Structures of phosphate and trivanadate complexes of Bacillus stearothermophilus phosphatase PhoE: structural and functional analysis in the cofactor-dependent phosphoglycerate mutase superfamily

Bacillus stearothermophilus phosphatase PhoE is a member of the cofactor-dependent phosphoglycerate mutase superfamily possessing broad specificity phosphatase activity. Its previous structural determination in complex with glycerol revealed probable bases for its efficient hydrolysis of both large, hydrophobic, and smaller, hydrophilic substrates. Here we report two further structures of PhoE complexes, to higher resolution of diffraction, which yield a better and thorough understanding of its catalytic mechanism. The environment of the phosphate ion in the catalytic site of the first complex strongly suggests an acid-base catalytic function for Glu83. It also reveals how the C-terminal tail ordering is linked to enzyme activation on phosphate binding by a different mechanism to that seen in Escherichia coli phosphoglycerate mutase. The second complex structure with an unusual doubly covalently bound trivanadate shows how covalent modification of the phosphorylable His10 is accompanied by small structural changes, presumably to catalytic advantage. When compared with structures of related proteins in the cofactor-dependent phosphoglycerate mutase superfamily, an additional phosphate ligand, Gln22, is observed in PhoE. Functional constraints lead to the corresponding residue being conserved as Gly in fructose-2,6-bisphosphatases and Thr/Ser/Cys in phosphoglycerate mutases. A number of sequence annotation errors in databases are highlighted by this analysis. B. stearothermophilus PhoE is evolutionarily related to a group of enzymes primarily present in Gram-positive bacilli. Even within this group substrate specificity is clearly variable highlighting the difficulties of computational functional annotation in the cofactor-dependent phosphoglycerate mutase superfamily.     

Cloning and sequencing of a cDNA encoding 2,3-bisphosphoglycerate-independent phosphoglycerate mutase from maize. Possible relationship to the alkaline phosphatase family.

The primary sequence of maize 2,3-bisphosphoglycerate-independent phosphoglycerate mutase was deduced from cDNAs isolated from maize cDNA libraries by screening with specific antibodies to the cofactor-independent enzyme and from a maize genomic clone. The genomic clone provided the 5'-nucleotide sequence encoding the N-terminal amino acids which could not be obtained from the cDNA. Confirmation that the nucleotide sequence was for the cofactor-independent phosphoglycerate mutase was obtained by sequencing the peptides generated from cyanogen bromide cleavage of the purified protein. This is the first report of the amino acid sequence of a 2,3-bisphosphoglycerate cofactor-independent phosphoglycerate mutase, which consists of 559 amino acids and is twice the molecular size of the mammalian cofactor-dependent enzyme subunit. Analysis of the cofactor-independent phosphoglycerate mutase amino acid sequence revealed no identity with the cofactor-dependent mutase types. Northern blot analysis confirmed this difference since the maize cofactor-independent phosphoglycerate mutase cDNA did not hybridize with mRNA of the cofactor-dependent mutase. The lack of amino acid identity between cofactor-dependent and -independent enzymes is consistent with their different catalytic mechanisms and suggests that both enzymes are unrelated evolutionarily and arose from two independent ancestral genes. However, a constellation of residues which are involved in metal ion binding in various alkaline phosphatases is conserved in the maize cofactor-independent phosphoglycerate mutase, which suggests that the enzyme is a member of the alkaline phosphatase family of enzymes.     

Human bisphosphoglycerate mutase. Expression in Escherichia coli and use of site-directed mutagenesis in the evaluation of the role of the carboxyl-terminal region in the enzymatic mechanism

Bisphosphoglycerate mutase is an erythrocyte-specific enzyme whose main function is to synthesize 2,3-diphosphoglycerate, the allosteric effector of hemoglobin. In addition to its main 2,3-diphosphoglycerate synthase activity, the enzyme displays phosphatase and mutase activities both involving 2,3-diphosphoglycerate in their reaction. The three activities have been demonstrated to be catalysed at a unique active site. To study the structure of such an active site we have developed a recombinant system producing mutants of human bisphosphoglycerate mutase in Escherichia coli, by site-directed mutagenesis. For this purpose the human bisphosphoglycerate mutase cDNA that we had previously cloned has been used to construct a procaryotic high level expression vector bearing the "tac" promoter. Human bisphosphoglycerate mutase produced in E. coli, a species which does not normally synthesize this enzyme, represented 8% of the total soluble bacterial protein and displayed the three catalytic activities (synthase, mutase, and phosphatase) characteristic of the enzyme. Since it has been suggested that the carboxyl-terminal region may be implicated in the catalytic activity of the enzyme, three variants deleted in this part of the protein were produced. Our results indicate that a minimal deletion of 7 amino acid residues in the carboxyl-terminal portion of the human bisphosphoglycerate mutase completely abolished the three catalytic activities of the enzyme. In contrast, the effects of the deletion of the last two lysine residues were limited to a 38% reduction in the synthase activity. These results show that the carboxyl-terminal amino acid residues are either directly or indirectly implicated in the three catalytic functions of the human bisphosphoglycerate mutase, and that the two terminal lysine residues are not essential for the major part of the enzymatic mechanism of the enzyme.     

Human erythrocyte phosphoglycerate mutase: Evidence for normal catalysis in the absence of added 2,3-bisphospho-D-glycerate

Kinetic analyses indicate that human erythrocyte phosphoglycerate mutase catalyzes the normal, reversible isomerization of D-glycerate-3-P and D-glycerate-2-P in the absence of added D-glycerate-2,3-P₂. The reaction is impeded, however, by a potent inhibitor which occurs as a natural component of commericial D-glycerate-3-P. Inhibition may be overcome through substrate purification or by adding D-glycerate-2,3-P₂ to the reaction medium containing the contaminant. In surmounting the inhibition, bisphosphoglycerate performs as a non-essential activator and not as a cofactor. The latter concept is corroborated by the observation that D-glycerate-2,3-P₂ has absolutely no influence on mutase catalysis conducted in the presence of pure substrate. The data presented here and elsewhere, however, suggest that the red cell enzyme is readily phosphorylated by mono- as well as bisphosphoglycerate. Additional findings show that at concentrations in excess of 3mM, D-glycerate-3-P accelerates phosphoglycerate mutase catalysis in the absence of cofactor, suggesting that the mutase molecule possesses a normal catalytic site and an allosteric activator site.     

Molecular cloning and nucleotide sequence of murine 2,3-bisphosphoglycerate mutase cDNA

Cloning and sequencing of a murine cDNA with the entire coding region of 2,3-bisphosphoglycerate mutase is reported, as a prerequisite for further expression studies of this erythroid specific enzyme in Friend mouse erythroleukemia cells. A comparison between species of the deduced amino acid sequences of these proteins shows 20 substitutions between mouse and human and 21 between mouse and rabbit: none of these substitutions are in positions assumed to be in the active site. Amino acid alignment with the other related enzymes, the phosphoglycerate mutases, in combination with crystallographic data from yeast phosphoglycerate mutase, gives some insight into the structure/function correlation for this protein family. Amino acid residues which are most likely critical for either 2,3-bisphosphoglycerate mutase or phosphoglycerate mutase function are pointed out. Concerning the phylogenetic analysis, phosphoglycerate mutases B and M from mammalians appear to have diverged with the yeast enzyme from a common ancestor, before the emergence of the 2,3-bisphosphoglycerate mutases.     

Cofactor-independent phosphoglycerate mutase has an essential role in Caenorhabditis elegans and is conserved in parasitic nematodes

Phosphoglycerate mutases catalyze the interconversion of 2- and 3-phosphoglycerate in the glycolytic and gluconeogenic pathways. They exist in two unrelated forms that are either cofactor (2,3-diphosphoglycerate)-dependent or cofactor-independent. The two enzymes have no similarity in amino acid sequence, tertiary structure, or catalytic mechanism. Certain organisms including vertebrates have only the cofactor-dependent form, whereas other organisms can possess the independent form or both. Caenorhabditis elegans has been predicted to have only independent phosphoglycerate mutase. In this study, we have cloned and produced recombinant, independent phosphoglycerate mutases from C. elegans and the human-parasitic nematode Brugia malayi. They are 70% identical to each other and related to known bacterial, fungal, and protozoan enzymes. The nematode enzymes possess the catalytic serine, and other key amino acids proposed for catalysis and recombinant enzymes showed typical phosphoglycerate mutase activities in both the glycolytic and gluconeogenic directions. The gene is essential in C. elegans, because the reduction of its activity by RNA interference led to embryonic lethality, larval lethality, and abnormal body morphology. Promoter reporter analysis indicated widespread expression in larval and adult C. elegans with the highest levels apparent in the nerve ring, intestine, and body wall muscles. The enzyme was found in a diverse group of nematodes representing the major clades, indicating that it is conserved throughout this phylum. Our results demonstrate that nematodes, unlike vertebrates, utilize independent phosphoglycerate mutase in glycolytic and gluconeogenic pathways and that the enzyme is probably essential for all nematodes.     

Relative phosphate contents of phosphofructokinase and other soluble phosphoproteins in skeletal muscle

In vivo phosphorylation of muscle proteins has been studied by incorporation of [³²P]phosphate with emphasis placed upon the phosphorylation of glycolytic enzymes. Of the approximately 25 soluble proteins resolved by two-dimensional electrophoresis that contain significant ³²P, phosphofructokinase was the sole glycolytic enzyme identified as a phosphoprotein. The extent of phosphorylation found for this enzyme was the same as determined previously for purified phosphofructokinase and was about the same as the extent of phosphorylation of phosphorylase in resting muscle. Subsequent partial purification of several glycolytic enzymes confirmed the absence of significant amount of phosphate. However, phosphoglycerate mutase contained small amounts of covalently bound ³²P that was exchangeable with 3-phosphoglycerate and therefore, most likely was incorporated during the catalytic reaction cycle. Analogous results were obtained for phosphoglucomutase. Both mutases were also phosphorylated at the same sites by the catalytic subunit of cyclic AMP-dependent protein kinase.     

Structure and mechanism of action of a novel phosphoglycerate mutase from Bacillus stearothermophilus

Bacillus stearothermophilus phosphoglycerate mutase (PGM), which interconverts 2- and 3-phosphoglyceric acid (PGA), does not require 2,3-diphosphoglyceric acid for activity. However, this enzyme does have an absolute and specific requirement for Mn(2+) ions for catalysis. Here we report the crystal structure of this enzyme complexed with 3PGA and manganese ions to 1.9 A resolution; this is the first crystal structure of a diphosphoglycerate-independent PGM to be determined. This information, plus the location of the two bound Mn(2+) ions and the 3PGA have allowed formulation of a possible catalytic mechanism for this PGM. In this mechanism Mn(2+) ions facilitate the transfer of the substrate's phosphate group to Ser62 to form a phosphoserine intermediate. In the subsequent phosphotransferase part of the reaction, the phosphate group is transferred from Ser62 to the O2 or O3 positions of the reoriented glycerate to yield the PGA product. Site-directed mutagenesis studies were used to confirm our mechanism and the involvement of specific enzyme residues in Mn(2+) binding and catalysis.     

Mechanism of catalysis of the cofactor-independent phosphoglycerate mutase from Bacillus stearothermophilus. Crystal structure of the complex with 2-phosphoglycerate

The structure of the complex between the 2, 3-diphosphoglycerate-independent phosphoglycerate mutase (iPGM) from Bacillus stearothermophilus and its 3-phosphoglycerate substrate has recently been solved, and analysis of this structure allowed formulation of a mechanism for iPGM catalysis. In order to obtain further evidence for this mechanism, we have solved the structure of this iPGM complexed with 2-phosphoglycerate and two Mn(2+) ions at 1. 7-A resolution. The structure consists of two different domains connected by two loops and interacting through a network of hydrogen bonds. This structure is consistent with the proposed mechanism for iPGM catalysis, with the two main steps in catalysis being a phosphatase reaction removing the phosphate from 2- or 3-phosphoglycerate, generating an enzyme-bound phosphoserine intermediate, followed by a phosphotransferase reaction as the phosphate is transferred from the enzyme back to the glycerate moiety. The structure also allowed the assignment of the function of the two domains of the enzyme, one of which participates in the phosphatase reaction and formation of the phosphoserine enzyme intermediate, with the other involved in the phosphotransferase reaction regenerating phosphoglycerate. Significant structural similarity has also been found between the active site of the iPGM domain catalyzing the phosphatase reaction and Escherichia coli alkaline phosphatase.     

Phylogeny and ontogeny of the phosphoglycerate mutases.--V. Inactivation of phosphoglycerate mutase isozymes by histidine-specific reagents

1. The three isozymes of glycerate-2,3-P2 dependent phosphoglycerate mutase present in tissues of mammals and reptiles were inactivated by both treatment with diethylpyrocarbonate and photooxidation with rose bengal. 2. Inactivation of type M isozyme purified from rabbit muscle was complete when two histidine residues per enzyme subunit were carboethoxylated. Hydroxylamine removed the carboethoxy groups, with partial recovery of the enzymatic activity. The cofactor protected the enzyme against inactivation. 3. The inactivation of rabbit muscle phosphoglycerate mutase by photooxidation with methylene blue and rose bengal was sharply pH dependent. The pH profile of enzyme inactivation followed the titration curve of histidine, suggesting that this amino acid was critical for enzyme activity. Glycerate-2,3-P2 did not protect phosphoglycerate mutase against photoinactivation.     

Detection of an essential sulfhydryl group in phosphoglycerate mutase with an affinity-labeling reagent.

N-Bromoacetylethanolamine phosphate rapidly and irreversibly inactivates rabbit muscle phosphoglycerate mutase. At high molar ratios of reagent to enzyme, loss of activity (both mutase and phosphatase) approximates pseudo-first order kinetics. A rate-saturation effect is observed with half-maximal rate of inactivation occurring at 0.32 mM reagent, a value close to the Km for 3-phosphoglyceric acid. This datum and the dissociation constant of the 2,3-bisphosphoglycerate-enzyme complex, as determined from inactivation kinetics in the presence of the bisphosphate, suggest that the reagent reacts at the substrate binding site. Inactivation results from the covalent incorporation of about 0.8 mol of reagent/mol of catalytic subunit as determined with 14C-labeled reagent. Incorporation is negligible in the presence of substrate and is reduced 8-fold in the presence of 6 M urea. From amino acid analyses on acid hydrolysates of the inactivated enzyme, we have identified a sulfhydryl group as the site of alkylation. A peptide containing the essential sulfhydryl group has been isolated from a tryptic digest of the enzyme inactivated with labeled reagent; its amino acid composition is Trp1, Lys1,-Cys(Cm)1, Asp1, Ser1, Glu2, Gly1, Ala1, Leu1, Phe2.     

Structure and mechanism of action of a cofactor-dependent phosphoglycerate mutase homolog from Bacillus stearothermophilus with broad specificity phosphatase activity

The crystal structure of Bacillus stearothermophilus PhoE (originally termed YhfR), a broad specificity monomeric phosphatase with a molecular mass of approximately 24 kDa, has been solved at 2.3 A resolution in order to investigate its structure and function. PhoE, already identified as a homolog of a cofactor-dependent phosphoglycerate mutase, shares with the latter an alpha/beta/alpha sandwich structure spanning, as a structural excursion, a smaller subdomain composed of two alpha-helices and one short beta-strand. The active site contains residues from both the alpha/beta/alpha sandwich and the sub-domain. With the exception of the hydrophilic catalytic machinery conserved throughout the cofactor-dependent phosphoglycerate mutase family, the active-site cleft is strikingly hydrophobic. Docking studies with two diverse, favored substrates show that 3-phosphoglycerate may bind to the catalytic core, while alpha-napthylphosphate binding also involves the hydrophobic portion of the active-site cleft. Combining a highly favorable phospho group binding site common to these substrate binding modes and data from related enzymes, a catalytic mechanism can be proposed that involves formation of a phosphohistidine intermediate on His10 and likely acid-base behavior of Glu83. Other structural factors contributing to the broad substrate specificity of PhoE can be identified. The dynamic independence of the subdomain may enable the active-site cleft to accommodate substrates of different sizes, although similar motions are present in simulations of cofactor-dependent phosphoglycerate mutases, perhaps favoring a more general functional role. A significant number of entries in protein sequence databases, particularly from unfinished microbial genomes, are more similar to PhoE than to cofactor-dependent phosphoglycerate mutases or to fructose-2,6-bisphosphatases. This PhoE structure will therefore serve as a valuable basis for inference of structural and functional characteristics of these proteins.     

Characterization of active-site mutants of Schizosaccharomyces pombe phosphoglycerate mutase. Elucidation of the roles of amino acids involved in substrate binding and catalysis.

The roles of a number of amino acids present at the active site of the monomeric phosphoglycerate mutase from the fission yeast Schizosaccharomyces pombe have been explored by site-directed mutagenesis. The amino acids examined could be divided broadly into those presumed from previous related structural studies to be important in the catalytic process (R14, S62 and E93) and those thought to be important in substrate binding (R94, R120 and R121). Most of these residues have not previously been studied by site-directed mutagenesis. All the mutants except R14 were expressed in an engineered null strain of Saccharomyces cerevisiae (S150-gpm:HIS) in good yield. The R14Q mutant was expressed in good yield in the transformed AH22 strain of S. cerevisiae. The S62A mutant was markedly unstable, preventing purification. The various mutants were purified to homogeneity and characterized in terms of kinetic parameters, CD and fluorescence spectra, stability towards denaturation by guanidinium chloride, and stability of phosphorylated enzyme intermediate. In addition, the binding of substrate (3-phosphoglycerate) to wild-type, E93D and R120,121Q enzymes was measured by isothermal titration calorimetry. The results provide evidence for the proposed roles of each of these amino acids in the catalytic cycle and in substrate binding, and will support the current investigation of the structure and dynamics of the enzyme using multidimensional NMR techniques.     

Isolation and partial characterization of monophosphoglycerate mutase from human erythrocytes.

Monophosphoglycerate mutase has been purified to homogeneity from outdated human erythrocytes as indicated by exclusion chromatography, polyacrylamide gel electrophoresis, and equilibrium centrifugation. Occasionally, the recommended purification procedure yields a small amount (3% or less) of a single extraneous protein which can be deleted from the enzyme preparation by employing an additional purification step. The native enzyme has a molecular weight of 54,000 to 56,000 as determined by equilibrium centrifugation and exclusion chromatography. Disc gel electrophoresis in the presence of sodium dodecyl sulfate yields a single protein band with a molecular weight of 28,600, indicating that the native macromolecule is a dimer composed of subunits of similar mass. Homogeneous monophosphoglycerate mutase is free of diphosphoglycerate mutase, enolase, and nonspecific phosphatase activities; however, the enzyme manifests intrinsic 2,3-diphospho-D-glycerate phosphatase activity as shown by thermal denaturation studies. The diphosphatase activity is stimulated by PPi and glycolate-2-P, but is inhibited by Cl-, HSO3-, and Pi. The pH optimum for both the diphosphatase and the mutase is 6.8. The Km for 2,3-diphospho-D-glycerate in the phosphatase reaction is 82 muM at 37 degrees and pH 7.2. The amino acid composition of homogeneous monophosphoglycerate mutase is given.     

High resolution structure of the phosphohistidine-activated form of Escherichia coli cofactor-dependent phosphoglycerate mutase

The active conformation of the dimeric cofactor-dependent phosphoglycerate mutase (dPGM) from Escherichia coli has been elucidated by crystallographic methods to a resolution of 1.25 A (R-factor 0.121; R-free 0.168). The active site residue His(10), central in the catalytic mechanism of dPGM, is present as a phosphohistidine with occupancy of 0.28. The structural changes on histidine phosphorylation highlight various features that are significant in the catalytic mechanism. The C-terminal 10-residue tail, which is not observed in previous dPGM structures, is well ordered and interacts with residues implicated in substrate binding; the displacement of a loop adjacent to the active histidine brings previously overlooked residues into positions where they may directly influence catalysis. E. coli dPGM, like the mammalian dPGMs, is a dimer, whereas previous structural work has concentrated on monomeric and tetrameric yeast forms. We can now analyze the sequence differences that cause this variation of quaternary structure.     

The microdetermination of 2,3-diphosphoglycerate

A procedure for microestimation of 2,3-diphosphoglycerate, utilizing its role as coenzyme in the phosphoglycerate mutase reaction is described. The coenzymic activity was determined by assaying phosphoglycerate mutase polarimetrically without a coupled enzyme. This method is applicable to samples containing as little as 0.002 μmole of 2,3-diphosphoglycerate/ml. The content in various biological extracts was determined.     

Functional homology of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase, phosphoglycerate mutase, and 2,3-bisphosphoglycerate mutase

The bisphosphatase domain of the rat liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase has been shown to exhibit a structural similarity to yeast phosphoglycerate mutase and human red blood cell 2,3-bisphosphoglycerate mutase including very similar active site sequences with a histidyl residue being involved in phospho group transfer. The liver bifunctional enzyme was found to catalyze the hydrolysis of glycerate 1,3-bisphosphate to glycerate 3-phosphate and inorganic phosphate. The Km for glycerate 1,3-bisphosphate was 320 microM and the Vmax was 11.5 milliunits/mg. Incubation of the rat liver enzyme with [1-32P]glycerate 1,3-bisphosphate resulted in the formation of a phosphoenzyme intermediate, and the labeled amino acid was identified as 3-phosphohistidine. Tryptic and endoproteinase Lys-C peptide maps of the 32P-phosphoenzyme labeled either with [2-32P]fructose 2,6-bisphosphate or [1-32P]glycerate 1,3-bisphosphate revealed that 32P-radioactivity was found in the same peptide, proving that the same histidyl group accepts phosphate from both substrates. Fructose 2,6-bisphosphate inhibited competitively the formation of phosphoenzyme from [1-32P]glycerate 1,3-bisphosphate. Effectors of fructose-2,6-bisphosphatase also inhibited phosphoenzyme formation. Substrates and products of phosphoglycerate mutase and 2,3-bisphosphoglycerate mutase also modulated the activities of the bifunctional enzyme. These results demonstrate that, in addition to a structural homology, the bisphosphatase domain of the bifunctional enzyme has a functional similarity to phosphoglycerate mutase and 2,3-bisphosphoglycerate mutase and support the concept of an evolutionary relationship between the three enzyme activities.     

Active site phosphohistidine peptides from red cell bisphosphoglycerate synthase and yeast phosphoglycerate mutase.

Bisphosphoglycerate synthase (glycerate-1,3-P2 yields glycerate-2,3-P2) and phosphoglycerate mutase (glycerate-3-P formed from glycerate-2-P) are both phosphorylated by substrates at a histidine residue forming covalent intermediates which have been shown to function in the phosphoryl transfer reactions catalyzed by these enzymes (Rose, Z. B., and Dube, S. (1976) J. Biol. Chem. 251, 4817--4822). We have phosphorylated bisphosphoglycerate synthase from horse red blood cells with [U-32P]glycerate-2,3-P2, digested with trypsin, and purified the phosphopeptide. The amino acid sequence of the phosphohistidine peptide has been determined to be: His-Gly-Gln-Gly-Ala-Trp-Asn-Lys. In like manner, a phosphohistidyl peptide has now been purified from yeast phosphoglycerate mutase, for which the amino acid sequence is known (Winn, S. I., Watson, H. C., Fothergill, L. A., and Harkins, R. N. (1977) Biochem. Soc. Trans. 5, 657-659). The amino acid composition of the phosphopeptide indicates that histidine-8 was phosphorylated. The sequence of this peptide is closely homologous with the active site peptide from bisphosphoglycerate synthase. In yeast phosphoglycerate mutase, the denatured phosphoenzyme hydrolyzes with a single rate constant of 2.02 X 10(-4) s-1 at pH 3, 45 degrees C. The relevance of these observations to the enzymatic mechanism is discussed.     

Purification and characterization of phosphoglycerate mutase from methanol-grown Hyphomicrobium X and Pseudomonas AM1.

Phosphoglycerate mutase has been purified from methanol-grown Hyphomicrobium X and Pseudomonas AMI by acid precipitation, heat treatment, ammonium sulphate fractionation, Sephadex G-50 gel filtration and DEAE-cellulose column chromatography. The purification attained using the Hyphomicrobium X extract was 72-fold, and using the Pseudomonas AMI extract, 140-fold. The enzyme purity, as shown by analytical polyacrylamide gel electrophoresis, was 50% from Hyphomicrobium X and 40% from Pseudomonas AMI. The enzyme activity was associated with one band. The purified preparations did not contain detectable amounts of phosphoglycerate kinase, phosphopyruvate hydratase, phosphoglycerate dehydrogenase or glycerate kinase activity. The molecular weight of the enzymic preparation was 32000 +/- 3000. The enzyme from both organisms was stable at low temperatures and, in the presence of 2,3-diphosphoglyceric acid, could withstand exposure to high temperatures. The enzyme from Pseudomonas AMI has a broad pH optimum at 7-0 to 7-6 whilst the enzyme from Hyphomicrobium X has an optimal activity at pH 7-3. The cofactor 2,3-diphosphoglyceric acid was required for maximum enzyme activity and high concentrations of 2-phosphoglyceric acid were inhibitory. The Km values for the Hyphomicrobium X enzyme were: 3-phosphoglyceric acid, 6-0 X 10(-3) M: 2-phosphoglyceric acid, 6-9 X 10(-4) M; 2,3-diphosphoglyceric acid, 8-0 X 10(-6) M; and for the Pseudomonas AMI ENzyme: 3-4 X 10(-3) M, 3-7 X 10(-4) M and 10 X 10(-6) M respectively. The equilibrium constant for the reaction was 11-3 +/- 2-5 in the direction of 2-phosphoglyceric acid to 3-phosphoglyceric acid and 0-09 +/- 0-02 in the reverse direction. The standard free energy for the reaction proceeding from 2-phosphoglyceric acid to 3-phosphoglyceric acid was -5-84 kJ mol(-1) and in the reverse direction +5-81 kJ mol(-1).