Conversion of allosteric inhibition to activation in phosphofructokinase by protein engineering

Structure of the apo form of Bacillus stearothermophilus phosphofructokinase

The crystal structure of the unliganded form of Bacillus stearothermophilus phosphofructokinase (BsPFK) was determined using molecular replacement to 2.8 Å resolution (Protein Data Bank entry 3U39 ). The apo BsPFK structure serves as the basis for the interpretation of any structural changes seen in the binary or ternary complexes. When the apo BsPFK structure is compared with the previously published liganded structures of BsPFK, the structural impact that the binding of the ligands produces is revealed. This comparison shows that the apo form of BsPFK resembles the substrate-bound form of BsPFK, a finding that differs from previous predictions.

DNA topoisomerase II and its growing repertoire of biological functions

DNA topoisomerases are enzymes that disentangle the topological problems that arise in double-stranded DNA. Many of these can be solved by the generation of either single or double strand breaks. However, where there is a clear requirement to alter DNA topology by introducing transient double strand breaks, only DNA topoisomerase II (TOP2) can carry out this reaction. Extensive biochemical and structural studies have provided detailed models of how TOP2 alters DNA structure, and recent molecular studies have greatly expanded knowledge of the biological contexts in which TOP2 functions, such as DNA replication, transcription and chromosome segregation -- processes that are essential for preventing tumorigenesis.

Recent developments in enzyme and microbial biotechnology--strategies in bioprocess design

Microorganisms have been used traditionally by industry as sources of natural products, or as sources of enzymes capable of mediating specific chemical transformations. This situation has changed radically in recent years, a time during which we have seen a dramatic increase in the number and range of potential biotechnological applications of enzymes and their genetically-engineered variants. An increasing number of enzymes, receptors and other proteins have now been structurally characterized, and their genes isolated as a basis for producing recombinant proteins for genetic analysis of their structure and function. These innovations have necessitated development of associated technologies for large-scale production of proteins in bioreactors, appropriate strategies for quality control, and new analytical tools for structural characterization of recombinant gene products. Some recombinant proteins are already in an advanced stage of development for use either as new-generation therapeutics, as target molecules for "intelligent" drug screening, or as biological components of biosensors. As the predictive power of protein model building improves, the diversity of applications of such technology will increase further as it becomes feasible to generate totally synthetic proteins with specifically-tailored properties.

Inversion of the allosteric response of Escherichia coli glucosamine-6-P deaminase to N-acetylglucosamine 6-P, by single amino acid replacements

Amino acid replacements in the active site of glucosamine-6-P deaminase from Escherichia coli (GlcN6P deaminase, EC 3.5.99.6) involving the residues D141 and E148 produce atypical allosteric kinetics. These residues are located in the chain segment 139-156 which is part of the active site and which also forms several intersubunit contacts close to the allosteric site. In the D141N and E148Q mutant forms of this deaminase, there is an inversion of the effect of its physiological allosteric effector, N-acetylglucosamine 6-P, which becomes an inhibitor at substrate concentrations above a critical value. For both mutants, this particular point appears at low substrate concentration and the inhibition by the allosteric activator is the dominant effect in velocity versus substrate curves. These effects are analyzed as a particular case of the concerted allosteric model, assuming that the R state, the conformer displaying the higher affinity for the substrate, is the less catalytic state, thus producing an inverted allosteric response.

Quantification of allosteric influence of Escherichia coli phosphofructokinase by frequency domain fluorescence

The allosteric properties of the wild-type Escherichia coli phosphofructokinase were compared to the E187A mutant by using frequency-domain techniques. Tryptophan-shifted mutants comprising of double (W311Y/Y55W and W/311F/F188W) and triple (W311Y/Y55W/E187A and W311F/F188W/E187A) amino acid residue changes, which allowed for better fluorescence probing at targeted sites, were also compared to the wild-type and E187A. The additive nature of multiple mutations allowed one to partition the net effect of modifying residue 187. In general, the mutant enzymes displayed greater heterogeneity in sub-state population than did the wild-type enzyme. The semi-cone angle model was used to quantify the extent of depolarization of the fluorophore. Use of the model presupposes that the extent of depolarization directly correlates with the degree of flexibility of the fluorophore. A relationship has been established between the values determined from the semi-cone angle calculations and the thermodynamic components responsible for the allosteric linkage between the regulatory and substrate binding. Coupling interactions giving rise to positive entropy components are manifested by increasing flexibility of the ternary complexes rather than the binary complexes.

Pre-steady state quantification of the allosteric influence of Escherichia coli phosphofructokinase

Stopped-flow kinetics was utilized to determine how allosteric activators and inhibitors of wild-type Escherichia coli phosphofructokinase influenced the kinetic rate and equilibrium constants of the binding of substrate fructose 6-phosphate. Monitoring pre-steady state fluorescence intensity emission changes upon an addition of a ligand to the enzyme was possible by a unique tryptophan per subunit of the enzyme. Binding of fructose 6-phosphate to the enzyme displayed a two-step process, with a fast complex formation step followed by a relatively slower isomerization step. Systematic addition of fructose 6-phosphate to phosphofructokinase in the absence and presence of several fixed concentrations of phosphoenolpyruvate indicated that the inhibitor binds to the enzyme concurrently with the substrate, forming a ternary complex and inducing a conformational change, rather than a displacement of the equilibrium as predicted by the classical two-state model (Monod, J., Wyman, J., and Changeux, J. P. (1965) J. Mol. Biol. 12, 88-118). The activator, MgADP, also altered the affinity of fructose 6-phosphate to the enzyme by forming a ternary complex. Furthermore, both phosphoenolpyruvate and MgADP act by influencing the fast complex formation step while leaving the slower enzyme isomerization step essentially unchanged.

Chemical modification of SH groups of E. coli phosphofructokinase-2 induces subunit dissociation: monomers are inactive but preserve ligand binding properties

Modification of Escherichia coli phosphofructokinase-2 (Pfk-2) with N-(1-pyrenil)maleimide results in an enzyme form that is inactive. However, the rate of modification is drastically reduced in the presence of the allosteric effector MgATP. The stoichiometry of the label incorporation was found to be 2.03 +/- 0.035 mol of the reagent/mol of subunit, in agreement with the number of titratable SH groups by 5,5'-dithiobis(2-nitrobenzoic acid) in the labeled protein. HPLC gel filtration experiments demonstrate that native Pfk-2 is a dimer in the absence of ligands, while in the presence of MgATP a dimer-tetramer transition is promoted. In contrast, the modified enzyme eluted as a monomer and the presence of MgATP was not able to induce aggregation. Although the modified monomers are inactive, the dissociation constants for the substrates and the allosteric effector MgATP, measured by following the fluorescence of the binding probe, are the same as for the native enzyme. Quenching of pyrene fluorescence emission of labeled phosphofructokinase-2 monomers by acrylamide gave downward curved Stern-Volmer plots, with very similar quenching efficiencies for the control and for the fructose-6-P and MgATP-enzyme complexes. These results show the presence of SH groups in the interface of Pfk-2 subunits, critical for subunit interactions, and that conformational changes occurring through the dimers are essential for catalytic activity.

Metabolic fluxes, pools, and enzyme measurements suggest a tighter coupling of energetics and biosynthetic reactions associated with reduced pyruvate kinase flux

In this study, it is found that, for Bacillus subtilis, citrate-glucose cometabolism leads to zero acid production over a wide range of growth rates and nearly theoretical carbon yield. Experimental results are presented that point to pyruvate kinase (PYK) as a site of citrate-mediated glycolytic flux attenuation. First, the measured fluxes show that, compared with cultures grown on glucose, the PYK flux drops by more than tenfold when citrate is added. Second, relative to cultures metabolizing glucose, the phosphoenolpyruvate (PEP) pool elevates substantially, whereas the pyruvate pool drops, when citrate is present. Finally, our modeling results indicate that maximizing carbon yield corresponds to nearly eliminating pyruvate kinase (PYK) flux and that the pyruvate supplied by the PEP-consuming glucose transport system can supply the biosynthetic requirements. A literature review suggests some mechanisms for how PYK attenuation by citrate addition can occur. At this juncture, we hypothesize that direct PYK inhibition occurs which, in turn, also leads to phosphofructokinase inhibition via the elevated PEP pool. These two inhibition events combine to throttle glycolytic flux; minimize acid formation; and substantially increase cellular, product, and energetic yields.

An essential methionine residue involved in substrate binding by phosphofructokinases

An alignment of all PPi-dependent phosphofructokinases and all allosteric ATP-dependent PFKs shows relatively few residues that are fully conserved. One residue that is conserved is a methionine residue that appears from the crystal structure of Escherichia coli PFK to be interacting with fructose 6-P. Very conservative substitutions for this methionine with leucine or isoleucine by site-directed mutagenesis of E. coli ATP-PFK and Entamoeba histolytica PPi-PFK produced profound decreases either in the apparent affinity for fructose 6-P or in maximal velocity, or both. Methionine provides a highly specific interaction with fructose 6-P for binding and for transition state stabilization.

Studies on the enzymatic and transcriptional activity of the dimerization cofactor for hepatocyte nuclear factor 1

The relationship between the enzymatic and the transcriptional activity of the bifunctional protein pterin-4a-carbinolamine dehydratase/dimerization cofactor for hepatocyte nuclear factor 1 (DCoH) has been elucidated by site-directed mutagenesis. DCoH dimers harbor a binding site for hepatocyte nuclear factor 1 (HNF1), two active centers that bind pterins, and a saddle-shaped surface that resembles nucleic acid binding domains. Two domains of the protein have been selectively targeted to determine if a change in one activity affects the other. No strong correlation has been found, supporting the idea that carbinolamine dehydratase activity is not required for HNF1 binding in vitro or transcriptional coactivation in vivo. Double mutations in the active center, however, influence the in vivo transcriptional activity but not HNF1 binding. This finding suggests that some active center residues also are used during transcription, possibly for binding of another (macro)molecule. Several mutations in the saddle led to a surprising increase in transcription, therefore linking this domain to transcriptional regulation as well. The transcriptional function of DCoH therefore is composed of two parts, HNF1 binding and another contributing effect that involves the active site and, indirectly, the saddle.

Contribution of individual side-chains to the stability of BPTI examined by alanine-scanning mutagenesis

Bovine pancreatic trypsin inhibitor (BPTI) serves as an important model system for the examination of almost all aspects of protein structure. Systematic studies of the effects of mutation on the thermodynamic stability of BPTI, however, have been limited by the extreme stability of the protein. A derivative of BPTI containing only the 5-55 disulfide bond, termed [5-55]Ala, has been shown previously to fold into a structure very similar to that of native BPTI and to be a functional trypsin inhibitor. [5-55]Ala undergoes a reversible thermal unfolding transition with a melting temperature of 39 degrees C, and is therefore well suited for stability studies. Using an alanine-scanning mutagenesis approach, we have examined the contribution to stability of each side-chain in the [5-55]Ala derivative of BPTI. These studies demonstrate the importance of the two hydrophobic cores composed largely of clusters of aromatic residues, as well as the internal hydrogen-bonding network, in stabilizing BPTI. Overall, there is a strong relationship between change in buried surface area and stability for both polar and hydrophobic residues, with proportionality constants of 50 and 20 cal/A2, respectively. None of the alanine substitutions substantially stabilized [5-55]Ala. Nonetheless, approximately 60% (28/46) of the alanine mutants were destabilized by less than 10 degrees C, suggesting that a form of BPTI with up to half of its residues being alanine could fold into a stable structure resembling the native one.

Slow ligand-induced transitions in the allosteric phosphofructokinase from Escherichia coli

The fluorescence of the unique tryptophan residue of the allosteric phosphofructokinase from Escherichia coli varies upon binding of any ligand, whether substrate or effector, suggesting that the protein undergoes a conformational change. This fluorescent probe has been exploited to determine the rates of the structural transitions that occur upon ligand binding and that are responsible for the remarkable allosteric behavior of this enzyme. The kinetics of fluorescence changes measured after rapidly mixing phosphofructokinase with one of its ligands show the presence of several allosteric transitions with widely different rates, ranging from a few hundred s-1 to less than 0.1 s-1. The rate of each conformational change increases with the concentration of the ligand used to trigger it, suggesting that ligands induce a conformational change and do not displace a pre-existing equilibrium. The hypothesis that each ligand stabilizes a different conformational state of the protein is confirmed by the kinetics of displacement of one ligand by another: for instance, the binary complexes between phosphofructokinase and either its substrate, fructose-6-phosphate, or its allosteric activator, ADP, have the same low fluorescence and should be in the same active state, but they show different rates of conformational transition upon binding the inhibitor phosphoenolpyruvate. It appears that phosphofructokinase can exist in more than two states. Some conformational changes between these multiple states are slow enough to play an important role in the kinetics of the reaction catalyzed by phosphofructokinase, and could even explain part of its allosteric behavior. These results show that steady-state measurements are not sufficient to analyze the regulatory properties of E. coli phosphofructokinase.

The role of Glu187 in the regulation of phosphofructokinase by phosphoenolpyruvate

In bacterial phosphofructokinases, either a glutamic or an aspartic residue is present at position 187, and the mechanism of inhibition by phosphoenolpyruvate seems to be correlated to the nature of residue 187. Upon binding phosphoenolpyruvate, only the enzymes with a Glu187 would undergo a major allosteric conformational change from an active into an inactive state, whereas the enzymes with an Asp187 would only show a simple upward shift in their pH-profile of activity. The phosphofructokinase from Spiroplasma citri, which has an Asp187, has been purified and its properties follow this pattern. The behaviour of mutants of the enzyme from Escherichia coli in which Glu187 is replaced by either aspartate or leucine confirms the importance of residue 187. The major allosteric transition of E. coli phosphofructokinase is abolished by the substitution Glu187-->Asp, suggesting that a glutamate at position 187 is necessary (but not sufficient) for the protein to undergo the change from the active into the inactive state induced by phosphenolpyruvate. In addition, the presence of an acidic residue, aspartate or glutamate, at position 187 is required (but not sufficient) for the binding of ADP (or GDP). This requirement of a negative charge for ADP binding could explain the striking conservation of an aspartate residue at position 187 in all the eukaryotic phosphofructokinases.

Critical amino acids responsible for converting specificities of proteins and for enhancing enzyme evolution are located around beta-turn potentials: data-based prediction

Various reports have described that amino acid substitutions can alter substrate, positional, inhibitory, and target gene specificities of proteins. By using the method of Chou and Fasman, the present work predicts that critical amino acids for converting these substrate specificities of trypsin, L-lactate dehydrogenase, aspartate aminotransferase, beta-lactamase, and cytochrome P-450 are found to exist within regions predicted as beta-turns. The ratios of hydroxylation and oxygenation positions of substrates by cytochrome P-450 and lipoxygenase, respectively, are varied by changes of the protein structures, probably around turn conformations. Inhibitory specificities of bovine pancreatic trypsin inhibitor and alpha 1-antitrypsin and target gene specificity of glucocorticoid receptor are converted by changing turn structures. Occurrence of beta-turn probabilities can be predicted around the amino acid alteration positions of an evolutionally antecedent protein of a nylon degradation enzyme. These findings will have relevance to work on protein engineering and enzyme evolution.

Genetic Approaches to Protein Structure and Function: Point Mutations as Modifiers of Protein Function

In this review, I summarize data in the biological literature which underscore the utility of a genetic approach to protein structure/function problems, with emphasis on binding phenomena, particularly of cytokine and growth factor/receptor interactions. Useful parallels or contrasts to chemical ligand/receptor systems and DNA binding protein interactions are examined where they simplify the analysis of protein ligand/receptor interactions. This approach was prompted by the fact that purely rational approaches, based on resolution of the three dimensional structure of proteins, are limited because such data is available for fewer than 3% of the 17,000 proteins for which the amino acid sequence has been deduced by molecular biology techniques.

Mutational studies of protein structures and their stabilities

The fundamental relationship between structure and function has served to guide investigations into the workings of living systems at all levels - from the whole organism to individual cells on down to individual molecules. When X-ray crystallography began to reveal the three-dimensional structures of proteins like myoglobin, lysozyme and RNase A, protein chemists were well prepared to draw inferences about functional mechanisms from the precise positioning of amino acid residues they could see. The close proximity between an amino acid side chain and a chemical group on a bound ligand strongly suggests a functional role for that side chain in binding affinity and specificity. Likewise, the nearly universal finding of large clusters of hydrophobic side chains buried in the core of proteins strongly supports a major functional role of hydrophobic interactions in protein folding and stability. Even though eminently plausible hypotheses like these, grounded in the most fundamental principles of chemistry and the logic of structure–function relationships, become widely accepted and make their way into textbooks, protein chemists have felt compelled to search for ways to test them and put them on a more quantitative basis.

A conformational transition involved in antagonistic substrate binding to the allosteric phosphofructokinase from Escherichia coli

The binding of fructose 6-phosphate, ATP or its nonhydrolyzable analogue adenylyl 5'-(beta,gamma-methylenediphosphonate), ADP, and phosphoenolpyruvate to Escherichia coli phosphofructokinase has been studied by changes in the protein fluorescence and/or equilibrium dialysis. The results lead to the following conclusions: (1) tetrameric phosphofructokinase can bind four ATP but only two fructose-6-phosphate, and this binding occurs without cooperativity; (2) only two conformational states, T and R, with respectively a high and a low fluorescence, seem accessible to phosphofructokinase, which exists as a mixture of one-third R and two-third T states in the absence of ligand; (3) the substrate fructose 6-phosphate and the allosteric activator ADP bind preferentially to the low-fluorescence R state, while the other substrate, ATP [or its nonhydrolyzable analogue adenylyl 5'-(beta,gamma-methylenediphosphonate)], and the allosteric inhibitor phosphoenolpyruvate bind to the high-fluorescence T state; (4) the binding of a given ligand is cooperative, with a Hill coefficient of 2, only when this binding is accompanied by a complete shift from one state to the other; for instance, the binding of the ATP analogue adenylyl 5'-(beta,gamma-methylenediphosphonate) to the T state is cooperative only in the presence of fructose 6-phosphate which favors the R state. This behavior is qualitatively consistent with a concerted transition, but quite different from that described earlier for phosphofructokinase from steady-state activity measurements (Blangy et al., 1968). This discrepancy suggests that the allosteric properties of phosphofructokinase are due in part to ligand binding and in part to the kinetics of the enzymatic reaction.

Heterotropic interactions in aspartate transcarbamoylase: turning allosteric ATP activation into inhibition as a consequence of a single tyrosine to phenylalanine mutation

Aspartate transcarbamoylase (EC 2.1.3.2) is extensively studied as a model for cooperativity and allostery. This enzyme shows cooperativity between the catalytic sites, and its activity is feedback inhibited by CTP and activated by ATP. These regulatory processes involve several interfaces between catalytic and regulatory chains as well as between domains within these two types of chains. As far as the regulatory chain is concerned, its two domains are in contact through a hydrophobic interface, in which a tyrosine residue is inserted in a pocket involving two leucine residues of the allosteric domain and a valine and a leucine residue of the zinc domain. To probe the possible implication of this hydrophobic core in the CTP and ATP regulatory effect, the tyrosine was replaced by a phenylalanine through oligonucleotide-directed mutagenesis. Interestingly, the resulting mutant shows a complete inversion of the ATP effect; it is now inhibited by ATP instead of being activated by this nucleotide triphosphate. This mutant remains normally sensitive to the feedback inhibitor CTP. This result shows that the hydrophobic interface between the two domains of the regulatory chain plays an important role in the discrimination between the regulatory signals promoted by the two allosteric effectors.

Phosphofructokinase from Plasmodium berghei: a kinetic model of allosteric regulation

As in mammalian cells, phosphofructokinase (PFK) is of major regulatory importance in the glucose metabolism of Plasmodium berghei. The malarial enzyme shows allosteric properties similar to PFK from various sources; it is activated by fructose-6-phosphate and inhibited by ATP, but differs with respect to allosteric regulation. Enzyme activity is only marginally increased by AMP, a potent activator of many phosphofructokinases. Phosphoenolpyruvate, which is reported to inhibit PFK activity, efficiency activates the malarial enzyme. No activation by ADP was observed. Instead, ADP inhibits the enzyme non-allosterically and competitively to the substrate MgATP. Phosphate stimulates the catalytic activity of malarial PFK independently of the activation by F6P and PEP.

Dissection of the effector-binding site and complementation studies of Escherichia coli phosphofructokinase using site-directed mutagenesis

A systematic study by site-directed mutagenesis has been conducted on the effector site of phosphofructokinase from Escherichia coli to delineate the role of side chains in binding the allosteric activator, GDP, and inhibitor, PEP, and to search for key residues in the allosteric transtion. Target residues were identified from the crystal structure of the enzyme-nucleoside diphosphate complex. It is found that both activator and inhibitor bind to the same set of amino acid side chains. Deletion of positively charged groups (Arg21, Arg25, Arg54, Arg154, and Lys213 mutated to alanine) weakens binding of both effectors by 2-3 kcal/mol, consistent with the disruption of charged hydrogen bonds. Residue Glu187, which is known from the crystal structure to bind the coordinated Mg2+ ion of GDP, is found to have a unique behavior on mutation and appears to be crucial in triggering the allosteric transition. All other residues mutated simply weaken binding of both PEP and GDP in a parallel manner. However, mutation of Glu----Ala187 reverses the roles of GDP and PEP, causing GDP to become an allosteric inhibitor and PEP an activator. Mutation of Glu----Gln187 has only a small effect on the binding of PEP, and both PEP and GDP are inhibitors. Studies are described in which mutations in different subunits of a tetrameric complex complement each other. The effector site is composed of residues from two subunits. In particular, Arg21 and Lys213 in each site are from different subunits. Mutations of either one of these residues abolishes activation by GDP of the homotetramer.(ABSTRACT TRUNCATED AT 250 WORDS)

Urea-induced inactivation, dissociation, and unfolding of the allosteric phosphofructokinase from Escherichia coli

The influence of urea on the allosteric phosphofructokinase from Escherichia coli has been studied by measuring the changes in enzymatic activity, protein fluorescence, circular dichroism, and retention in size-exclusion chromatography. Tetrameric, dimeric, and monomeric forms of the protein can be discriminated by their elution from a high-performance liquid chromatography gel filtration column. Three successive steps can be detected during the urea-induced denaturation of phosphofructokinase: (i) the dissociation of the native tetramer into dimers which abolishes the activity; (ii) the dissociation of dimers into monomers which exposes the unique tryptophan, Trp-311, to the aqueous solvent; (iii) the unfolding of the monomers which disrupts most of the secondary structure. This pathway involves the ordered dissociation of the interfaces between subunits and supports a previous hypothesis (Deville-Bonne et al., 1989). Phosphofructokinase can be quantitatively renatured from urea solutions, provided that precautions are taken to avoid the aggregation of one insoluble monomeric state. The renaturation of phosphofructokinase from urea implies three steps: an initial folding reaction within the monomeric state is followed by two successive association steps. The faster association step restores the native fluorescence, and the slower regenerates the active enzyme. The renaturation and denaturation of phosphofructokinase correspond to the complex pathway: tetramer in equilibrium dimer in equilibrium folded monomer in equilibrium unfolded monomer. It is found that the subunit interface which forms the regulatory site is more stable and associates 40 times more rapidly than the subunit interface which forms the active site.

Role of coupling entropy in establishing the nature and magnitude of allosteric response

The coupling free energy between an allosteric ligand and a substrate, delta Gax, is an explicit measure of the nature as well as the magnitude of impact that an allosteric ligand has on the binding of the substrate ligand to the enzyme, with positive values indicating inhibition and negative values indicating activation. By measuring the variation with temperature of the coupling free energy between the allosteric ligand and the substrate, it is possible to determine the enthalpic and entropic components that give rise to the coupling free energy. We have performed this analysis on two different K-type allosteric systems: the allosteric inhibition of rat liver phosphofructokinase by MgATP, and the allosteric activation of beef heart NAD+-dependent isocitrate dehydrogenase by ADP. In both cases the coupling free energy arises as the net result of opposing enthalpic and entropic components, with the coupling enthalpy (delta Hax) favoring activation and the coupling entropy (delta Sax) favoring inhibition. For phosphofructokinase at 25 degrees C, the absolute value of T delta Sax is greater than the absolute value of delta Hax, and net inhibition of rat liver phosphofructokinase by MgATP is realized. For isocitrate dehydrogenase, delta Hax dominates; however, the net activation is substantially mitigated by the magnitude of T delta Sax. Hence, the coupling entropy plays an important role in establishing both the nature and magnitude of the allosteric response. We hypothesize that the negative coupling entropy arises from the particular constraint placed upon the internal dynamical properties of the enzyme by the simultaneous binding of both allosteric and substrate ligands.

Mechanisms of cooperativity and allosteric regulation in proteins

AUosteric proteins control and coordinate chemical events in the living cell. When Monod conceived that idea he said that he had discovered the second secret of life. The first was the structure of DNA. The theory as published by Monodet al.(1963) was concerned chiefly with cooperativity and feedback inhibition of enzymes, such as the inhibition of threonine deaminase, the first enzyme in the pathway of the synthesis of isoleucine, by isoleucine, and its activation by valine. Two years later the theory was formalized by Monodet al.(1965).

Crystal structure of the complex of phosphofructokinase from Escherichia coli with its reaction products

The crystal structure of Escherichia coli phosphofructokinase complexed with its reaction products fructose 1,6-bisphosphate (Fru1,6P) and ADP/Mg2+, and the allosteric activator ADP/Mg2+, has been determined at 2.4 A resolution. The structure was solved by molecular replacement using the known structure of Bacillus stearothermophilus phosphofructokinase, and has been refined to a crystallographic R-factor of 0.165 for all data. The crystallization mixture contained the substrate fructose 6-phosphate, but the electron density maps showed clearly the presence of the product fructose 1,6-bisphosphate, presumably formed by the enzyme reaction with contaminating ATP. The crystal consists of tetrameric molecules with subunits in two different conformations despite their chemical identity. The magnesium ion in the "closed" subunit bridges the phosphate groups of the two products. In the "open" subunit, the products are about 1.5 A further apart, with the Mg2+ bound only to ADP. These two conformations probably represent two successive stages along the reaction pathway, in which the closure of the subunit is required to bring the substrates sufficiently close to react. This conformational change within the subunit is distinct from the quaternary structure change seen previously in the inactive T-state conformation. It is probably not involved in the co-operativity or allosteric control of the enzyme, since the co-operative product fructose 1,6-bisphosphate is not moved, nor are the subunit interfaces changed. The structure of the enzyme is similar to that of B. stearothermophilus phosphofructokinase, and confirms the location of the sites for the two reaction products (or substrates), and of the effector site binding the activator ADP/Mg2+. However, this structure gives a clearer picture of the active site, and of the interactions between the enzyme and its reaction products.

High-level expression of Bacillus stearothermophilus 6-phosphofructo-1-kinase in Escherichia coli

The 6-phosphofructo-1-kinase (PFK) gene from Bacillus stearothermophilus has been expressed at high levels in Escherichia coli. This expression has been demonstrated by complementation studies, sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, and PFK assays of cell extracts. A level of B. stearothermophilus PFK expression corresponding to 20% of the total extracted protein was calculated from densitometric scans of an SDS-polyacrylamide gel. The high level of recombinant gene expression will enable this laboratory to determine structure-function relationships in B. stearothermophilus PFK by the method of oligodeoxynucleotide-directed mutagenesis.