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PubMed1996

Molecular basis for dysfunction of some mutant forms of methylmalonyl-CoA mutase: deductions from the structure of methionine synthase.

Inherited defects in the gene for methylmalonyl-CoA mutase (EC 5.4.99.2) result in the mut forms of methylmalonic aciduria. mut- mutations lead to the absence of detectable mutase activity and are not corrected by excess cobalamin, whereas mut- mutations exhibit residual activity when exposed to excess cobalamin. Many of the mutations that cause methylmalonic aciduria in humans affect residues in the C-terminal region of the methylmalonyl-CoA mutase. This portion of the methylmalonyl-CoA mutase sequence can be aligned with regions in other B12 (cobalamin)-dependent enzymes, including the C-terminal portion of the cobalamin-binding region of methionine synthase. The alignments allow the mutations of human methylmalonyl-CoA mutase to be mapped onto the structure of the cobalamin-binding fragment of methionine synthase from Escherichia coli (EC 2.1.1.13), which has recently been determined by x-ray crystallography. In this structure, the dimethylbenzimidazole ligand to the cobalt in free cobalamin has been displaced by a histidine ligand, and the dimethylbenzimidazole nucleotide "tail" is thrust into a deep hydrophobic pocket in the protein. Previously identified mut0 and mut- mutations (Gly-623 --> Arg, Gly-626 --> Cys, and Gly-648 --> Asp) of the mutase are predicted to interfere with the structure and/or stability of the loop that carries His-627, the presumed lower axial ligand to the cobalt of adenosylcobalamin. Two mutants that lead to severe impairment (mut0) are Gly-630 --> Glu and Gly-703 --> Arg, which map to the binding site for the dimethylbenzimidazole nucleotide substituent of adenosylcobalamin. The substitution of larger residues for glycine is predicted to block the binding of adenosylcobalamin.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗

PubMed2006

Energetics of interaction between the G-protein chaperone, MeaB, and B12-dependent methylmalonyl-CoA mutase.

MeaB is an auxiliary protein that supports the function of the radical B(12)-dependent enzyme, methylmalonyl-CoA mutase, although its precise role is not understood. Mutations in the human homolog of MeaB, MMAA, lead to methylmalonic aciduria, an inborn error of metabolism that can be fatal. To obtain insights into the function of this recently discovered protein, we have characterized the entropic and enthalpic contributions to DeltaGdegree (assoc) for complexation of MeaB (in the presence and absence of nucleotides) with methylmalonyl-CoA mutase (in the presence and absence of cofactor). The dissociation constant for binding of methylmalonyl-CoA mutase and MeaB ranges from 34 +/- 4 to 524 +/- 66 nm, depending on the combination of nucleotide and mutase form. Holomutase binds MeaB 15-fold more tightly when the nonhydrolyzable GTP analog, GMPPNP, is bound versus GDP. In contrast, the apomutase binds MeaB with similar affinity in the presence of either nucleotide. Our studies reveal that a large structural rearrangement accompanies interaction between these proteins and buries between approximately 4000 and 8600A(2) of surface area, depending on the combination of ligands in the active sites of the two proteins. Furthermore, we demonstrate that MeaB binds GTP and GDP with similar affinity (K(d) of 7.3 +/- 1.9 and 6.2 +/- 0.7 microm, respectively at 20 degrees C) and has low intrinsic GTPase activity (approximately 0.04 min(-1) at 37 degrees C), which is stimulated approximately 100-fold by methylmalonyl-CoA mutase. These studies provide insights into the energetics of interaction between the radical enzyme methylmalonyl-CoA mutase and MeaB, which are discussed.

Bacterial Proteins↗

PubMed1990

Methylmalonyl-CoA mutase (EC 5.4.99.2) and methionine synthetase (EC 2.1.1.13) in the tissues of cobalt-vitamin B12 deficient sheep.

The changes in the activities of the two vitamin B12-dependent enzymes methylmalonyl-CoA mutase (EC 5.4.99.2) and methionine synthetase (5-methyltetrahydrofolate-homocysteine methyltransferase, EC 2.1.1.13) are described in two groups of sheep maintained for 20 weeks on either a cobalt-deficient or a Co-sufficient whole-barley diet. At the end of that period, the plasma concentrations of vitamin B12 were depressed and those of methylmalonic acid were raised in the Co-deficient group. During the course of the experiment hepatic holo-mutase activity, measured on biopsy samples, declined in Co-deficient animals with a half-life of 73 d. There was a similar, but slower decline in lymphocyte holo-mutase activity which fell with a half-life of 125 d. At slaughter, there was no difference between Co-sufficient and Co-deficient animals in total mutase activity in liver, kidney, brain and spinal cord. In contrast, the total-synthetase activity of liver and kidney was reduced by 60 and 30% respectively in the Co-deficient animals. There was no change in either group of animals in total-synthetase activity, or in either holo-mutase or holo-synthetase activity, in brain and spinal cord. In the Co-deficient animals, holo-mutase and holo-synthetase activities in liver, the tissue with the greatest activity of both enzymes, fell to 25 and 39% respectively, of that of Co-sufficient animals. The corresponding reductions for kidney were 12 and 51% respectively. These results indicated that activity of both holoenzymes is greatly reduced in Co-deficient sheep.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗

PubMed1993

Amino acid residues involved in the catalytic site of human erythrocyte bisphosphoglycerate mutase. Functional consequences of substitutions of His10, His187 and Arg89.

Human bisphosphoglycerate mutase (GriP2 mutase) is a trifunctional enzyme which synthesizes and degrades GriP2 in red cells. Among the amino acid residues involved in its active site there are two conserved histidine residues, His10 which is phosphorylated during the catalytic process and His187 for which only speculative data have been made about the potential role during the reactions. Another amino acid residue, Arg89, had not been described as part of this active site but we have recently shown that a natural mutant Arg89-->Cys was highly thermolabile and showed severe perturbations of its enzymatic properties. To understand better the exact role of these residues, replacements of His10 by Gly (H10G) or Asp (H10D), His187 by Asn (H187N), Tyr (H187Y) or Asp (H187D) and Arg89 by Cys (R89C), Ser (R89S), Gly (R89G) or Lys (R89K) were performed by site-directed mutagenesis. The results obtained in this report show that replacement of the His10 residue completely abolished the enzymatic activities. Concerning the His187 residue, our results afford arguments that it plays an essential role in the three catalytic activities. Indeed all these activities are abolished in the two H187Y and H187D variants, whereas they are detectable though strongly diminished, for the H187N variant. In addition mutations at His187 could be distinguishable from those at His10 since the former resulted in a thermolabile enzyme, whereas no significant change in heat stability was observed for the latter. It is noteworthy that the H187N variant is protected against thermal instability by glycerate 2,3-bisphosphate (GriP2). Concerning the Arg89 mutants, R89C, R89S and R89G, the three variants showed characteristics identical to those found in the natural R89C mutant, i.e. loss of 99% of synthase activity, consistent decrease of mutase and 2-phosphoglycolate-stimulated phosphatase activities whereas the unstimulated phosphatase activity was normal. Moreover these mutants were unstable at 55 degrees C but GriP2 was able to protect them against thermal instability. In contrast, the R89K mutant was stable at 55 degrees C. Its synthase and unstimulated phosphatase activities were normal but its mutase and 2-phosphoglycolate-stimulated phosphatase activities were decreased. In addition, Km values for monophosphoglycerates were increased (3.2-fold) in the synthase but normal in mutase activities, whereas Km values for GriP2 were normal in mutase and phosphatase activities.(ABSTRACT TRUNCATED AT 400 WORDS)

Arginine↗

PubMed1999

The role of the C-terminal region in phosphoglycerate mutase.

Removal of the C-terminal seven residues from phosphoglycerate mutase from Saccharomyces cerevisiae by limited proteolysis is associated with loss of mutase activity, but no change in phosphatase activity. The presence of the cofactor 2, 3-bisphosphoglycerate, or of the cofactor and substrate 3-phosphoglycerate together, confers protection against proteolysis. The substrate alone offers no protection. Replacement of either or both of the two lysines at the C-terminus by glycines has only limited effects on the kinetic properties of phosphoglycerate mutase, indicating that these residues are unlikely to be involved in crucial electrostatic interactions with the substrate, intermediate or product in the reaction. However, the double-mutant form of the enzyme is more sensitive to proteolysis and is no longer protected against proteolysis by the presence of cofactor. The proteolysed wild-type and two of the mutated forms of the enzyme show a reduced response to 2-phosphoglycollate, which enhances the instability of the phospho form of the native enzyme. The phosphoglycerate mutase from Schizosaccharomyces pombe, which lacks the analogous C-terminal tail, has an inherently lower mutase activity and is also less responsive to stimulation by 2-phosphoglycollate. It is proposed that the C-terminal region of phosphoglycerate mutase helps to maintain the enzyme in its active phosphorylated form and assists in the retention of the bisphosphoglycerate intermediate at the active site. However, its role seems not to be to contribute directly to ligand binding, but rather to exert indirect effects on the transfer of the phospho group between substrate, enzyme, intermediate and product.

Amino Acid Sequence↗

PubMed2005

Stabilisation of methylene radicals by cob(II)alamin in coenzyme B12 dependent mutases.

Coenzyme B12 initiates radical chemistry in two types of enzymatic reactions, the irreversible eliminases (e.g., diol dehydratases) and the reversible mutases (e.g., methylmalonyl-CoA mutase). Whereas eliminases that use radical generators other than coenzyme B12 are known, no alternative coenzyme B12 independent mutases have been detected for substrates in which a methyl group is reversibly converted to a methylene radical. We predict that such mutases do not exist. However, coenzyme B12 independent pathways have been detected that circumvent the need for glutamate, beta-lysine or methylmalonyl-CoA mutases by proceeding via different intermediates. In humans the methylcitrate cycle, which is ostensibly an alternative to the coenzyme B12 dependent methylmalonyl-CoA pathway for propionate oxidation, is not used because it would interfere with the Krebs cycle and thereby compromise the high-energy requirement of the nervous system. In the diol dehydratases the 5'-deoxyadenosyl radical generated by homolysis of the carbon-cobalt bond of coenzyme B12 moves about 10 A away from the cobalt atom in cob(II)alamin. The substrate and product radicals are generated at a similar distance from cob(II)alamin, which acts solely as spectator of the catalysis. In glutamate and methylmalonyl-CoA mutases the 5'-deoxyadenosyl radical remains within 3-4 A of the cobalt atom, with the substrate and product radicals approximately 3 A further away. It is suggested that cob(II)alamin acts as a conductor by stabilising both the 5'-deoxyadenosyl radical and the product-related methylene radicals.

Cobamides↗

PubMed2005

Searching for intermediates in the carbon skeleton rearrangement of 2-methyleneglutarate to (R)-3-methylitaconate catalyzed by coenzyme B12-dependent 2-methyleneglutarate mutase from Eubacterium barkeri.

Coenzyme B(12)-dependent 2-methyleneglutarate mutase from the strict anaerobe Eubacterium barkeri catalyzes the equilibration of 2-methyleneglutarate with (R)-3-methylitaconate. Proteins with mutations in the highly conserved coenzyme binding-motif DXH(X)(2)G(X)(41)GG (D483N and H485Q) exhibited decreased substrate turnover by 2000-fold and >4000-fold, respectively. These findings are consistent with the notion of H485 hydrogen-bonded to D483 being the lower axial ligand of adenosylcobalamin in 2-methyleneglutarate mutase. (E)- and (Z)-2-methylpent-2-enedioate and all four stereoisomers of 1-methylcyclopropane-1,2-dicarboxylate were synthesized and tested, along with acrylate, with respect to their inhibitory potential. Acrylate and the 2-methylpent-2-enedioates were noninhibitory. Among the 1-methylcyclopropane-1,2-dicarboxylates only the (1R,2R)-isomer displayed weak inhibition (noncompetitive, K(i) = 13 mM). Short incubation (5 min) of 2-methyleneglutarate mutase with 2-methyleneglutarate under anaerobic conditions generated an electron paramagnetic resonance (EPR) signal (g(xy) approximately 2.1; g(z) approximately 2.0), which by analogy with the findings on glutamate mutase from Clostridium cochlearium [Biochemistry, 1998, 37, 4105-4113] was assigned to cob(II)alamin coupled to a carbon-centered radical. At longer incubation times (>1 h), inactivation of the mutase occurred concomitant with the formation of oxygen-insensitive cob(II)alamin (g(xy) approximately 2.25; g(z) approximately 2.0). In order to identify the carbon-centered radical, various (13)C- and one (2)H-labeled substrate/product molecules were synthesized. Broadening (0.5 mT) of the EPR signal around g = 2.1 was observed only when C2 and/or C4 of 2-methyleneglutarate was labeled. No effect on the EPR signals was seen when [5'-(13)C]adenosylcobalamin was used as coenzyme. The inhibition and EPR data are discussed in the context of the addition-elimination and fragmentation-recombination mechanisms proposed for 2-methyleneglutarate mutase.

Carbon↗

PubMed2001

UDP-galactopyranose mutase has a novel structure and mechanism.

Uridine diphosphogalactofuranose (UDP-Galf ) is the precursor of the d-galactofuranose (Galf ) residues found in bacterial and parasitic cell walls, including those of many pathogens, such as Mycobacterium tuberculosis and Trypanosoma cruzi. UDP-Galf is made from UDP-galactopyranose (UDP-Galp) by the enzyme UDP-galactopyranose mutase (mutase). The mutase enzyme is essential for the viability of mycobacteria and is not found in humans, making it a viable therapeutic target. The mechanism by which mutase achieves the unprecedented ring contraction of a nonreducing sugar is unclear. We have solved the crystal structure of Escherichia coli mutase to 2.4 A resolution. The novel structure shows that the flavin nucleotide is located in a cleft lined with conserved residues. Site-directed mutagenesis studies indicate that this cleft contains the active site, with the sugar ring of the substrate UDP-galactose adjacent to the exposed isoalloxazine ring of FAD. Assay results establish that the enzyme is active only when flavin is reduced. We conclude that mutase most likely functions by transient reduction of substrate.

Amino Acid Sequence↗

PubMed1973

Chorismate mutase from Streptomyces aureofaciens: a heat-stable enzyme.

Chorismate mutase from Streptomyces aureofaciens was purified 12-fold. This enzyme preparation did not show any activity when tested for anthranilate synthetase, prephenate dehydrogenase, or prephenate dehydratase. The catalytic activity of chorismate mutase has a broad optimum between pH 7 and 8. The initial velocity data followed regular Michaelis-Menten kinetics with a K(m) of 5.3 x 10(-4) M, and the molecular weight of the enzyme was determined by sucrose gradient centrifugation to be 50,000. Heat inactivation of chorismate mutase, which occurs above temperatures of 60 C, is reversible. The enzyme activity can be restored even when chorismate mutase is treated at the temperature of a boiling-water bath for 15 min. Heat-denatured and renatured enzymes showed the same Michaelis constant and the same molecular weight as the native enzyme. l-Phenylalanine, l-tyrosine, l-tryptophan, and metabolites of the aromatic amino acid pathway were tested as potential modifiers of chorismate mutase activity. The activity of the enzyme was inhibited by none of these substances. Chorismate mutase of S. aureofaciens was not repressed in cells grown in minimal medium supplemented with l-phenylalanine, l-tyrosine, or l-tryptophan.

Cell-Free System↗

PubMed1989

Isolation, characterization, and structure of a mutant 89 Arg----Cys bisphosphoglycerate mutase. Implication of the active site in the mutation.

Bisphosphoglycerate mutase (EC 5.4.2.4.) is a trifunctional enzyme which displays synthase, mutase, and phosphatase activities. The purification, characterization, and structural study of an abnormal form of the enzyme, isolated from a patient which we reported earlier (Rosa, R., Prehu, M. O., Beuzard, Y., and Rosa, J. (1978) J. Clin. Invest. 62, 907-915), is described. The abnormal enzyme, present at 50% of the level of the normal enzyme as estimated by immunological methods, showed elevated electrophoretic mobility and hybridized with erythrocyte phosphoglycerate mutase (EC 5.4.2.1.) in the same manner as the normal control. The mutant enzyme was unstable at 55 degrees C and could be protected against thermal instability by 0.5 mM glycerate 2,3-bisphoshate but not by either glycerate 3-phosphate or glycolate 2-phosphate. Two of the three functions of the mutant enzyme were distinct from those of the normal protein. The specific activity of the synthase was 0.57% of normal and that of the mutase 4.1%. By contrast, the specific phosphatase activity was not affected by the mutation. However, the phosphatase activity of the mutated protein was markedly less stimulated by glycolate-2-phosphate than that of the control. High performance liquid chromatography analysis of tryptic peptides derived from the mutant enzyme showed an abnormal profile with the absence of two peaks normally containing the T12 and T13 peptides and without the appearance of a supplementary peak. Amino acid sequence and mass spectrometric analysis demonstrated the substitution of Arg----Cys residue in position 89 producing an uncleaved T12-T13 present in the same peak as the T6. Considered together, our data suggest that Arg-89 is located at or near the active site of bisphosphoglycerate mutase and that this residue is probably involved in the binding of monophosphoglycerates.

Amino Acid Sequence↗

PubMed1996

Homology modeling of human methylmalonyl-CoA mutase: a structural basis for point mutations causing methylmalonic aciduria.

Point mutations in the human gene encoding coenzyme B12 (adenosylcobalamin)-dependent methylmalonyl-CoA mutase give rise to an inherited disorder of propionic acid metabolism termed mut methylmalonic aciduria. Almost all such mutations alter amino acids in the homodimeric human enzyme that are identical to residues in the catalytic alpha-subunit of the heterodimeric methylmalonyl-CoA mutase from the bacterium Propionibacterium shermanii, to which the mature human enzyme shows an overall 65% sequence identity. To explore how specific mutations might cause the observed clinical phenotype, 12 known mutations were mapped onto a three-dimensional homology model of the subunit of the human enzyme, generated using the program MODELLER on the basis of the recently published 2.0 A X-ray crystal structure of the P. shermanii methylmalonyl-CoA mutase. Eight mutations are found in the C-terminal B12-binding domain, of which 4 (G623R, G626C, G630E, G703R) are in direct contact with the corrin and are clustered around the histidine ligand (H627) provided by the protein to coordinate the cobalt atom of the B12 cofactor. Introduction of a side chain, particularly one that is charged, at any of these positions is expected to disrupt the flavodoxin-like fold and severely impair its binding of B12. Mutation at either of two other highly conserved glycine residues in this domain (G648D, G717V) also disrupts critical elements in the fold as would the introduction of an additional positive charge in the mutation H678R. Mutation of an arginine in a solvent-exposed loop to a hydrophobic residue (R694W) is also pathogenic. The remaining mutations have been mapped to the N-terminal region of the mutase, two of which introduce a buried, uncompensated charge, either near the subunit interface (A377E), or near the narrow channel through which acyl-CoA esters gain access to the active site (W105R). The extreme N-terminus of methylmalonyl-CoA mutase is predicted to make extensive contacts with the other subunit, and a mutant in this region (R93H) may prevent the correct assembly of the dimer.

Amino Acid Metabolism, Inborn Errors↗

PubMed1995

Cooperative manganese (II) activation of 3-phosphoglycerate mutase of Bacillus megaterium: a biological pH-sensing mechanism in bacterial spore formation and germination.

The conversion of 3-P-glycerate mutase of Bacillus megaterium from a catalytically inactive to an active form was markedly more effective with buffered Mn2+ than with just added Mn2+. The previously reported stimulation by threonine disappeared when buffered Mn2+ was used. Activation of mutase showed a sigmoid dependence on Mn2+ concentration when buffered with tetramethylenediamine tetraacetate. The curve obeyed Hill kinetics with a coefficient of 2.1 +/- 0.1. At 0.5 microM free Mn2+, buffered with trimethylenediamine tetraacetate, activation of mutase increased about 73-fold over the pH range 6.6 to 7.4. Plotted against [OH-], the activation showed a strongly sigmoid response with Hill coefficient of 3.5 +/- 0.1. When mutase activated at pH 6.4 and 0.5 microM free Mn2+ in the presence of substrate was transferred to a similar medium at pH 7.4, the rate of product accumulation increased 360-fold within a few minutes. The pH sensitivity conferred upon mutase by low [Mn2+] may account for its large activity decrease during sporulation, and later increase during spore germination, when spore pH, respectively, declines and rises by about 1 unit. These changes result in the accumulation, and later reutilization, of 3-P-glycerate reserves in the spore. Such a pH-sensing function of Mn2+ may have wider biological uses.

Bacillus megaterium↗

PubMed1992

Nuclear location of phosphoglycerate mutase BB isozyme in rat tissues.

We have previously reported (Ureña et al. Eur. J. Cell Biol. 1990) that in skeletal muscle, type MM phosphoglycerate mutase isozyme is present in the nucleus as well as in the cytosol. To determine whether type BB phosphoglycerate mutase isozyme is also present in nucleus, the subcellular location of this isozyme was studied in different rat tissues by cell fractionation and immunogold techniques. With the aid of high affinity-purified anti-phosphoglycerate mutase BB isozyme antibodies, the isozyme was located in the nucleus of neuronal, astroglial and liver cells but not in the nucleus of oligodendroglial and endothelial cells. Biochemical studies on purified nuclear fractions also demonstrated the presence of phosphoglycerate mutase activity in the nucleus. Both immunocytochemical and biochemical techniques showed that nuclear phosphoglycerate mutase-specific activity depended on the type of cell.

Animals↗

PubMed1993

Cloning and expression in yeast of a higher plant chorismate mutase. Molecular cloning, sequencing of the cDNA and characterization of the Arabidopsis thaliana enzyme expressed in yeast.

Chorismate mutase (EC 5.4.99.5) catalyzes the first step in the branch of the shikimate pathway which leads to the aromatic amino acids, phenylalanine and tyrosine. We have isolated a cDNA for this enzyme from the higher plant, Arabidopsis thaliana, by complementing a yeast strain (aro7) with a cDNA library from A. thaliana. This is the first chorismate mutase cDNA isolated from a plant. It encodes a protein of 334 amino acids. The identity of the deduced amino acid sequence is 41% to the chorismate mutase sequence from Saccharomyces cerevisiae. The N-terminal portion of the deduced amino acid sequence has no homology to the S. cerevisiae sequence but resembles known plastid-specific transit peptides. The A. thaliana chorismate mutase expressed in yeast revealed allosteric control by the three aromatic amino acids, as previously described for plastidic chorismate mutase isozymes.

Amino Acid Sequence↗

PubMed1984

Purification and characterization of methylmalonyl-CoA mutase from Ascaris lumbricoides.

Methylmalonyl CoA mutase from Ascaris lumbricoides has been purified to homogeneity. The mutase is homogeneous as judged by equilibrium sedimentation and polyacrylamide gel electrophoresis. The worm mutase is a glycoprotein with a mol. wt of 147,000 +/- 3500 composed of two identical or very similar subunits. One molecule of adenosylcobalamin is tightly bound to each subunit. The mutase from Ascaris is not affected by exposure to light, cyanide ion or intrinsic factor and is not inhibited by iodoacetate and rho-hydroxymercuribenzoate. The kinetic constants of this mutase for (R,S)methylmalonyl CoA are Km = 4.2 X 10(-5) M and Vmax = 4.73 mumol/mg/min.

Amino Acids↗

PubMed2003

Polymorphisms within the vitamin B12 dependent methylmalonyl-coA mutase are not risk factors for neural tube defects.

Methionine synthase and methylmalonyl-CoA mutase (mutase) are the only two known vitamin B(12) (B(12)) dependent enzymes in humans. A lower level of B(12) has been shown to be an independent maternal risk factor for neural tube defects (NTDs) prompting an investigation of common genetic variants within B(12) dependent enzymes. To investigate the role of methylmalonyl-CoA mutase variants we studied 279 complete NTD triads (NTD affected case and both parents) and 256 controls. Based on case-control and family based (transmission disequilibrium test) analyses we did not find an association between the mutase single nucleotide polymorphisms (SNPs) K212K (636A-->G), H532R (1595A-->G) and V671I (2011G-->A) and NTDs. However, there was a significant difference in the frequencies of these polymorphisms between a group of African Americans and American Caucasians (K212K, P=0.002; H532R, P</=0.001; V671I, P=0.006). In conclusion, common variants in the mutase gene do not appear to be risk factors for NTDs but their allele frequencies are significantly different between ethnic groups.

Case-Control Studies↗

PubMed2003

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.

Amino Acid Sequence↗

PubMed1998

Use of site-directed mutagenesis to identify residues specific for each reaction catalyzed by chorismate mutase-prephenate dehydrogenase from Escherichia coli.

Site-directed mutagenesis was performed on the bifunctional enzyme chorismate mutase-prephenate dehydrogenase in order to identify groups important for each of the two reactions. We selected two residues for mutagenesis, Lys37 and His131, identified previously by differential peptide mapping to be essential for activity [Christendat, D., and Turnbull, J. (1996) Biochemistry 35, 4468-4479]. Kinetic studies reveal that K37Q exhibits no mutase activity while retaining wild-type dehydrogenase activity, verifying that Lys37 plays a key role in the mutase. By contrast His131 is not critical for the dehydrogenase; H131A is a reasonably efficient catalyst exhibiting 10% dehydrogenase and 30% mutase activity compared to the wild-type enzyme. Chemical modification of H131A by diethyl pyrocarbonate further inactivated the dehydrogenase, suggesting that a different histidine is now accessible to modification. To identify this group, the protein's remaining eight histidines were changed to alanine or asparagine. A single substitution, H197N, decreased the dehydrogenase activity by 5 orders of magnitude while full mutase activity was retained. In H197N, the Michaelis constants for prephenate and NAD+ and the mutant's elution profile from Sepharose-AMP were similar to those of wild-type enzyme, indicating that catalysis rather than substrate binding is altered. Log V for the dehydrogenase reaction catalyzed by H197N is pH-independent and is in contrast to wild-type enzyme, which shows a decrease in activity at low pH and pK of about 6.5. We conclude that His197 is an essential catalytic residue in the dehydrogenase reaction.

Alanine↗