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R G Matthews

Publications and source records attributed to R G Matthews.

At least 37 records · Page 2Linked to original sources
PubMed1998

Methionine synthase exists in two distinct conformations that differ in reactivity toward methyltetrahydrofolate, adenosylmethionine, and flavodoxin.

Methionine synthase (MetH) from Escherichia coli catalyzes the synthesis of methionine from homocysteine and methyltetrahydrofolate via two methyl transfer reactions that are mediated by the endogenous cobalamin cofactor. After binding both substrates in a ternary complex, the enzyme transfers a methyl group from the methylcobalamin cofactor to homocysteine, generating cob(I)alamin enzyme and methionine. The enzyme then catalyzes methyl transfer from methyltetrahydrofolate to the cob(I)alamin cofactor, forming methylcobalamin cofactor and tetrahydrofolate prior to the release of both products. The cob(I)alamin form of the enzyme occasionally undergoes oxidation to an inactive cob(II)alamin species; the enzyme also catalyzes its own reactivation. Electron transfer from reduced flavodoxin to the cob(II)alamin cofactor is thought to generate cob(I)alamin enzyme, which is then trapped by methyl transfer from adenosylmethionine to the cobalt, restoring the enzyme to the active methylcobalamin form. Thus the enzyme is potentially able to catalyze two methyl transfers to the cob(I)alamin cofactor: methyl transfer from methyltetrahydrofolate during primary turnover and methyl transfer from adenosylmethionine during activation. It has recently been shown that methionine synthase is constructed from at least four separable regions that are responsible for binding each of the three substrates and the cobalamin cofactor, and it has been proposed that changes in positioning of the substrate binding regions vis-à-vis the cobalamin binding region could allow the enzyme to control which substrate has access to the cofactor. In this paper, we offer evidence that methionine synthase exists in two different conformations that interconvert in the cob(II)alamin oxidation state. In the primary turnover conformation, the enzyme reacts with homocysteine and methyltetrahydrofolate but is unreactive toward adenosylmethionine and flavodoxin. In the reactivation conformation, the enzyme is active toward adenosylmethionine and flavodoxin but unreactive toward methyltetrahydrofolate. The two conformations differ in the susceptibility of the substrate-binding regions to tryptic proteolysis. We propose a model in which conformational changes control access to the cobalamin cofactor and are the primary means of controlling cobalamin reactivity in methionine synthase.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1998

Methylenetetrahydrofolate reductase and methionine synthase: biochemistry and molecular biology.

Methylenetetrahydrofolate reductase and cobalamin-dependent methionine synthase catalyze the penultimate and ultimate steps in the biosynthesis of methionine in prokaryotes, and are required for the regeneration of the methyl group of methionine in mammals. Defects in either of these enzymes can lead to hyperhomocysteinemia. The sequences of the human methylenetetrahydrofolate reductase and methionine synthase are now known, and show clear homology with their bacterial analogues. Mutations in both enzymes that are known to occur in humans and to be associated with hyperhomocysteinemia affect residues that are conserved in the bacterial enzymes. Structure/function studies on the bacterial proteins, summarized in this review, are therefore relevant to the function of the human enzymes; in particular studies on the effects of bacterial mutations analogous to those causing hyperhomocysteinemia in human may shed light on the defects associated with these mutations.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1998

Cobalamin-dependent methionine synthase and serine hydroxymethyltransferase: targets for chemotherapeutic intervention?

Chemotherapeutic drugs targeted at folate-dependent reactions have typically been directed at a limited number of target enzymes: dihydrofolate reductase, thymidylate synthase, and GAR and AICAR transformylase. This review discusses two other potential targets for chemotherapeutic inhibition: cobalamin-dependent methionine synthase and serine hydroxymethyltransferase. Brief reviews of the catalytic properties of these two enzymes are presented, and possible strategies for chemotherapeutic intervention are discussed.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1997

Changes in protonation associated with substrate binding and Cob(I)alamin formation in cobalamin-dependent methionine synthase.

Methionine synthase catalyzes the transfer of a methyl group from methylcobalamin enzyme to homocysteine, generating methionine and cob(I)alamin enzyme, and then from methyltetrahydrofolate to cob(I)alamin enzyme, generating tetrahydrofolate and regenerating the methylcobalamin enzyme. The reactions catalyzed by methionine synthase require deprotonation of the substrate, homocysteine, and protonation of the product tetrahydrofolate, with no net change in proton stoichiometry for a complete turnover cycle. In addition, formation of the intermediate cob(I)alamin enzyme requires a change in the cobalt ligand geometry from 6-coordinate to 4-coordinate, and this rearrangement may require the transient protonation of protein residues to stabilize the cob(I)alamin enzyme. In the E. coli enzyme, the lower face of the methylcobalamin cofactor is coordinated by histidine 759, which is hydrogen bonded to aspartate 757 and then to serine 810, forming a "ligand triad". It has previously been shown that reduction of cob(II)alamin enzyme to cob(I)alamin is associated with the uptake of a proton from solution, and it has been postulated that this proton resides within the His759-Asp757 pair. Cob(I)alamin can also be generated by demethylation of methylcobalamin enzyme by homocysteine; it was not known whether this mode of cob(I)alamin formation was associated with proton uptake. In this paper, we use equilibrium titrations and kinetic analyses in the presence of the pH indicator dye phenol red, along with studies of the pH dependence of oxidation/reduction equilibria, to identify and characterize mechanistic steps associated with proton uptake and release in both the turnover and reactivation of the enzyme. We confirm that cob(I)alamin formation by reduction of cob(II)alamin enzyme is associated with proton uptake and show that mutation of Asp757 to Glu abolishes the pH dependence of this reduction. Demethylation of methylcobalamin enzyme also leads to cob(I)alamin formation and is also shown to be associated with proton uptake. By observing pre-steady-state reactions with homocysteine and methyltetrahydrofolate in the presence of phenol red, we show that this proton uptake occurs at a rate that is equal to the rate of formation of the cob(I)alamin enzyme. In addition, we show that binding of homocysteine to the enzyme results in the rapid release of a proton, presumably the homocysteine thiol proton. In contrast, binding methyltetrahydrofolate to the enzyme does not result in proton uptake, suggesting that the proton destined for the product tetrahydrofolate is already present on the free methylcobalamin enzyme.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1997

Cobalamin-dependent methionine synthase from Escherichia coli: involvement of zinc in homocysteine activation.

Methionine synthase (MetH) is a modular protein with at least four distinct regions; amino acids 2-353 comprise a region responsible for binding and activation of homocysteine, amino acids 345-649 are thought to be involved in the binding and activation of methyltetrahydrofolate, amino acids 650-896 are responsible for binding of the prosthetic group methylcobalamin, and amino acids 897-1227 are involved in binding adensylmethionine and are required for reductive activation of enzyme in the cob(II)alamin form. Previous studies have shown that mutations of Cys310 or Cys311 to either alanine or serine result in loss of all detectable catalytic activity. These mutant proteins retain the ability to catalyze methyl transfer from methyltetrahydrofolate to exogenous cob(I)alamin, but have lost the ability to transfer methyl groups from exogenous methylcobalamin to homocysteine [Goulding, C. W., Postigo, D., and Matthews, R. G. (1997) Biochemistry 36, 8082-8091]. We now demonstrate that both MetH holoenzyme and a truncated MetH(2-649) protein, which lacks a cobalamin prosthetic group, contain 0.9 equiv of zinc, while the Cys310Ser and Cys311Ser mutant proteins contain less than 0.05 equiv of zinc. Addition of l-homocysteine to MetH(2-649) is accompanied by release of 1 equiv of protons/mol of protein, while addition of l-homocysteine to the Cys310Ser and Cys311Ser mutant truncated proteins does not result in proton release. The Cys310Ala and Cys311Ala mutant methylcobalamin holoenzymes have completely lost the ability to transfer the methyl group from methylcobalamin to homocysteine, suggesting that zinc is required for this reaction. Further evidence that zinc is required for catalytic activity comes from experiments in which the zinc is removed from MetH(2-1227). Removal of zinc from methylated wild-type holoenzyme by treatment with methyl methanethiolsulfonate and then with dithiothreitol and EDTA results in loss of the ability of the protein to catalyze methyl transfer from methyltetrahydrofolate to homocysteine. Reconstitution of the zinc-depleted holoenzyme results in incorporation of 0.4 equiv of zinc/mol of protein and partial restoration of the ability of the protein to catalyze homocysteine methylation.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1997

A nucleoprotein activation complex between the leucine-responsive regulatory protein and DNA upstream of the gltBDF operon in Escherichia coli.

The global regulator Lrp (leucine-responsive regulatory protein), in some cases modulated by its co-regulator leucine, has been shown to regulate more than 40 genes and operons in Escherichia coli. Leucine modulates Lrp regulation of leucine-responsive operons. The level of sensitivity of these operons to leucine varies greatly, but the basis for this variation is only partially understood. One operon controlled by Lrp that is relatively insensitive to leucine is gltBDF, which includes genes specifying the large (GltB) and small (GltD) subunits of glutamate synthase. Earlier gel mobility shift assays have demonstrated that Lrp binds to a fragment of DNA containing the gltBDF promoter region. To further define the nature of this Lrp-gltBDF interaction, DNase I footprinting experiments were performed. The results indicate that Lrp binds cooperatively to three sites quite far upstream, spanning the region from -140 to -260 base-pairs relative to the start of transcription. Phased hypersensitivity is observed throughout the entire binding region, suggesting that Lrp bends the DNA. To determine the relative importance of these three sites for the transcriptional activation of gltBDF, a series of site-directed mutations was generated. The effects of these mutations on Lrp binding were determined both by DNase I footprinting and by quantitative mobility shift assays, while their effects on transcription in vivo were examined by measuring beta-galactosidase activity levels of chromosomal gltB::lacZ operon fusions. Our results indicate that all three sites are required for maximal gene expression, as is the proper phasing of the sites with one another and with the start of transcription. Our results suggest that Lrp binds a central palindromic site, interacting predominantly with the major groove of its DNA target, and that additional dimers bind to flanking sites to form a nucleoprotein activation complex.

Bacterial Proteins↗
PubMed1997

Cobalamin-dependent methionine synthase is a modular protein with distinct regions for binding homocysteine, methyltetrahydrofolate, cobalamin, and adenosylmethionine.

Methionine synthase (MetH) catalyzes the transfer of a methyl group from bound methylcobalamin to homocysteine, yielding enzyme-bound cob(I)alamin and methionine. The cofactor is then remethylated by methyltetrahydrofolate. We now demonstrate that MetH is able to catalyze methylation of free cob(I)alamin with methyltetrahydrofolate. MetH had previously been shown to catalyze methylation of homocysteine with free methylcobalamin as the methyl donor, in a reaction that is first-order in added methylcobalamin, and we have confirmed this observation using homogenous enzyme. A truncated polypeptide lacking the cobalamin-binding region of the holoenzyme, MetH(2-649), was overexpressed and purified to homogeneity. MetH(2-649) catalyzes the methylation of free cob(I)alamin by methyltetrahydrofolate and the methylation of homocysteine by free methylcobalamin. Furthermore, a protein comprising residues 2-353 of the holoenzyme has now been overexpressed and purified to homogeneity, and this protein catalyzes methyl transfer from free methylcobalamin to homocysteine but not from methyltetrahydrofolate to free cob(I)alamin. The mutations Cys310Ala and Cys311Ala in MetH(2-649) completely abolish methyl transfer from exogenous methylcobalamin to homocysteine but do not affect methyl transfer from methyltetrahydrofolate to exogenous cob(I)alamin, consistent with a modular construction for MetH. We infer that MetH is a modular protein comprising four separate regions: a homocysteine binding region (residues 2-353), a methyltetrahydrofolate binding region (residues 354-649), a region responsible for binding the cobalamin prosthetic group (residues 650-896), and an AdoMet-binding domain (residues 897-1227).

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1997

Interaction of Escherichia coli cobalamin-dependent methionine synthase and its physiological partner flavodoxin: binding of flavodoxin leads to axial ligand dissociation from the cobalamin cofactor.

Cobalamin-dependent methionine synthase from Escherichia coli catalyzes the last step in de novo methionine biosynthesis. Conversion of the inactive cob(II)alamin form of the enzyme, formed by the occasional oxidation of cob(I)alamin during turnover, to an active methylcobalamin-containing form requires a reductive methylation of the cofactor in which an electron is supplied by reduced flavodoxin and the methyl group is derived from S-adenosyl-L-methionine. E. coli flavodoxin acts specifically in this activation reaction, and neither E. coli ferredoxin nor flavodoxin from the cyanobacterium Synechococcus will substitute, despite their highly similar midpoint potentials for one-electron transfer. As assessed by EPR spectroscopy, the binding of flavodoxin to cob(II)alamin methionine synthase results in a change in the coordination geometry of the cobalt from five-coordinate to four-coordinate. Histidine 759 of methionine synthase, which replaces the normal lower ligand dimethylbenzimidazole on binding of methylcobalamin to methionine synthase, is dissociated from the cobalt of the cobalamin by the binding of flavodoxin. The association of flavodoxin and methionine synthase depends on ionic strength and pH; the pH dependence corresponds to the uptake of one proton on association. The formation of a complex between flavodoxin and methionine synthase perturbs the midpoint potentials of the flavin and cobalamin cofactors only marginally and without any significant thermodynamic advantage for electron transfer to the cobalamin of methionine synthase. No significant binding was seen between oxidized flavodoxin and methylcobalamin methionine synthase. A model for the interaction of methionine synthase with flavodoxin is proposed in which flavodoxin binding leads to changes in the distribution of methionine synthase conformations.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1997

Enzyme-catalyzed methyl transfers to thiols: the role of zinc.

Zinc has been identified as a cofactor in a growing number of proteins that utilize thiols as nucleophiles, including proteins that catalyze the transfer of methyl groups to thiols. The latter category includes the Ada protein involved in the response of E. coli to DNA alkylation, cobalamin-independent and cobalamin-dependent methionine synthase, and enzymes involved in the formation of methylcoenzyme M in methanogenesis. Farnesyl-protein transferase and geranylgeranyl-protein transferase also contain zinc and an X-ray structure of farnesyl-protein transferase has recently been determined. Within the past year, studies on the role of zinc in these proteins and in model compounds have shown that the thiol substrates are coordinated to the zinc as thiolates, suggesting a role for zinc in maintenance of thiol reactivity at neutral pH.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1997

Use of an inducible regulatory protein to identify members of a regulon: application to the regulon controlled by the leucine-responsive regulatory protein (Lrp) in Escherichia coli.

Procedures were developed to facilitate the identification of genes that belong to a given regulon and characterization of their responses to the regulator. The regulon controlled by the Escherichia coli leucine-responsive regulatory protein (Lrp) was studied by isolating random transcriptional fusions to lacZ, using lambda placMu53 and a strain in which lrp is under isopropylthio-beta-D-galactopyranoside (IPTG)-inducible control. Fusions exhibiting IPTG-responsive beta-galactosidase activity were cloned by integrating the suicide vector pIVET1 via homologous recombination at lacZ, followed by self-ligating digested chromosomal DNA. We verified the patterns of lacZ expression after using the plasmid clones to generate merodiploid strains with interrupted and uninterrupted copies of the same sequence. If the merodiploid expression pattern was unchanged from that shown by the original fusion strain, then the cloned fusion was responsible for the regulatory pattern of interest; a difference in the expression pattern could indicate that the original strain carried multiple fusions or that there were autogenous effects of having interrupted the fused gene. Using these procedures, we generated a fusion library of approximately 5 x 10(6) strains; approximately 3,000 of these strains were screened, yielding 84 Lrp-responsive fusions, and 10 of the 84 were phenotypically stable and were characterized. The responses of different fusions in a given operon to in vivo Lrp titrations revealed variations in expression with the position of insertion. Among the newly identified members of the regulon is an open reading frame (orf3) between rpiA and serA. Also, expression of a fusion just downstream of dinF was found to be Lrp dependent only in stationary phase.

Bacterial Proteins↗
PubMed1997

Structure-based perspectives on B12-dependent enzymes.

Two X-ray structures of cobalamin (B12) bound to proteins have now been determined. These structures reveal that the B12 cofactor undergoes a major conformational change on binding to the apoenzymes of methionine synthase and methylmalonyl-coenzyme A mutase: The dimethylbenzimidazole ligand to the cobalt is displaced by a histidine residue from the protein. Two methyltransferases from archaebacteria that catalyze methylation of mercaptoethanesulfonate (coenzyme M) during methanogenesis have also been shown to contain histidine-ligated cobamides. In corrinoid iron-sulfur methyltransferases from acetogenic and methanogenic organisms, benzimidazole is dissociated from cobalt, but without replacement by histidine. Thus, dimethylbenzimidazole displacement appears to be an emerging theme in cobamide-containing methyltransferases. In methionine synthase, the best studied of the methyltransferases, the histidine ligand appears to be required for competent methyl transfer between methyl-tetrahydrofolate and homocysteine but dissociates for reductive reactivation of the inactive oxidized enzyme. Replacement of dimethylbenzimidazole by histidine may allow switching between the catalytic and activation cycles. The best-characterized B12-dependent mutases that catalyze carbon skeleton rearrangement, for which methylmalonyl-coenzyme A mutase is the prototype, also bind cobalamin cofactors with histidine as the cobalt ligand, although other cobalamin-dependent mutases do not appear to utilize histidine ligation. It is intriguing to find that mutases, which catalyze homolytic rather than heterolytic cleavage of the carbon-cobalt bond, can use this structural motif. In methylmalonylCoA mutase a significant feature, which may be important in facilitating homolytic cleavage, is the long cobalt-nitrogen bond linking histidine to the co-factor. The intermediate radical species generated in catalysis are sequestered in the relatively hydrophilic core of an alpha/beta barrel domain of the mutase.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1996

The structure of the C-terminal domain of methionine synthase: presenting S-adenosylmethionine for reductive methylation of B12.

BACKGROUND: In both mammalian and microbial species, B12-dependent methionine synthase catalyzes methyl transfer from methyltetrahydrofolate (CH3-H4folate) to homocysteine. The B12 (cobalamin) cofactor plays an essential role in this reaction, accepting the methyl group from CH3-H4folate to form methylcob(III)alamin and in turn donating the methyl group to homocysteine to generate methionine and cob(I)alamin. Occasionally the highly reactive cob(I)alamin intermediate is oxidized to the catalytically inactive cob(II)alamin form. Reactivation to sustain enzyme activity is achieved by a reductive methylation, requiring S-adenosylmethionine (AdoMet) as the methyl donor and, in Esherichia coli, flavodoxin as an electron donor. The intact system is controlled and organized so that AdoMet, rather than methyltetrahydrofolate, is the methyl donor in the reactivation reaction. AdoMet is not wasted as a methyl donor in the catalytic cycle in which methionine is synthesized from homocysteine. The structures of the AdoMet binding site and the cobalamin-binding domains (previously determined) provide a starting point for understanding the methyl transfer reactions of methionine synthase. RESULTS: We report the crystal structure of the 38 kDa C-terminal fragment of E.coli methionine synthase that comprises the AdoMet-binding site and is essential for reactivation. The structure, which includes residues 901-1227 of methionine synthase, is a C-shaped single domain whose central feature is a bent antiparallel betasheet. Database searches indicate that the observed polypeptide has no close relatives. AdoMet binds near the center of the inner surface of the domain and is held in place by both side chain and backbone interactions. CONCLUSIONS: The conformation of bound AdoMet, and the interactions that determine its binding, differ from those found in other AdoMet-dependent enzymes. The sequence Arg-x-x-x-Gly-Tyr is critical for the binding of AdoMet to methionine synthase. The position of bound AdoMet suggests that large areas of the C-terminal and cobalamin-binding fragments must come in contact in order to transfer the methyl group of AdoMet to cobalamin. The catalytic and activation cycles may be turned off and on by alternating physical separation and approach of the reactants.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1996

Cobalamin-independent methionine synthase from Escherichia coli: a zinc metalloenzyme.

Cobalamin-independent methionine synthase (MetE) from Escherichia coli catalyzes the transfer of a methyl group from methyltetrahydrofolate to homocysteine. Previous work had shown the existence of a reactive thiol group, cysteine 726, whose alkylation led to loss of all detectable enzymatic activity [González, J.C., et al. (1992) Biochemistry 31, 6045-6056]. A site-directed mutation of MetE, Cys726Ser, was constructed to investigate the possible role of this cysteine. The Cys726Ser protein was purified to homogeneity, affording a protein with no detectable activity. To assess the possibility that cysteine726 functions as a metal ligand, inductively coupled plasma-atomic emission spectrometry was performed. The wild-type enzyme contains 1.02 equiv of zinc per subunit; the Cys726Ser mutant does not contain zinc, supporting the view that cysteine726 is required for metal binding. A loss of enzymatic activity is observed upon removal of zinc from the wild-type MetE by incubation in urea and EDTA; activity can subsequently be restored by zinc reconstitution, suggesting that zinc is required for catalysis. Circular dichroism measurements further suggest that there are no major differences in the secondary structures of the wild-type and the Cys726Ser mutant enzymes. Extended X-ray absorption fine structure analysis has established that the average zinc environment is different in the presence of homocysteine than in its absence and is consistent with the changes expected for displacement of an oxygen or nitrogen ligand by the sulfur of homocysteine. A possible model for zinc-dependent activation of homocysteine by MetE is presented.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
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↗
PubMed1996

The reactivity of B12 cofactors: the proteins make a difference.

Determination of the structure of intact methylmalonyl-CoA mutase from Propionibacterium shermanii, and comparisons with the structure of the cobalamin-binding fragment of methionine synthase from Escherichia coli, afford a first glimpse at the similarities and distinctions between the two principal classes of B12-dependent enzymes: the mutases and the methyltransferases.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1996

A synthetic module for the metH gene permits facile mutagenesis of the cobalamin-binding region of Escherichia coli methionine synthase: initial characterization of seven mutant proteins.

Cobalamin-dependent methionine synthase from Escherichia coli is a monomeric 136 kDa protein composed of multiple functional regions. The X-ray structure of the cobalamin-binding region of methionine synthase reveals that the cofactor is sandwiched between an alpha-helical domain that contacts the upper face of the cobalamin and an alpha/beta (Rossmann) domain that interacts with the lower face. An unexpected conformational change accompanies binding of the methylcobalamin cofactor. The dimethylbenzimidazole ligand to the lower axial position of the cobalt in the free cofactor is displaced by histidine 759 from the Rossmann domain [Drennan, C. L., Huang, S., Drummond, J. T., Matthews, R. G., & Ludwig, M. L. (1994) Science 266, 1669]. In order to facilitate studies of the roles of amino acid residues in the cobalamin-binding region of methionine synthase, we have constructed a synthetic module corresponding to nucleotides (nt) 1741-2668 in the metH gene and incorporated it into the wild-type metH gene. This module contains unique restriction sites at approximately 80 base pair intervals and was synthesized by overlap extension of 22 synthetic oligonucleotides ranging in length from 70 to 105 nt and subsequent amplification using two sets of primers. Expression of methionine synthase from a plasmid containing the modified gene was shown to be unaffected by the introduction of the synthetic module. E. coli does not synthesize cobalamin, and overexpression of MetH holoenzyme requires accelerated cobalamin transport. Growth conditions are described that enable the production of holoenzyme rather than apoenzyme. We describe the construction and initial characterization of seven mutants. Four mutations (His759Gly, Asp757Glu, Asp757Asn, and Ser810Ala) alter residues in the hydrogen-bonded network His-Asp-Ser that connects the histidine ligand of the cobalt to solvent. Three mutations (Phe708Ala, Phe714Ala, and Leu715Ala) alter residues in the cap region that covers the upper face of the cobalamin. The His759Gly mutation has profound effects, essentially abolishing steady-state activity, while the Asp757, Ser810, Phe708, and Leu715 mutations lead to decreases in activity. These mutations asses the importance of individual residues in modulating cobalamin reactivity.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗
PubMed1996

Mutations in the B12-binding region of methionine synthase: how the protein controls methylcobalamin reactivity.

Vitamin B12-dependent methionine synthase catalyzes the transfer of a methyl group from methyltetrahydrofolate to homocysteine via the enzyme-bound cofactor methylcobalamin. To carry out this reaction, the enzyme must alternately stabilize six-coordinate methylcobalamin and four-coordinate cob(I)alamin oxidation states. The lower axial ligand to the cobalt in free methylcobalamin is the dimethylbenzimidazole nucleotide substituent of the corrin ring; when methylcobalamin binds to methionine synthase, the ligand is replaced by histidine 759, which in turn is linked by hydrogen bonds to aspartate 757 and thence to serine 810. We have proposed that these residues control the reactivity of the enzyme-bound cofactor both by increasing the coordination strength of the imidazole ligand and by allowing stabilization of cob(I)alamin via protonation of the His-Asp-Ser triad. In this paper we report results of mutation studies focusing on these catalytic residues. We have used visible absorbance spectroscopy and electron paramagnetic resonance spectroscopy to probe the coordination state of the cofactor and have used stopped-flow kinetic measurements to explore the reactivity of each mutant. We show that mutation of histidine 759 blocks turnover, while mutations of aspartate 757 or serine 810 decrease the reactivity of the methylcobalamin cofactor. In contrast, we show that mutations of these same residues increase the rate of AdoMet-dependent reactivation of cob(II)alamin enzyme. We propose that the reaction with AdoMet proceeds via a different transition state than the reactions with homocysteine and methyltetrahydrofolate. These results provide a glimpse at how a protein can control the reactivity of methylcobalamin.

5-Methyltetrahydrofolate-Homocysteine S-Methyltran↗