e lo w f l u o r e s c e n c e s ta te T 8 % T 1 i i itl 1 r -9.5% I -15% -14% [AMPPNP] (uM) 0 1 5 20 50 [Fru-6P] (uM) 500 500 500 500 500 FIGURE 3.8. Combined Effects o f Fru-6 P and AM PPNP on Intrinsic Fluorescence o f the Chimeric PFK. (A) Effect o f Adding a Saturating Amount o f Fru-6 P (500 uM) to ChiPFK Incubated with Increasing Amounts o f AMPPNP. The thin arrows show the initial effect o f AMPPNP on the fluorescence, and the thick arrows show the subsequent effect o f Fru-6 P. AMPPNP places ChiPFK in a high-fluorescence state, Fru-6 P a lowfluorescence state. (B) Effect of Adding a Saturating Amount o f AMPPNP (20 uM) to ChiPFK Incubated with Increasing Amounts o f Fru-6 P. The thin arrows show the initial effect o f Fru-6 P on the fluorescence, and the thick arrows show the subsequent effect o f AMPPNP. The number at each arrowhead is the overall percent change in fluorescence represented by the arrow it accompanies. Each number is the average of two measurements. The concentrations o f AMPPNP and Fru-6 P are indicated. 101 B h ig h f l u o r e s c e n c e s ta te u n l i g a n d e d s ta te lo w f l u o r e s c e n c e s ta te +14% +18% +21% +22% +22% +3% -4% -7% 8%8% 8%8% [Fru-6P](|iM ) 0 2 [A M P P N P K u M ) 20 20 10 50 100 20 20 20 500 20 1000 20 102 conformational state as AMPPNP levels increase. Fig. 3.8B shows that AMPPNP (20 jiM) can bring ChiPFK to the high-fluorescence state when Fru-6 P levels are low to moderate, but is less effective in doing so at high Fru-6 P levels. Discussion Here we report studies on a chimeric bacterial phosphofructokinase (ChiPFK), which has been successfully purified after high-level expression in PFK-deficient E. coli cells. ChiPFK exists as an active tetramer similar in size to tetramers o f the native enzymes from which it is derived. Its catalytic rate constant is about half that o f either of the two native enzymes. Its affinities for substrates, as measured by the KmATPand S i/2 Fru'6 P-values, are similar to those of BsPFK and the activated form of EcPFK. Although hybrid EcPFK tetramers have been studied before (Lau & Fersht, 1989), this is the first time a chimeric PFK, containing parts o f two different PFKs grafted together, has been constructed and studied. Our results attest to the great structural and functional similarity between BsPFK and EcPFK, especially at their active sites. However, the results also emphasize differences in their mechanisms o f heterotropic regulation. Though stable as a tetramer in the sucrose gradient sedimentation experiment, ChiPFK is somewhat unstable under the conditions o f the activity assay. The two native enzymes are stable under these conditions. The loss o f ChiPFK activity over time is paralleled by a quenching o f its Trp 311 fluorescence in titration experiments. The fact that similar quenching profiles were observed in the titration experiments irrespective o f the ligand added suggests that the effect is time-dependent rather than ligand-dependent. It most likely reflects dissociation o f the active tetramer when present in dilute solutons. This dissociation is to monomers rather than dimers since quenching o f Trp 311 fluorescence occurs when the EcPFK tetramer dissociates into monomers, but not dimers (Deville-Bonne e ta l., 1989; LeBras et al., 1989). Phosphoenolpyruvate does not inhibit ChiPFK. PEP binds to the enzyme, but the binding causes no measurable change in Trp 311 intrinsic fluorescence. This 103 suggests that conformational changes normally associated with allosteric inhibition are absent in ChiPFK. The observation that GDP inhibits ChiPFK noncompetitively with respect to MgATP indicates that GDP binds to ChiPFK. This binding may occur in both the active and effector sites. The inability of GDP to protect ChiPFK against thermal inactivation suggests that GDP does not bind the effector site. However, GDP binding in the active site o f BsPFK has been shown to destabilize BsPFK in thermal inactivation experiments (Zhu et al., manuscript in preparation). Thus, such binding in the active site o f ChiPFK could destabilize the enzyme and mask the stabilization offered by its binding in the effector site. The inability of GDP to induce a fluorescence decrease in ChiPFK could thus be due either to poor binding in the effector site or to an inability o f the ChiPFK tetramer to transmit allosteric changes. A third possibility is that the ChiPFK tetramer is already locked in an activated state and cannot be further activated by GDP. The active site o f EcPFK appears to close when AMPPNP binds to it, while the active site o f ChiPFK is locked in an "open" conformation. EcPFK bound tightly to the Cibacron Blue affinity column during purification, and AMPPNP strongly inhibited subsequent Fru-6 P binding to EcPFK in fluorescence studies. The latter result suggests that closure of the active site when AMPPNP binds in the absence o f Fru-6 P blocks subsequent binding of Fru-6 P. Unlike EcPFK, ChiPFK associated loosely with the column during purification, and Fru-6 P could bind to and cause a fluorescence decrease in ChiPFK after the enzyme had bound AMPPNP (20 pM). The active site o f ChiPFK is thus locked open. Our results also suggest that the ChiPFK active site resembles that o f the activated form of EcPFK. This is shown by similarities between ChiPFK and activated-EcPFK in their kinetic parameters (Table 3.1), their saturation by MgATP in the presence o f low Fru-6 P (Fig. 3.4B), and their AMPPNP versus Fru-6 P inhibition patterns. Thus, the active site o f ChiPFK is locked in an open structure similar to that of the activated form of EcPFK. 104 The results in Table 3.3 indicate that ChiPFK can exist in two conformational states: a high-fluorescence state induced by the binding o f ATP or AMPPNP, and a low-fluorescence state induced by Fru-6 P binding. The smaller fluorescence decrease seen when Fru-6 P binds to ChiPFK compared to EcPFK ( 8 % versus 19%, respectively) suggests that the conformational change is less, and perhaps different, in ChiPFK. As revealed in Figs. 3.8A & B, the balance between highand low-fluorescence conformational states depends on the relative amounts o f AMPPNP and Fru-6 P present. Furthermore, AMPPNP is more potent than Fru-6 P in its ability to reverse the fluorescence state (low versus high) o f the ChiPFK molecule. These binding results parallel the kinetic results showing that the relative amounts o f Fru-6 P and ATP present determine the reaction rate. There are at least three possible explanations for the sigmoidal Fru-6 P saturation kinetics o f ChiPFK: (1) the sigmoidicity is due to apparent cooperativity arising from the kinetic mechanism. Both of the native enzymes from which ChiPFK is derived obey a steady-state random Bi Bi kinetic mechanism (Byrnes et al., 1994; Zheng & Kemp, 1992). This mechanism can allow two kinetically-distinct pathways, and sigmoidal saturation curves (Ferdinand, 1966). Although we have not rigorously determined the kinetic mechanism o f ChiPFK, our results suggest that ChiPFK also obeys a steady-state random mechanism. (2 ) a slow isomerization occurs in the active site when Fru-6 P binds in the presence o f AIT. If the isomerization is slower than the catalytic rate, cooperativity can result (Ainslie et al., 1972). (3) only two of the four active sites o f ChiPFK are allosterically-linked by a cooperative structural transition (Monod et al., 1965) that is triggered by interactions between ATP and Fru-6 P in the active site. The fact that two conformational states exist for ChiPFK casts doubt on the notion that its cooperativity is entirely kinetic in origin. Presumably, the ChiPFK molecule goes through a conformational transition as it moves from one state to the 105 other according to the balance o f substrate concentrations. Regarding this transition, the questions are: ( 1 ) is it a rapid and concerted one, involving more than one active site? or (2) is it slow relative to catalysis? Although one would expect homotropic linkage to be absent in ChiPFK since it is not heterotropically-regulated, this may not be the case. Indeed, a mutant EcPFK enzyme (Leu 178- >Trp) has been reported to exhibit a highly sigmoidal Fru-6 P saturation profile in the absence o f heterotropic regulation (Serre et al., 1990). A determination o f whether the conformational transition is fast or slow relative to catalysis will have to await stopped-flow fluorescence measurements. The mechanism by which AMPPNP abolishes the cooperativity o f ChiPFK (Fig. 3.7) is not known. However, it could be related to an interaction between the imidophosphate group of AMPPNP and Arg 72, an active site residue important in the cooperative behavior o f EcPFK (Berger & Evans, 1990). Arg 72 is involved in neutralizing the negative charge on the transferred y-phosphate o f ATP in the transition state o f EcPFK (Zheng & Kemp, 1994). The imidophosphate group o f AMPPNP is less acidic than the y-phosphate o f ATP (Yount et al., 1971). As a result, at pH 8.2 the Arg 72-imidophosphate interaction may be weakened relative to the Arg 72-phosphate interaction. A strengthening o f the interaction with Arg 72 has been proposed to occur when the ATP analog adenosine 5'-[y-thio] triphosphate (ATP-y-S) is used instead of ATP, in this case by perturbation o f an interaction with Thr 125 (Auzat et al., 1994). Whether the conformational transition associated with ChiPFK cooperativity is concerted or involves slow isomerization, the altered interaction with Arg 72 could be important in explaining the effect o f AMPPNP on ChiPFK cooperative behavior. In conclusion, we have found that the active site of the the chimeric enzyme is locked in an "open" conformation similar to that o f the activated form of E. coli PFK. Presumably, a conformational change responsible for closure o f the active site has been disrupted in the chimeric enzyme, whose ATP-binding domain is from BsPFK. Yet, despite the "openness" of its active site, ChiPFK exhibits sigmoidal kinetics with respect 106 to Fru-6 P. We propose that the lower cooperativity of the chimeric PFK is related to its inability to undergo the open-to-closed transition involving its ATP-binding domain. Furthermore, the fact that some cooperativity remains in ChiPFK suggests that mechanisms in addition to one involving closure and opening o f the active site are important for E. coli PFK cooperative behavior. Indeed, the complete mechanism for allosteric regulation o f E. coli PFK is probably complex, involving binding cooperativity (Blangy et al., 1968), cooerativity due to slow transitions (Deville-Bonne et al., 1991a), and kinetic cooperativity arising from the kinetic mechanism (Zheng & Kemp, 1992). Chapter 4 The Role of Residue 161 in the Allosteric Transitions of Two Bacterial Phosphofructokinases 107 108 Phosphofructokinase (PFK, EC 2.7.1.11) catalyzes the first committed step o f glycolysis, the transfer o f the y-phosphate o f ATP to fructose 6 -phosphate (Fru-6 P) to produce ADP and fructose 1,6-bisphosphate. A divalent Mg++ ion, as well as a monovalent NH4 + or K+ ion, is required for catalysis. In bacteria, PFK is allosterically inhibited by phosphoenolpyruvate (PEP) and activated by ADP (or GDP). Bacterial PFK is a tetramer o f identical subunits each 36,000 daltons in size. There are four active sites, and four effector sites into which both PEP and ADP (or GDP) bind. The PFKs from the two bacteria Escherichia coli and Bacillus stearothermophilus are remarkably similar in structure (Evans e ta l., 1981; Shirakihara & Evans, 1988): their subunits have all the same secondary structural elements, their subunit a-carbon traces are nearly superimposable, and they share 55% amino acid identity. Nevertheless, PEP and GDP regulate the two enzymes somewhat differently. For example, the PEP inhibition profile for EcPFK is highly sigmoidal, while the profile for BsPFK is only slightly sigmoidal. In addition, whereas GDP strongly activates EcPFK in the presence of saturating MgATP and a Fru-6 P concentration equal to the S i/2 -value, it weakly activates BsPFK under these conditions (Byrnes e ta l., manuscript submitted). This difference is related to the fact that MgATP "allosterically" inhibits EcPFK (Evans, 1992), but not BsPFK, at low Fru-6 P concentration (Byrnes et al., 1994). However, GDP strongly activates BsPFK when the enzyme is inhibited by PEP (Valdez et al., 1989). Finally, GDP can super-activate EcPFK in the presence of saturating Fru-6 P levels, raising the kcat 10-20% higher than its value without GDP (Deville-Bonne et al., 1991a), but it cannot super-activate BsPFK. The heterotropic regulation o f EcPFK by PEP and GDP has been explained in terms o f the concerted two-state model o f Monod, Wyman, and Changeux (MWC model, Monod et al., 1965), in which the protein molecule exists in equilibrium between two states: a low-activity tense (T) state that binds PEP, and a high-activity relaxed (R) state that binds GDP (Blangy et al., 1968). We will assume that BsPFK 109 also obeys the MWC model, although recent evidence suggests it does not (Zhu et al., manuscript submitted). Schirmer & Evans (1990) have uncovered the structural basis for the allosteric transition o f BsPFK by comparing crystal structures o f the enzyme obtained in the presence o f inhibiting versus activating conditions. The BsPFK tetramer can be viewed as a dimer o f rigid dimers. During the transition from the Tto the In- state, the rigid dimers rotate 7° relative to each other. This rotation is coupled with a coordinated movement o f two loops, the 8 H and 6 F loops, located between the effector and active sites. At the effector site, the major movement involves a change in the position o f the 8 H loop, which lines one edge o f the effector cleft. In the absence of effector, the cleft is open. Binding o f the smaller molecule PEP closes the cleft, shifting the enzyme to its T-state. On the other hand, binding of the larger ADP (or GDP) molecule pushes the 8 H loop away from the effector site, causing it to contact the 6 F loop which transmits a structural change to the active site. This structural change involves reorganization o f the 6 F loop, which is composed or residues Thr 156 to Arg 162 (Fig. 4.1). Arg 162, which points away from the active site in the T-state, swings back into the active site during the T-to-R transition, and binds the phosphate group of Fru-6 P in the R-state. At the same time, Glu 161 which projects across the dimer-dimer interface into the active site in the T-state, has rotated away in the R-state. The 6 F loop and residues Arg 162 and Glu 161 within it are thus proposed to be important in the allosteric transition of BsPFK. Most of the residues within the 6 F loop, including Arg 162, are conserved between BsPFK and EcPFK (French & Chang, 1987; Hellinga & Evans, 1985), but Glu 161 is not. In EcPFK, residue 161 is a glutamine (Fig. 4.2). How important is Glu 161 for PEP inhibition and GDP activation o f BsPFK? Does Gin 161 of EcPFK play a role in its regulation by either PEP or GDP? Can any differences between the two enzymes be traced to the difference at position 161, i.e., Glu versus Gin? To answer these questions, we have constructed a set o f mutants of both BsPFK and EcPFK with changes at position 161. Glu 161 of BsPFK has been subunit A Asp12 . 6 6 ^ Ser159,° Arg7 2 - -'0H' 155 -NH H,0 Arg252 V / \ „„/ . \ 'iNHj' + Alai 57 !*H His160 Glu241 Arg162 o Giu161 163 NH Arg243 subunit D subunit A A denosine /'ADP o o. ./ NH °7 \ 9*' / l^ Ar9 7 2/■ o 0 "*NH2 + NHj NH HjO Ar92 5 2 K H ! / \ A s p 1 2 A v.®%-V NH NHi.x + ; \ n h , _ _ ^ n h 'o'' . nhj Arg162 OH ° NHj F6P + V / An y /- oh Ser159 H i s 1 6 0 Glu161Glu241 163 Ar9243 subunit D FIGURE 4.1. Reorganization o f the 6 F Loop (156-162) of B. stearothermophilus PFK. The wavy line represents the boundary between dimers. (A) T-state structure, showing Glu 161 in the Fru-6 P site. (B) R-state structure, showing the phosphate of Fru-6 P bound by Arg 162 and Arg 243. (Taken with permission from Schirmer, T., & Evans, P. R. (1990) Nature 3 4 3 ,140-145. Copyright 1990 Macmillan Magazines, Limited). I l l B. stearothermophilus E161Q E161A E. coli Q161E Q161R Q161A 161 T A T S H E R T A T S H Q R T A T S H A R T S S S H Q R T S S S H E R T S S S H R R T S S S H A R FIGURE 4.2. The Amino Acid Sequences o f the 6 F Loops o f B. stearothermophilus PFK, E. coli PFK, and the Five M utant Enzymes. Residue 161 is shown in enlarged, bold print. 112 changed to glutamine, and to alanine which lacks hydrogen bonding ability and is smaller. Gin 161 o f EcPFK has been changed to glutamate, to alanine, and to arginine in order to have a positively-charged residue at position 161. The abilities o f PEP and GDP to inhibit and activate, respectively, the five mutant and the two wild-type enzymes were studied using steady-state kinetics. The results o f these studies are presented. M aterials and Methods Enzymes, Chemicals, and Oligonucleotides-Restriction endonucleases, T4 DNA Ligase, DNA Polymerase (Klenow fragment), T4 Polynucleotide Kinase, and other enzymes used for cloning or in vitro mutagenesis were from New England Biolabs (Beverly, MA), Gibco-BRL (Grand Island, NY), or United States Biochemical (Cleveland, OH). X-gal and IPTG were from Gibco-BRL. The auxiliary enzymes aldolase, a-glycerophosphate dehydrogenase, and triosephosphate isomerase were from Sigma Chemical Co. (St. Louis, MO). Substrates ATP and Fru-6 P, and effectors PEP and GDP, as well as the Cibacron Blue 3GA agarose (type 3000-CL-L) were also from Sigma. The mutagenic oligonucleotides and the sequencing primers were synthesized on an Applied Biosystems 380A DNA Synthesizer. All oligonucleotides were purified in a two-step procedure involving (1) reverse-phase chromatography on an OligoPak TM (Millipore, Milford, MA) purification column and (2) preparative 20% polyacrylamide gel electrophoresis. The purity and length o f each oligonucleotide was verified by first 5' end-labeling the oligo using [3 2 P] y-ATP and T4 polynucleotide kinase, then subjecting it to electrophoresis on an analytical 2 0 % polyacrylamide gel and autoradiography. Site-directed Mutagenesis-The EcPFK gene was excised from a recombinant ecpfk/pUCl& plasmid (Hellinga & Evans, 1985), and subcloned into M 13m pl9 via its Nar I and Kpn I restriction sites. Similarly, the BsPFK gene contained within a 2.5 kilobase EcoR 1/ Cla I fragment that had been cloned into pBR322 (French & Chang, 113 1987) was subcloned into M 13mpl8 via its EcoR I and Acc I sites. The single-stranded DNA produced from either M13 vector contained the coding strand of the pfk gene. Competent D H 5aF E. coli cells (Gibco-BRL) were transformed with the recombinant M l3 RF DNA, then plated by the top-agar method onto YT (yeast extract/tryptone) agar plates containing X-gal and IPTG, which allow blue/white color selection. After overnight incubation at 37°C, a clear (turbid) plaque harboring the ecpfk or bspfk insert was picked and used to infect DH5ocF cells. The infected cells were grown in liquid YT medium for 6 hours at 37°C. After centrifugation, the supernatant o f the cell culture, which contained M13 phage particles, was used to infect fresh E. coli BW313 cells. (BW313 cells are du r ung~). The infected BW313 cells were grown overnight at 37°C in YT containing uridine at 25 (ig/ml. Uracil-containing single-stranded DNA was then prepared from the supernatant o f the infected cell culture (Kunkel et al„ 1987), and used as a template for in vitro mutagenesis as described by Zoller & Smith (1983). The mutagenic oligonucleotides were designed to change codon 161 in bspfk from that for a glutamate (GAG) to that for either a glutamine (CAG) or an alanine (GCG). In ecpfk, the mutagenic oligos were designed to change codon 161 from that for a glutamine (CAG) to that for a glutamate (GAA), an arginine (CGT), and an alanine (GCT). Table 4.1 lists the oligos and their sequences. After mutagenesis, competent DH5aF' cells were transformed with the reaction mixture containing the mutated DNA, then plated onto YT agar plates. Transformants were screened for the mutation by ( 1 ) plaque hybridization using the mutagenic oligonucleotide labeled with 32P as a probe (Seong & RajBhandary, 1987), and (2) partial sequencing o f the single- stranded DNA from positives identified in the plaque hybridization. A single positive transformant was then selected, single stranded DNA was prepared from it, and the entire coding region of the mutated gene was sequenced by the dideoxy method (Sanger, 1977) using a SequenaseTM kit (United States Biochemical) and a battery of 114 TABLE 4.1 Mutagenic Oligonucleotides Oligo Designation Position o f 3' End (codon number) Sequence cs'->y) BsE161Q 157 * CGTACGTCCGCTGGTGCGACGTCGC BsE161A 157 4c cgtacgtccgcgcgtgcgacgtcgc Ec Q161E 156 Jic * CGGAAATACGTTCGTGAGAAGAAGAGG Ec Q161R 156 * * CGG AAAT ACG ACGGTG AG AAG AAG AGG * * * Ec Q161A 156 CGGAAATACGACCGTGAGAAGAAGAGG Bs, B. stearothermophilus', Ec, E. coli. The mutagenic oligonucleotides are complementary to the bspfk or ecpfk coding strands. Oligos Bs E161Q and E161A are designed to change the codon for glutamate 161 o f bspfk (GAA) to those for a glutamine (CAG) and an alanine (GCG), while oligos Ec Q161E, Q161R, and Q161A are designed to change the codon for glutamine 161 o f ecpfk (CAG) to those for a glutamate (GAA), an arginine (CGT), and an alanine (GCT). The underlined nucleotides are those complementary to codon 161. The nucleotides with asterisks above them are the ones that are different in the mutated gene. 115 intragenic primers to verify that no unintentional mutations had been introduced. This was done for all five mutated genes. Expression and Purification ofPFKs-The mutated BsPFK genes were excised from M 13m pl8 and inserted into the EcoR V Hind III sites of the plasmid pUC18. Similarly, the mutated EcPFK genes were excised from M 13mpl9 and inserted into the Hind HI site o f pUC18. The recombinant pUC18 plasmids were then transformed into competent cells o f a PFK-deficient E. coli strain (DF 1020) for expression. The transformed DF 1020 cells were grown to stationary phase in Luria Broth containing ampicillin (50 |ig/ml) and the cells were pelleted, resuspended in 50 mM Tris-Cl, pH 7.4, 1 mM DTT, 1 mM EDTA, and 1 mM phenylmethylsulfonyl fluoride (Buffer A), and sonicated. The procedures used to purify BsPFK and EcPFK were somewhat different. BsPFK and its mutants were purified from the crude extract in a two-step procedure that involved heat-treatment at 70°C followed by affinity chromatography on a Cibacron Blue 3GA agarose column (Valdez et al., 1989). The wild-type and two mutant BsPFK enzymes were eluted from the column with a 0.25 to 1.5 M NaCl gradient; all three eluted at approximately 1.1 M NaCl. Like wild-type BsPFK, the EQ161 and EA161 mutant enzymes were both stable when incubated at 70°C during the heat step. Heat-treatment was not used in purifying the wild-type and mutant EcPFK enzymes since they are not thermostable. Rather, the crude extract obtained after sonication was loaded directly onto a Cibacron Blue 3GA agarose column and purified essentially as described (Hellinga & Evans, 1987). The column was washed with 1 M NaCl, then with Buffer A, and the native or modified EcPFK was eluted with 2 mM ATP/ 10 mM MgCl2 in Buffer A. For all enzymes, column fractions having the highest PFK activity were pooled. Each enzyme preparation was shown to be pure by the presence o f a single band on a 12% SDS-polyacrylamide gel stained with Coomassie Brilliant Blue. Purified wild-type or mutant BsPFK enzymes were concentrated by dialysis against 50% glycerol in Buffer A at 4°C. Purified wild-type or mutant EcPFK 116 enzymes, on the other hand, were concentrated by either (1) dialysis at 4°C against 50% glycerol in Buffer A containing 2 mM ATP/10 mM MgCl2 , or (2) ultrafiltration in an Amicon cell (Beverly, MA). The enzymes concentrated in 50% glycerol were stored at -20°C, and the enzymes concentrated by ultrafiltration were precipitated with 55% ammonium sulfate, then stored at 4°C. Activity Assays-PFK activity was measured at 30°C using an assay system (Kotlarz & Buc, 1982) that coupled the production o f fructose 1,6-bisphosphate to the oxidation o f NADH. The assay solution contained 100 mM Tris-Cl, pH 8 .2 ,10 mM MgCl2 , 5 mM NH4 CI, 0.20 mM NADH, and the coupling enzymes aldolase (20 pg/ml), triosephosphate isomerase ( 1 0 jig/ml) and a-glycerophosphate dehydrogenase ( 1 0 pg/ml). An ATP-regenerating system that utilizes creatine phosphate (1 mM) and creatine phosphokinase (10 pg/ml) was used in all EcPFK-catalyzed reactions to prevent activation by ADP produced in the reaction. When studying GDP-activation, the regenerating system was not used. It was found to be unnecessary for BsPFK and its mutants since they are not activated to any significant degree by GDP except in the presence o f PEP. For each assay, the change in absorbance at 340 nm was measured for at least 1-minute using an Hitachi UV-2000 spectrophotometer. Reactions were initiated by addition o f PFK. One initial velocity unit is defined as the number of pmoles o f fructose 1,6-bisphosphate formed per minute; there are 12.4 initial velocity units per AA3 4 o/min. unit. In order to obtain kinetic parameters, initial velocity data were fit to either the Michaelis-Menten equation for hyperbolic kinetics, or the Hill equation for sigmoidal kinetics. All curve-fitting was performed by nonlinear regression analysis using the program INPLOT (GraplxPad, Inc., San Diego, CA). Results Steady-state Kinetic Parameters-Substrate saturation o f the wild-type and mutant enzymes was studied using steady-state kinetics. Fig. 4.3 shows that the two wild-type enzymes have different Fru-6 P saturation curves: whereas the curve for R el at iv e A ct iv it y (v /V 1.2 2 1-0(Q E 0.8 0.6 0.4 0.2 0.0 L 0.0 M l 0.2 0.4 0.6 0.8 1.0 1.2 [ F r u 6 P ] (mM) FIGURE 4.3. Fructose 6 -phosphate Saturation Curves for B. stearothermophilus PFK (O) and E. coli PFK ( • ) . The MgATP concentration was saturating at 1.0 mM. 118 EcPFK is highly sigmoidal (Hill number 5.6) in the presence o f saturating MgATP concentration, the curve for BsPFK is essentially hyperbolic (Valdez e t a i , 1989), though not perfectly so (Bymes et al., 1994). Table 4.2 presents the kinetic parameters for the five mutant PFKs as well as the two wild-type enzymes. The kcat, KmATP, and KmFru"6P values for the two BsPFK mutants (EQ161 and EQ161) are similar to the values for wild-type BsPFK. The three EcPFK mutants (QE161, QR161, and QA161) have kinetic parameters similar to those for wild-type EcPFK, but show some variation in the sensitivity and cooperativity o f their responses to Fru-6 P (Table 4.2). These differences between the mutants and the wild-type enzyme are not great, however. Thus, alteration o f residue 161 does not significantly affect the response o f either BsPFK or EcPFK to substrates Fru-6 P and MgATP, indicating that this residue is not involved in substrate binding and catalysis. PEP Inhibition-Fig. 4.4 shows that both EcPFK and BsPFK are subject to inhibition by PEP. However, the sensitivity and cooperativity o f their responses differ. EcPFK is both less sensitive (Ii/2 PEP is 1.17 mM) and more cooperative (n is 4.3) in its response to increasing PEP concentration than is BsPFK (Ii/2 PEP is 0.28 and n is 1.6). Table 4.3, which presents the values for Ii/2 PEP and n for the five mutant enzymes as well as the wild-type ones, shows that mutation o f Glu 161 in BsPFK to either Gin or Ala decreases the sensitivity and increases the cooperativity o f the response to PEP. These changes, though significant, are not drastic. They suggest that residue 161 is involved in PEP inhibition o f BsPFK, but neither its hydrogen-bonding ability nor its size is critical. Mutation of Gin 161 in EcPFK to either Glu or Arg decreases the sensitivity and also decreases the cooperativity o f the response. It is interesting that mutants EQ161 o f BsPFK and QE161 o f EcPFK, in which residue 161 o f one enzyme has been replaced with that o f the other, have similar responses to PEP: both are less sensitive to PEP than their respective wild-type enzymes, and both display similar cooperative responses to PEP that are in-between those o f the two wild-type enzymes, TABLE 4.2 Steady-state Kinetic Parameters fo r Wild-type and Mutant PFKs Enzyme kcat (S' 1 ) 0 KmATP (jiM) KmFru'6P OiM)* S i/2 Fru'6 P(inM)c n WT BsPFK 206 70 ± 3 36 ± 2 N.A. h BsEQ161 208 1 1 2 ± 8 35 ± 3 N. A. h Bs EA161 168 123 ± 6 41 ± 3 N. A. h WT EcPFK 127 103 ± 2 N. A. 0.48 ±0.01 5.8 ± 0 .2 Ec QE161 126 77 ± 4 N. A. 1.87 ± 0 .0 4 5.6 ± 0.2 Ec QR161 133 47 ± 4 N. A. 0.91 ± 0 .03 5.0 ± 0.4 Ec QA161 134 55 ± 3 N. A. 0.92 ± 0.02 4.4 ± 0 .3 The kinetic parameters were obtained by fitting substrate saturation data to either the Michaelis-Menten equation or the Hill equation, kcat, the catalytic rate constant; KmATP, the ATP concentration at half-maximal velocity; KmFru"6p and S i/2 Fru'6p, the Fru6 P concentration at half-maximal velocity for Fm-6 P saturation data fit to the Michaelis-Menten and Hill equations, respectively; n, the Hill coefficient; N.A., not applicable; h, hyperbolic. KmATP was obtained in the presence o f saturating Fru-6 P concentration, and KmFru-6 P was obtained in the presence of saturating MgATP concentration, 'H'he kcat-values for BsPFK and its mutants appear to be higher than the values for EcPFK and its mutants, but this is a function o f their different Fru-6 P saturation profiles, i.e., hyperbolic versus sigmoidal. In the presence of GDP, their k^t values are similar. ^KmFru'6P is appropriate only for BsPFK and its mutants, which follow Michaelis-Menten Fru-6 P saturation kinetics. cSi/2 Fru'6p is appropriate only for EcPFK and its mutants. Pe rc en t A ct iv ity R em ai ni ng (% ) 100 50 0 0 2 4 6 [PEP] (mM) FIGURE 4.4. PEP Inhibition of B. stearothermophilus PFK (O ) and E. coli PFK ( • ) . The Fru-6 P concentration was 0.3 mM for BsPFK, and 1.5 mM for EcPFK. The MgATP concentration was 1.0 mM for both. 121 TABLE 4.3 PEP Inhibition o f Wild-type and Mutant PFKs Enzyme [Fru-6 P] (mM)fl I l/2 PEP (mM) n WT BsPFK 0.3 0.28 ± 0 . 0 1 1 . 6 ± 0 . 1 BsEQ161 0.3 3.9 ± 0 .1 3.0 ± 0 .2 Bs EA161 0.3 0.70 ± 0 .01 2 . 6 ± 0 . 1 WT EcPFK 1.5 1.17 ±0 .01 4.3 ± 0.2 Ec QE161 5.0 6.3 ± 0 .6 2.4 ± 0.5 Ec QR161 2.5 9.8 ± 0 .2 3.7 ± 0 .2 Ec QA161 5.0 NO INHIBITION Bs, B. stearothermophilus; Ec, E. coli;WT, wild-type; Ii/2 PEP, the PEP concentration at half-maximal (50%) inhibition; n, the Hill coefficient. The values for Il/2 PEP and n were obtained by fitting data from Activity versus PEP Concentration plots to the Hill equation. aThe Fru-6 P concentration used was a saturating amount chosen by inspection o f individual Activity versus Fru-6 P Concentration plots. The MgATP concentration was 1 mM for all assays. 122 i.e., the Hill coefficients are between 1.6 and 4.3. Most importantly, Table 4.3 shows that mutation o f Gin 161 to Ala completely abolishes PEP inhibition o f EcPFK. This indicates that the hydrogen-bonding ability o f residue 161 is required for the inhibition. The fact that the hydrogen-binding ability o f residue 161 is crucial for PEP inhibition o f EcPFK but not BsPFK suggests that the mechanisms by which PEP inhibits the two enzymes are somewhat different. GDP Activation o f EcPFK and Its Mutants-GDP activation o f EcPFK and its mutants was studied in two different ways: (1) by examining the ability o f GDP to "super-activate" the enzymes, i.e., by comparing Fru-6 P saturation curves in the absence and presence o f 2 mM GDP, and (2) by looking at GDP activation o f the ATP-inhibited enzymes, i.e., by plotting activity versus GDP concentration for the enzymes in the presence of saturating MgATP, and a Fru-6 P concentration equal to S i/2 Fru'6P. Fig. 4.5 and its inset show GDP activation o f EcPFK analyzed in these two ways. From plots such as those in Fig. 4.5, one can obtain the extent to which GDP increases the kcat beyond its value in the absence of GDP, i.e., the extent to which it super-activates. For wild-type EcPFK, the super-activation is 11% . On the other hand, from activation profiles such as the one in the inset o f Fig. 4.5,which is hyperbolic, one can obtain the GDP concentration at half-maximal activation, KactGDP. For wild-type EcPFK, the value is 13 ± 4 |iM. Table 4.4 presents the results of GDP activation studies of the three mutant enzymes QE161, QR161, and QA161, as well as wild-type EcPFK, analyzed in the two ways described above. All three mutant enzymes were super-activated by GDP to an extent similar to wild-type EcPFK, but their sensitivities to GDP activation, as measured by KaclGDP, were lower. Activation o f BsPFK and Its Mutants by GDP and Fru-6P-Unlike EcPFK, BsPFK is not subject to GDP activation to any significant extent unless the enzyme is already inhibited by PEP. Furthermore, GDP does not super-activate BsPFK. Therefore, GDP activation o f BsPFK was analyzed differently. BsPFK and its mutants . 2 0 CD 3 .15tn c 3 .10 MO u o > 40.0503 • pH 4-> • i- ic I-I U M 04m <*q 0 0 L0 0.0 0.5 1.0 1 [F ru -6P ] (mM) FIGURE 4.5. GDP Activation of E. coli PFK. Fru-6 P saturation curves in the absence (■ ) or presence (□ ) of 2 mM GDP, showing super-activation by GDP. The MgATP concentration was 1.0 mM. Inset, % activation versus GDP concentration for EcPFK inhibited by MgATP. The Fru-6 P concentration was equal to S i/2 Fru'6p (0.5 mM), and the MgATP concentration was saturating at 1.0 mM. 123 124 TABLE 4.4 Activation o f EcPFK and Its Mutants by GDP Enzyme kcatsuper (s -l) % Activation KactGDP OiM) WT EcPFK 140 (127) 1 1 1 3 ± 4 Ec QE161 89 (70) 28 51 ± 1 2 Ec QR161 147 (133) 1 0 84 ± 2 3 Ec QA161 157 (134) 17 95 ± 1 2 Ec, E. coli; WT, wild-type; kcalsuPer, the catalytic rate constant for the enzyme super-activated by 2 mM GDP (the number in parentheses is the kcat value for the enzyme in the absence o f GDP.); KactGDP, the concentration o f GDP at half-maximal activation. The values for kcatsuPer and the % Activation were obtained from plots such as the one in Fig. 4.5. KactGDP-values were obtained from plots such as the one in Fig. 4.5, inset. The MgATP concentration was 1.0 mM in all assays. In determining KactGDP, the Fru-6 P concentration was equal to the S i/2 Fru'6 P-value. See the text for further details. 125 were first inhibited by PEP, then the ability o f GDP to activate the PEP-inhibited enzyme was studied. (The PEP concentration was equal to its I i/2 PEP-value). Fig. 4.6A shows the activation profile obtained for wild-type BsPFK; it is hyperbolic and yields a KactGDP of 1 2 2 ± 1 1 jiM. The activation constants obtained from similar curves for the two mutant enzymes EQ161 and EA161 are presented in Table 4.5. Both are four-fold lower than the value for wild-type BsPFK, indicating that EQ161 and EA161 are more sensitive to GDP activation. The ability o f Fru-6 P to activate the PEP-inhibited wild-type and mutant BsPFK enzymes was also investigated. The Fru-6 P saturation profile for wild-type BsPFK in the presence o f PEP at a concentration equal to its Ii/2 PEP value (Fig. 4.6B) is somewhat cooperative (n is 1.6), and gives an S i/2 Fru-6P value o f 0.34 ± 0.01 fiM. As shown in Table 4.5, the EQ161 and EQ161 mutants are as sensitive as the wild-type enzyme is to Fru-6 P, but the cooperativity o f their responses are higher. It is interesting that, for each PEP-inhibited enzyme (wild-type, EQ161, and EA161), the cooperativity o f its response to Fru-6 P matches the cooperativity o f its response to PEP (Table 4.3). This symmetry between PEP inhibition and Fru-6 P activation for BsPFK and its mutants strongly suggests that the two processes occur through a common structural pathway. Discussion Using steady-state kinetics, we have studied two site-specific mutants o f B. stearothermophilus PFK (EQ161 and EA161) and three mutants o f E. coli PFK (QE161, QR161, and QA161) in order to better understand the role o f residue 161 in the allosteric regulation o f the wild-type enzymes. In the T-state structure o f BsPFK, Glu 161 is proposed to form a salt bridge with Arg 243, and to interact with Arg 252 across the dimer-dimer interface through a water molecule (Schirmer & Evans, 1990). The results of the PEP inhibition and GDP activation studies o f BsPFK presented here, however, show that neither the negative charge o f residue 161 nor its hydrogen-bonding ability is critical for allosteric inhibition and allosteric activation o f the enzyme. In view 126 A 50 40 co 30 > o < 20-p c CL) OL_ <D Q_ 0 .1 .2 .3 .4 5 6 [GDP] (mM) FIGURE 4.6. Activation o f PEP-inhibited B. stearothermophilus PFK. The enzyme was inhibited by PEP at a concentration equal to its Ii/2 PEP (0.28 mM). (A) Plot o f % Activation versus GDP concentration, showing the ability o f GDP to activate the PEPinhibited enzyme to 100%. The Fru-6 P concentration was 0.3 mM. (B) Effect of increasing Fru-6 P concentration on the activity o f the PEP-inhibited enzyme. The MgATP concentration was 1.0 mM in both (A) and (B). Initial Velocity (units/ug) o o o in }-* roin o in O' ° W 127 128 TABLE 4.5 Activation o f BsPFK and Its Mutants by GDP and Fru-6P Enzyme KactGDP(liM) S i/2Fru-6P (mM) n WT BsPFK 1 2 2 ± 1 1 0.34 ± 0.01 1.58 ± 0 .05 BsEQ161 23 ± 1 0.28 ± 0 . 0 1 3.0 ±0 .1 BsEA161 29 ± 2 0.28 ± 0 . 0 1 2 . 8 ± 0 . 2 The kinetic parameters in this table describe the activation o f PEP-inhibited BsPFK and its mutants by GDP and Fru-6 P. Bs, B. stearotkermophilus; WT, wild-type; KactGDP> the GDP concentration at half-maximal activation obtained from plots such as the one in Fig. 4.6A; S i/2 Fru"6P, the Fru-6 P concentration at half-maximal velocity obtained from plots such as the one in Fig. 4.6B; n, the Hill coefficient for the Fru-6 P saturation curve. The PEP concentration was equal to its Ii/2 PEP-value. The MgATP concentration was 1.0 mM in all assays. In determining Kact001*, the Fru-6 P concentration was 0.3 mM. 129 of these results, we propose that other interactions such as the one between Arg 72 and Glu 241 (Schirmer & Evans, 1990) are important in stabilizing the T-state o f BsPFK, and interactions among Arg 162, Arg 243, and Fru-6 P are important in stabilizing the In- state. In addition, it is significant that the EQ161 and EA161 mutants o f BsPFK were both less sensitive to PEP inhibition and more sensitive to GDP activation. These results indicate that in BsPFK the processes o f inhibition and activation are opposed to each other, and proceed through a common structural pathway, i.e., one that involves Glu 161. Our results are thus consistent with the model o f Schirmer & Evans (1990). The kinetic parameters obtained for the mutant enzymes (Table 4.2) are similar to those for their respective wild-type enzymes. Thus, residue 161 is not directly involved in the binding o f substrates, or in catalysis. The residue is also not important for homotropic regulation o f EcPFK, i.e., it plays no important role in the sigmoidal response of the enzyme to Fru-6 P (at saturating MgATP levels). The mechanism by which ATP inhibits EcPFK, causing its Fru-6 P saturation kinetics to be sigmoidal, has been investigated (Zheng & Kemp, 1992), but has not been conclusively determined. The mechanism is proposed to involve substrate antagonism between MgATP and Fru6 P in the active site (Zheng & Kemp, 1992; Deville-Bonne, 1991b; Johnson & Reinhart, 1992). There is evidence in the literature that the structural pathways by which ATP and PEP inhibit EcPFK are different. Several modified EcPFK enzymes have been reported to be insensitive to PEP inhibition but nevertheless have sigmoidal Fru-6 P saturation kinetics (Le Bras et al„ 1982; Serre et al, 1990; Byrnes et ah, manuscript submitted). Our results support the proposal that the mechanisms of ATP inhibition and allosteric PEP inhibition are distinct. Specifically, the cooperativity o f the EcPFK mutant QA161, which is insensitive to PEP inhibition, was not significantly different from the cooperativity o f wild-type EcPFK (Hill numbers 4.4 versus 5.6). This indicates that "allosteric" ATP inhibition is not affected by the mutation; therefore, its mechanism is distinct from PEP inhibition. The two processes, inhibition by ATP and 130 by PEP, most likely occur via different structural pathways altogether. (This is not to say that there is no overlap between them. Indeed, both inhibition processes affect catalysis at the active site. Thus, certain residues at the active site, e.g., Arg 72, Arg 162, and Arg 243, may participate in both). The results above showing that Gin 161 o f EcPFK is not important for ATP inhibition o f the enzyme are in contrast to those obtained for the PFK from rabbit muscle (Li et al„ 1993). Based on the fact that rabbit muscle PFK is about twice the size o f bacterial PFK and has clear Nand C-half internal homology, Poorman et al. (1984) have proposed that the rabbit muscle PFK gene arose from its procaryotic progenitor by gene duplication and divergence. Mutation o f Gin 200 o f rabbit muscle PFK, which corresponds to residue 161 o f bacterial PFK, showed that Q200 plays an important role in Fru-6 P binding and homotropic regulation by ATP, but is less important for heterotropic regulation by the effectors fructose 2 ,6 -bisphosphate and citrate. The effects o f the mutation were pH-dependent. Thus, Q200 in rabbit muscle PFK has evolved to play an important role in pH-dependent regulation o f the enzyme by ATP. In E. coli PFK, on the other hand, the corresponding residue Q161 is important for heterotropic regulation by PEP, but not homotropic regulation by ATP. Regulation by ATP of the larger eucaryotic PFK therefore proceeds via a mechanism that is different from, and undoubtedly more complex than, the mechanism by which ATP regulates the simpler procaryotic enzyme. Our results also suggest that GDP activation o f the ATP-inhibited EcPFK enzyme proceeds through a structural pathway different from the one followed when GDP activates the PEP-inhibited enzyme. Like BsPFK, activation o f EcPFK from its PEP-inhibited state most likely involves residue 161, and follows s mechanism similar to the one proposed by Schirmer and Evans (1990). However, the observation that the EcPFK mutant QA161 is activated by GDP despite its insensitivity to PEP inhibition indicates that activation o f EcPFK from its ATP-inhibited state does not involve residue 131 161, and proceeds through an entirely different pathway. The structural elements involved in this second pathway of activation are unknown. It is interesting that BsPFK does show a small degree o f activation by GDP (20%) in the absence o f PEP when Fru6 P is equal to S i/2 Fru'6P (Byrnes et al., manuscript submitted). This subtle activation of the more "rigid" BsPFK enzyme may be a scaled-down version o f the activation observed in the "floppier" EcPFK molecule inhibited by ATP. Four o f the seven residues along the 6 F loop o f EcPFK, which is comprised o f residues Thr 156 to Arg 162, have been previously mutated: Thr 156 has been changed to Gly/Ser, Ser 159 to Asn, His 160 to Asn (Kundrot & Evans, 1991), and Arg 162 to Ser (Berger & Evans, 1990). The mutation o f Q161 reported in this work now brings the number o f residues mutated to five. Each mutation has had a somewhat different effect on the enzyme. The T(GS)156 and SN159 mutants exhibit severely reduced catalytic rate constants and affinities for Fru-6 P, and linear kinetics (up to 100 mM) with respect to Fru-6 P. They are insensitive to GDP activation, have dissociation constants for PEP (KT(PEP)-values) similar to wild-type, and exhibit hyperbolic responses to PEP. T(GS)156 and SN159 are proposed to be locked in the T-state (Kundort & Evans, 1991). HN160 has a 10-fold lower affinity for Fru-6 P but retains full cooperativity. It has dissociation constants for GDP and PEP (Kr(GDP) and Kt(PEP), respectively) similar to those o f wild-type EcPFK, and gives a hyperbolic response to PEP. The RS162 mutant has a dramatically-reduced catalytic rate constant and affinity for Fru-6 P, has a two-fold lower cooperativity, and is inhibited by GDP even though GDP abolishes cooperativity. On the other hand, our results show that QA161 has a kcat and response to Fru-6 P similar to wild-type, is about 7-fold less sensitive to GDP activation, and is completely insensitive to PEP inhibition. It is difficult to correlate our results involving QA161 with those o f the other 6 F loop mutants, partly because of the different ways PEP inhibition and GDP activation were analyzed. Certainly, the major effect o f the Q161- >A mutation is to abolish PEP 132 inhibition. The structural basis for this could be the loss o f one or more hydrogen bonds in the vicinity of the active site and along the dimer-dimer interface in the T-state structure of EcPFK. In conclusion, our results show that the hydrogen-bonding ability o f residue 161 is critical for PEP inhibition o f EcPFK. The identity o f residue 161 is less important for PEP inhibition o f BsPFK, and other residues are apparendy important for stabilizing its T-state structure. The fact that GDP can activate the Ec QA161 mutant from its ATPinhibited state, despite the inability of the enzyme to be inhibited by PEP, suggests that a pathway o f activation that does not involve residue 161 exists in EcPFK. This second pathway o f GDP activation may be the one associated with super-activation. In contrast, the results showing that mutation o f residue 161 in BsPFK affects GDP activation as well as PEP inhibition suggests that the same structural elements are utilized in both processes, as suggested by Schirmer & Evans (1990). Finally, it appears that BsPFK, which has evolved to function at temperatures between 65° and 80°C, possesses a mechanism o f heterotropic regulation that involves minimal conformational change. Allosteric inhibition and activation utilize the same pathway. On the other hand, EcPFK, which has evolved under less stringent conditions, has a "floppier" structure, and more varied mechanisms o f heterotropic regulation. GDP activation and PEP inhibition follow divergent pathways. Chapter 5 Conclusions and Future Studies 133 134 The structural basis for kinetic and allosteric differences between the PFKs from Bacillus stearothermophilus and Escherichia coli has been investigated, and partially uncovered, in the studies presented in this dissertation. An analysis o f the results in the context o f the existing literature suggests that many o f the differences between the two PFKs can be attributed to differences in their conformational flexibility. E. coli PFK has a more "floppy" structure, while B. stearothermophilus PFK is more "rigid." The reasons for this are undoubtedly evolutionary. The bacteria from which the enzymes are isolated have evolved to live in very different ecological niches: thermophilic B. stearothermophilus in hot springs where temperatures are around 70°C, and mesophilic E. coli in the mammalian gut, where temperature is constant at 37°C. One would expect that the enzyme from a thermophilic organism would be much more conformationally conservative in its response to regulatory effectors than one from a mesophilic organism. Why?- because large conformational changes would be more apt to destabilize it. Indeed, BsPFK is conformationally conservative. The allosteric transition involves little movement: a rotation o f two rigid dimers by 7° relative to each other, a slight opening o f the dimer-dimer interface, and the concerted movement o f two loops positioned between the effector and active sites (Schirmer & Evans, 1990). On the other hand, EcPFK is more conformationally flexible than BsPFK despite the fact that the two enzymes are structurally nearly identical. Support for this conclusion comes from two lines o f evidence. First, EcPFK is allosterically inhibited by ATP, and the inhibition appears to be associated with closure o f the active site. The subunit o f the chimeric PFK (ChiPFK), which contains the rigid large domain of BsPFK grafted onto the remainder o f the EcPFK subunit (chapter 3), is locked in an open conformation. Correspondingly, ATP does not inhibit ChiPFK in the same manner it inhibits EcPFK: the inhibition is much less severe, and is more like that o f BsPFK. Thus, in EcPFK, there appears to be an association between closure of the active site- presumably via the open-to-closed transition discovered by Shirakihara & Evans 135 (1988)- and allosteric ATP inhibition. This association is absent in BsPFK, which apparently does not undergo the open-to-closed transition. ATP inhibition o f BsPFK occurs by a different mechanism. The inhibition is non-aliosteric, is much less pronounced, and arises out of the enzyme's steady-state alternative pathways kinetic mechanism (chapter 2). Thus, a conformational change that occurs in EcPFK apparently does not occur in BsPFK. A second line o f evidence indicating that EcPFK is more conformationally flexible comes from the fact that GDP can activate the enzyme through more than one structural pathway. GDP activates BsPFK through only one pathway, the one proposed by Schirmer & Evans (1990). Both PEP inhibition and GDP activation are affected by mutation o f residue 161 o f BsPFK (chapter 4), indicating that the structural pathways for the two processes overlap. In contrast, there appear to be two pathways by which GDP can activate EcPFK: one similar to that o f BsPFK which overlaps with the pathway o f PEP inhibition, plus a second one. Evidence for the existence o f a second pathway comes from studies o f the QA161 mutant (chapter 4). QA161 is activated normally by GDP, but is completely insensitive to PEP inhibition. This suggests that PEP inhibition and GDP activation o f the ATP-inhibited enzyme proceed through different structural routes. The presence in EcPFK of a pathway of GDP activation that is absent in BsPFK highlights the greater conformational flexibility o f EcPFK compared to BsPFK. Thus, EcPFK can be thought o f as being more "conformationally liberal." In conclusion, it appears that there are structural pathways o f inhibition (by ATP) and activation (by GDP) present in EcPFK that are absent in BsPFK. These processes may involve large conformational changes that cause cracking o f crystals grown for the purposes o f X-ray crystallography. Indeed, cracking appears to have been a problem for Evans and co-workers, who have been unable to obtain a "T" state structure o f EcPFK (Evans, 1992). 136 B. stearothermophilus PFK and E. coli PFK, once thought to be functionally indistinguishable because o f their similar structures, are now known to be quite different in their mechanisms o f regulation. The challenge at this point is to use structurefunction studies to further uncover the structural basis for these differences. In order to do this, attention will need to be focused in particular on EcPFK, on the mechanisms by which ATP allosterically inhibits the enzyme and GDP activates the ATP-inhibited enzyme. Studies o f site-specific mutants as well as o f chimeric Bs/Ec PFK mutants will most likely play an important role in these future efforts. In the following discussion, a collection o f possible future experiments is presented. Some o f these involve only BsPFK, some only EcPFK, and some both (chimeric PFKs). They are presented in order o f increasing time investment needed. Hopefully, they will stimulate future research on bacterial PFK. 1. Perform a pH study o f ChiPFK in an effort to determine the mechanism by which AMPPNP abolishes ChiPFK cooperativity (chapter 3). What effect does AMPPNP have on the sigmoidality o f the Fru-6 P saturation curve o f ChiPFK at different pHs, i.e., at pH 7.2 to pH 8.9? How does the kcat o f ChiPFK vary with pH? Does AMPPCP have an effect similar to that o f AMPPNP? 2. Could a high rate o f ATP hydrolysis be present in the active site o f ChiPFK because o f its openness? Is inorganic phosphate (Pi) produced by ATP hydrolysis, i.e., the reaction ATP > ADP + P^ responsible for the quenching observed when ChiFPK is titrated with ATP? Note that both Pi and Fru-6 P bind Arg 162 and Arg 243 (Evans eta l., 1986), and Fru-6 P binding to EcPFK causes quenching o f fluorescence. Could Pi also be causing quenching o f fluorescence by specifically binding in the Fru6 P site after being produced by ATP hydrolysis? This question could be answered by studying the rate o f ATP hydrolysis in ChiPFK, and correlating the results with those from fluorescence titrations using Pj. 137 3. Does PEP promote dissociation o f the BsPFK tetramer into dimers, as it does the thermostable PFK from Thermus thermophilus (Xu et al., 1990)? Note: as in B. stearothermophilus PFK, there is a glutamate at position 161 in T. thermophilus PFK. Is the QE161 mutant o f EcPFK less stable along its active dimer-dimer interface than wild-type? These questions could be answered by performing sucrose gradient sedimentation or gel permeation HPLC (Xu et al., 1990) studies o f BsPFK and QE161 in the absence and presence o f PEP. The resulting fractions could be analyzed for the presence o f tetramer or dimer using SDS-PAGE and silver staining. 4. Glutamate 241 has not been mutated in either EcPFK or BsPFK. Yet, this residue is very important for stabilizing the low-activity (T-state) forms o f the enzymes (see Fig. 1.9). But mutating Glu 241, it may be possible to obtain an altered enzyme that is locked in the "R" state, and is insensitive to PEP inhibition. Moreover, it would be very significant if the results showed that PEP inhibition and ATP inhibition were affected differently. The following mutations would be appropriate: Glu 241 to Ala, to Asp, to Arg, and to Gin. 5. The fluorescence o f the unique tryptophan o f BsPFK, Tip 179, is not sensitive to ligand binding while that o f Trp 311 o f EcPFK is. It would be useful to construct a double mutant o f BsPFK, WA179/YW 311, in order to remove the Trp from position 179 and place it at position 311, which is hopefully the right place for observing fluorescence changes due to conformational changes in the enzyme molecule. Note that the residue at position 179 in EcPFK is an alanine. The single mutants WA179 and YW 311 would also have to be made in order to determine the effect that each individual mutation has on BsPFK kinetics and regulation. 6 . Valdez et al. (1988) constructed a mutant BsPFK enzyme, RA25, that exhibited sigmoidal Fru-6 P saturation kinetics (n was 2.0) in the absence o f PEP. This result is very interesting since the wild-type enzyme is cooperative only in the presence of PEP. Arg 25 is proposed to bind the phosphate o f PEP and the P-phosphate o f ADP 138 in the effector site. It is located along the regulatory interface. Residue 25 should be further investigated by site-directed mutagenesis. By changing Arg 25 to other residues besides alanine, it may be possible to increase the Hill number beyond 2.0. The following mutations should be attempted: Arg 25- >Gln, - >Glu, and - >Lys. 7. Construct additional chimeric Bs/Ec PFK enzymes. By constructing a series of chimeras, it may be possible to define those regions o f EcPFK responsible for its sigmoidal Fru-6 P saturation kinetics. This could be done in a systematic way: one could move from the N-terminus to the C-terminus, grafting progressively longer Nterminal BsPFK segments onto the remainder o f the EcPFK subunit. Likewise, progressively longer N-terminal EcPFK segments could be grafted onto the BsPFK subunit Junctions between BsPFK and EcPFK portions o f the chimeric subunits could be carefully chosen to fall within loops between cc-helices and (3-strands so that the structure o f the enzyme is not disrupted. By examining Fru-6 P saturation profiles o f the resulting chimeric enzymes, those regions responsible for the sigmoidal Fru-6 P saturation kinetics could be identified. 8 . The regions responsible for sigmoidality in the Fru-6 P saturation curve of EcPFK may have already been fortuitously discovered; they may be the two regions that move during the open-to-closed transition. These two regions are: (i) a portion o f the large domain (residues 41 to 122; Shirakihara & Evans, 1988), and (ii) the 40-amino acid stretch between residues 280 and 319 near the C-terminus (Serre et al., 1990; Shirakihara & Evans, 1988). With this in mind, two chimeric PFKs can be constructed: (a) one in which the 40 amino acids near the C-terminus are replaced with the corresponding residues from BsPFK. Does the replacement abolish heterotropic regulation? Is homotropic cooperativity (n=2) retained? (b) a chimeric Bs/Ec PFK having residues 41 to 122 and 280 to 319 from EcPFK, and the rest from BsPFK. The goal here is to create a mutant BsPFK enzyme with the ability to undergo an open-to139 closed transition. Possibly it will exhibit homotropic cooperativity, i.e., a sigmoidal Fru-6 P saturation curve. 9. The JK loop o f E. coli PFK moves between the unliganded and active siteliganded forms o f the enzyme (Rypniewski & Evans, 1989). Upon binding ligands, the JK loop shifts toward the catalytic loop and away from its symmetry-related equivalent across the molecular p axis (see Fig. 1.3). Hydrogen bonds are formed between residues in the JK loop and the catalytic loop during this process. On the other hand, these same hydrogen bonds are broken, and others are formed, when ligands are released from the active site. Movement o f the JK loop may be associated with the cooperativity o f the enzyme. The hydrogen bonds between residues in the JK and catalytic loops that are formed and broken are between Asn 288 and Asp 134, Glu 286 and Asp 134, and Glu 286 and Thr 133. 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A s on e of the chapters o f m y dissertation, I w ou ld like to inclu de the m anuscript o f a paper published in your journal: Byrnes, M., Xhu, X., Y ounathan, E. S., and C hang, S. H . (1994) Biochemistry 3 3 ,3424-3431. M ay I have a letter from you stating that I have the journal's perm ission to include the m anuscript in m y dissertation? C ould y o u FAX the letter to the num ber listed in the heading? If there is a problem w ith m y request, p lease let m e know . Thank you. Sincerely, 6 0 ( .- , W. M alcolm Byrnes Dept, o f Biochem istry LSU Boron Ho a g e • ( o i u n o n o • 7 0 8 0 3 1 8 0 6 • Mi r • f A H S 0 4 3 8 8 S 3 J 1 SENT BYiXerox Telecopier 7020 I 929-94 I 15153 2020726060-1 504 388 53211# 2 American Chemical Society PUBLICATIONS DIVISION COPYRIGHT OFFICE DATE: September 29, 1994 MEMORANDUM 1155 SIXTEENTH STREET, N.W. WASHINGTON, D.C. 20038 Phone (202) 872-4367 or -4388 Fax (202) 872-6060 TO: W. Malcolm Byrnes . Department o t Biochemistry Louisiana State University FROM: C. Arisen Courtney ' RE: c u c n n I r -Acopyright Assistant C ^ 4 ^ Your letter dated September 29. 1994 Thank you for your recent letter, regarding your request for permission to include your paper(s), per your attached letter, in your thesis. Please note the following: * If your paper has already been published by ACS, I would be happy to grant you this permission royalty free provided that you print the required ACS copyright! credit line on the first page of your article: "Reprinted with permission from FULL REFERENCE CITATION. Copyright: YEAR American Chemical Society." j * If you plan to submit your thesis to UMI, please inform them that permission to include your already published ACS article as part of your thesis is granted for paper and microform copies only; the ACS copyright notice (see above) must appear on the first page of the ACS article. i * If your paper has not already been published by ACS, you may include it in your thesis provided that you pr^nt the following ACS copyright credit line on the first page of your article: "Reprinted with permission from JOURNAL NAME, in press. Unpublished work copyright CURRENT YEAR American Chemical Society." You may NOT include tlie ACS paper in the version that you submit to UMI until ACS has published your paper; permission for UMI to use thej ACS article as part of your thesis is then limited to microform and paper copies only. * other: ______________ Thank you for writing. If you have any questions, please call me at 2 0 2 / 8 7 2 -4 3 6 B . Vita Walton Malcolm Bymes was born on December 7, 1959 in Covington, Louisiana to Mrs. Christine FitzSimons Bymes and Mr. Vincent Malcolm Bymes. He attended college at Xavier University of Louisiana in New Orleans, receiving a B.S. in Chemistry with distinction in English (overall g.p.a. 4.0/4.0) in May, 1981. While at Xavier, Malcolm received several awards, including the ACS Analytical Award (1980), the ACS Award (1981), and the Mother Agatha M. Ryan Award (1981), Xavier's second-highest for overall achievement. After teaching general, analytical, and organic chemistry laboratories at his alma mater for a year, Malcolm entered a graduate program in chemistry and biochemistry at the University o f Illinois in Champaign-Urbana. However, he left after two years (in May, 1984) to teach chemistry at St. John's College in Belize City, Belize, Central America. After two years at SJC, having successfully prepared his students for their Cambridge O-level and A-level exams, Malcolm left Belize for Washington, D. C., where he taught chemistry and physics at Gonzaga College High School. Finally, in August, 1988, Malcolm entered the graduate program in the Biochemistry Department at Louisiana State University in order to complete his graduate studies. While at LSU, he has worked in the laboratory of Dr. Simon H. Chang, investigating the structural basis for differences between the phosphofructokinases from two bacteria, B. stearothermophilus and E. coli. He was a teaching assistant (3 yrs.) for the senior-level undergraduate biochemistry laboratory course, and was active as a member of the Biochemistry Graduate Student Council (3 yrs.), which organized the first and second annual Biochemistry Research Minisymposia. During his tenure at LSU, Malcolm has been a part of six publications: (1) Bym es, M., Zhu, X., Younathan, E. S., & Chang, S. H. (1994) "Kinetic Characteristics 148 149 of the Phosphofructokinase from Bacillus stearothermophilus: MgATP Nonallosterically Inhibits the Enzyme" Biochemistry 33, 3424-3431. (2) Bvmes. M., Hu, W., Younathan, E. S., & Chang, S. H. (1994) "A Chimeric Bacterial Phosphofructokinase Exhibits Cooperativity in the Absence of Heterotropic Regulation" J. Biol. Chem. (in press). (3) Byrnes. M., Auzat, I., Garel, J.-R., & Chang, S. H. "The Role o f Residue 161 in the Allosteric Transitions of Two Bacterial Phosphofructokinases" (in preparation). (4) Zhu, X., Bvmes. M., & Chang, S. H. "The Role o f Glycine 212 in the Allosteric Behavior of Phosphofructokinase from Bacillus stearothermophilus" (submitted to Biochemistry). (5) Li, J., Zhu, X., Bvmes. M,, Nelson, J. W., & Chang, S. H. (1993) "Site-directed Mutagenesis o f Rabbit Muscle Phosphofructokinase cDNA: Mutations at Glutamine 200 Affect the Allosteric Properties of the Enzyme" J. Biol. Chem. 268, 24599-24606. (6 ) Li, J., Chen, Z., Lu, L., Bymes. M., & Chang, S. H. (1990) "Sequence Diversity in the 5'-untranslated Region of Rabbit Muscle Phosphofructokinase mRNA" Biochem. Biophys. Res. Comm. 170, 1056-1060. Malcolm will be leaving LSU in October, 1994 to accept a position as post- doctoral associate in the Deptartment o f Pharmacology (College o f Veterinary Medicine) at Cornell University in Ithaca, New York, in the laboratory o f Dr. Robert Oswald. He and Dr. Oswald have written and submitted an application for an NIH individual research training fellowship. DOCTORAL EXAMINATION AND DISSERTATION REPORT Candidate: Walton Malcolm Byrnes Major Field: Biochemistry Title of Dissertation: ^he Structural Basis for Kinetic and Allosteric Differences between Two Bacterial Phosphofructokinases Approved: - " 7 / ~/ - ( , L c - <-.....r- ,0-Major Professor and Chairman / E X A M I N I N G C O M M I T T E E ; Y / / / » ! •/ A- ''Sr y xf. s£in/fl Date of Examination: October 11,