Methionine synthase reductase

Flavin Mononucleotide (FMN) Methionine synthase reductase, Chorismate synthase... 

The best characterized B 12-dependent methyltransferases is methionine synthase (Figure 15.11) from E. coli, which catalyses the transfer of a methyl group from methyltetrahydrofolate to homocysteine to form methionine and tetrahydrofolate. During the catalytic cycle, B12 cycles between CH3-Co(in) and Co(I). However, from time to time, Co(I) undergoes oxidative inactivation to Co(II), which requires reductive activation. During this process, the methyl donor is S-adenosylmethionine (AdoMet) and the electron donor is flavodoxin (Fid) in E. coli, or methionine synthase reductase (MSR) in humans. Methionine synthase... 

The loss of a methyl group from AdoMet in each of the reactions yields S-ad-enosylhomocysteine (AdoHcy) and this is subsequently hydrolysed to adenosine and Hey by AdoHcy-hydrolase. Hey sits at a metabolic branch point and can be remethylated to methionine by way of two reactions. One is the 5-methyltetrahydrofo-late dependent reaction catalysed by methionine synthase, which itself is reductively methylated by cobalamin (vitamin B12) and AdoMet, requiring methionine synthase reductase. 5-Methyltetrahydrofolate is generated from 5,10-methylenetetrahydrofo-late (MTHF) by MTHF reductase. The second remethylation reaction is catalysed by betaine methyltransferase, which is restricted to the liver, kidney and brain, while methionine synthase is widely distributed. 

A large elevation of Hey in body fluids and tissues is found in several genetic enzyme deficiencies, the homocystinurias. These include cystathionine /3-synlhase deficiency [9], the remethylation defects due to deficiency of MTHF reductase [10], methionine synthase and methionine synthase reductase deficiencies, as well as defects of intracellular cobalamin metabolism [11], namely the cblF, cblC and cblD defects. It is noteworthy that low levels of total Hey (tHcy) have been described in sulphite oxidase deficiency [12]. 

Figure 21-2. Metabolism of homocysteine. BHMT, betaineihomocysteine methyl-transferase CBS, cystathionine P-synthase Cob, cobalamin CTH, cystathionine y-lyase DHF, dihydrofolate DMG, dimethylglycine FAD, flavin adenine dinucleotide MAT, methionine adenosyltransferase 5-MTHF, 5-methyltetrahydrofolate 5,10-MTHF, 5,10-methylenetetrahydrofolate MTHFR, methylenetetrahydrofolate reductase MS, methionine synthase MTRR, methionine synthase reductase MTs, methyl transferases PLE pyridoxal phosphate SAH, S-adenosylhomocysteine SAHH, SAH hydrolase SAM, 5-adenosylmethionine SHMT, serine hydroxymethyltransferase THF, tetrahydrofolate Zn, zinc.

Figure 21-3. The methionine synthase reaction. Methionine synthase catalyzes the remethylation of homocysteine to methionine. In the first half reaction (1), a methyl group is transferred from 5-methyl tetrahydrofolate (5-MTHF) to the reduced form of cobalamin [Cob(I)], generating methyl-cobalamin [Methyl-Cob(III)] and tetrahydrofolate (THF). During the second half reaction (2), the methyl group is transferred from methylcobalamin to homocysteine, generating methionine. During the catalytic reaction, Cob(I) occasionally becomes oxidized, producing an inactive form of cobalamin, cob(II)alamin [Cob(II)]. The enzyme methionine synthase reductase (MTRR) then reactivates Cob(II) through reductive methylation, producing methyl-Cob(III). SAM, 5-adenosylmethionine SAH, 5-adeno-sylhomocysteine.

Methionine synthase is composed of five structural domains that provide for binding of its substrate HCY, the methyl donor 5-methyItetrahydrofolate, cobal-amin, and SAM (Fig. 4). In most tissues SAM is utilized to methylate oxidized cobalamin, in conjunction with electron donation by methionine synthase reductase, thereby restoring methylcobalamin and allowing resumption of activity. This mode of reactivation is required approximately every 100-1,000 turnovers, even under strictly anaerobic laboratory conditions (Bandarian et al., 2003). Under physiological conditions, oxidation of cobalamin is undoubtedly much more common, illustrating how vitamin B12 serves as a sensor of redox status. During oxidative stress, cobalamin is more frequently oxidized and more HCY is diverted toward cysteine and GSH synthesis. 

Figure 4.3. Schematic outline of the diflavin reductase family. Members contain an N-terminal FMN-binding flavodoxin-like domain and a C-terminal FAD/NADPH-binding ferredoxin reductase-like domain, which contains an additional linker region. Shown are CPR, which has an amino-terminal membrane anchor region (Anc) NRl (Novel reductase 1) MSR (methionine synthase reductase), which contains an additional inlerdomain sequence P450 BM3, which is fused to a P450 domain and NOS (nitric oxide synthases), which has linked to a heme-containing oxygenase domain that is structurally distinct from the P450s.

Leclerc, D., A. Wilson, R. Dumas, C. Gafuik, D. Song, D. Watkins et al. (1998). Cloning and mapping of a cDNA for methionine synthase reductase, a flavoprotein defective in patients with homocystinuria. Proc. Natl. Acad. Sci. USA 95, 3059-3064. 

Wolthers, K.R., J. Basran, A.W. Munro, and N.S. Scrutton (2003). Molecular dissection of human methionine synthase reductase Determination of the flavin redox potentials in full-length enzyme and isolated flavin-binding domains. Biochemistry 42, 3911-3920. 

Wolthers KR, Lou X, Toogood HS, Leys D, Scrutton NS (2007) Mechanism of coenzyme binding to human methionine synthase reductase revealed through the crystal structure of the FNR-like module and isothermal titration calorimetry. Biochemistry 46 11833-11844... 

Figure 44.1 Folate-mediated one carbon metabolism network. Enzymes and transport proteins are enclosed in rectangular boxes. AHCY S-adenosyDiomocys-teine hydrolase AICART 5-aminoimidazole carboxamide ribonucleotide transferase BHMT betaine homocysteine methyltransferase CBS cystathionine beta-synthase DHFR dihydrofolate reductase FR folate receptor FTCD formimidoyltransferase cyclodeaminase GART glycinamide ribonucleotide transformylase MATs (MATI/MATIII) adenosylmethionine transferase enzyme I/III MS methionine synthase MSR methionine synthase reductase MT methyltransferase MTHFD methylenetetrahydrofolate dehydrogenase MTHFR 5,10-methylenete-trahydrofolate reductase MTHFS 5,10-methylenetetrahydrofolate synthase. RFC reduced folate AdoMet 5-adenosylmethionine AdoHcy S-adenosylhomocysteine Hey homocysteine SHMT serine hydroxymethyltransferase TS thymidylate synthase.

Table 10J.2. Methionine synthase reductase deficiency (clbE) (approx. 12 patients)...

Methionine synthase deficiency (incl. methylmalonyl CoA mutase def.) 7.9/10.8 Methionine synthase reductase deficiency 10.7.2... 

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