5-Methyltetrahydrofolate-homocysteine methyltransferase

Researchers studying the metalloenzyme hydrogenase would like to design small compounds that mimic this enzyme s ability to reversibly reduce protons to H2 and H2 to 2H+, using an active center that contains iron and nickel. Cobalamins (vitamin and its derivatives) contain an easily activated Co-C bond that has a number of biological functions, one of which is as a methyl transferase, 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR). This enzyme converts homocysteine (an amino acid that has one more CH2 group in its alkyl side chain than cysteine see Figure 2.2) to methionine as methylcobalamin is converted to cobalamin. 

NO is known to react with the cobalt of cobalamins (Cbl). The structure of cobalamin is presented in Scheme 7. Cobalamins containing various axial ligands tram to dimethylbenzimidazole moiety are known as vitamin B12 and are important cofactors for 5-methyltetrahydrofolate-homocysteine methyltransferase and methylmalonyl-coA mutase playing a key role in the normal functioning of the brain and nervous system and in red blood cell formation [330, 331]. 

Homocysteine lies at a metabolic crossroad it may condense with serine to form cystathionine, or it may undergo remethylation, thereby conserving methionine. There are two pathways for remethylation in humans. In one, betaine provides the methyl groups, while in the other 5-methyltetrahydrofolate is the methyl donor. This latter reaction is catalyzed by a Bj -containing enzyme, 5-methyltetrahydrofolate homocysteine methyltransferase. Two defects in this latter mechanism may account for the inability to carry out remethylation. In one of them, patients are unable to synthesize or accumulate methylcobalamin, while others cannot produce the second cofactor, 5 -methyltetrahydrofolate, because of adefect in 5,10-methylenetrahydrofolate reductase. 

Fig. 20.3 Pathway of methionine metabolism. The numbers represent the following enzymes or sequences (1) methionine adenosyltransferase (2) S-adenosylmethionine-dependent transmethylation reactions (3) glycine methyltransferase (4) S-adenosylhomocysteine hydrolase (5) betaine-homocysteine methyltransferase (6) 5-methyltetrahydrofolate homocysteine methyltransferase (7) serine hydroxymethyltransferase (8) 5,10-methylenetetrahydrofolate reductase (9) S-adenosylmethionine decarboxylase (10) spermidine and spermine synthases (11) methylthio-adenosine phosphorylase (12) conversion of methylthioribose to methionine (13) cystathionine P-synthase (14) cystathionine y-lyase (15) cysteine dioxygenase (16) cysteine suplhinate decarboxylase (17) hypotaurine NAD oxidoreductase (18) cysteine sulphintite a-oxoglutarate aminotransferase (19) sulfine oxidase. MeCbl = methylcobalamin PLP = pyridoxal phosphate...

N -Methyl-FH4 serves as a source of methyl groups for conversion of L-homocysteine to L-me-thionine (5-methyltetrahydrofolate-homocysteine methyltransferase, EC 2.1.1.13) (see L-Methio-nine). 

Methylcobalamin is involved in a critically important physiological transformation, namely the methylation of homocysteine (8) to methionine (9) (eq. 2) catalyzed by A/ -methyltetrahydrofolate homocysteine methyltransferase. The reaction sequence involves transfer of a methyl group first from... 

The fibroblasts do not convert cyanocobalamin or hydroxocobalamin to methylcobalamin or adenosyl-cobalamin, resulting in diminished activity of both N5-methyltetrahydrofolate homocysteine methyltransferase and methylmalonyl-CoA mutase. Supplementation with hydroxocobalamin rectifies the aberrant biochemistry. The precise nature of the underlying defect remains obscure. Diagnosis should be suspected in a child with homocystinuria, methylmalonic aciduria, megaloblastic anemia, hypomethioninemia and normal blood levels of folate and vitamin B12. A definitive diagnosis requires demonstration of these abnormalities in fibroblasts. Prenatal diagnosis is possible. 

N -Methyltetrahydrofolate-Homocysteine Methyltransferases Robert T. Taylor and Herbert Weissbach... 

Serine-hydroxymethyl transferase, methylenetetrahydrofolate reductase, and methyltetrahydrofolate-homocysteine methyltransferase, mechanism of biological methylation with 90CRV1275. 

Reduced serum folate concentrations have been demonstrated in patients with homocystinuria taking pyridoxine. The mechanism of this effect may involve removal of substrate inhibition of the enzyme, A5-methyltetrahydrofolate homocysteine methyltransferase, due to pyridoxine-induced reduction of the substrate, homocysteine (27). 

Homocystinuria is a biochemical abnormality caused either by a deficiency of cystathionine P-syn-thase or impaired activity of N -methyltetrahydrofolate-homocysteine methyltransferase. The classical homocystinuria occurs when the conversion of homocysteine to cystathionine is limited by a deficiency of cystathionine P-synthase, with accumulation of methionine and homocysteine and a decrease in cysteine. 

C. N -Methyltetrahydrofolate-Homocysteine Methyltransferases and de novo Synthesis of the Methyl... 

Methylcobalamin-dependent reactions, involved in the synthesis of methionine in animals and micro-organisms, and in the formation of acetate and methane in bacteria, have been reviewed by Poston and Stadtman (43) and more recently by Taylor (40). As stated before, methylcobalamin is formed from reduced Co -cobalamin in the course of the methyltetrahydrofolate homocysteine methyltransferase reaction (Fig. 6). Presumably, the Co -cobalamin is bound by the apoen-... 

K. Sato, E. Hiei, S. Shimizu, and R. Abeles (1978), Affinity chromatography of V -methyltetrahydrofolate-homocysteine methyltransferase on a cobalamin-Sepharose. FEES Lett. 85, 73-76. 

Cobalamin is a complex molecule containing a Co atom. In the mamalian synthesis of methionine, cobalamin acts as a coenzyme by accepting the methyl group from N5-methyltetrahydrofolate and transferring it to homocysteine. The reaction is catalyzed by cobalamin-N%-methyl-THF homocysteine methyltransferase. The overall reaction is... 

Methionine can be regenerated by the transfer of a methyl group to homocysteine fromTV -methyltetrahydrofolate, a reaction catalyzed by methionine synthase (also known as homocysteine methyltransferase). 

Figure 8 Extended folate metabolism, including compartmentation. MTHFR, methylenetetrahydrofolate reductase SHMT, serine hydroxymethyltransferase BHMT, betaine homocysteine methyltransferase, MAT, methionine adenosyltransferase SAH-hydrolase, S-adenosylhomocysteine hydrolase MT, methyltransferase CBS, cystathionine /i-synthase SAM, S-adenosylmethionine SAH, S-aden-osylhomocysteine THF, tetrahydrofolate and 5-MeTHF, 5-methyltetrahydrofolate. (Reproduced from Van der Put etal. (2001) Folate, homocysteine and neural tube defects An overview. Experimental Biology and Medicine 226 243-270.)...

Homocysteine can be recycled back to methionine either by transfer of a methyl group from betaine catalyzed by betaine-homocysteine methyltransferase, or from N -methyltetrahydrofolate(N -methyl-FH4)catalyzedbyN -methyl-FH4-methyltransferase, which requires methyl cobalamin ... 

Transmethylation Methionine adenosyl transferase Betaine-homocysteine methyltransferase iV -methyltetrahydrofolate-homocysteine 5-methyltransferase... 

Because of the complexity of the enzymatic systems involved in coenzyme Bi2 chemistry there are several reports on the purification of B 12-dependent enzymes or B 12-binding proteins by vitamin B12 affinity adsorbents. In fact, for purification of enzymes or proteins, affinity chomatography has been widely used as one of the most attractive methods (270). For that purpose, the synthesis of a cobalamin-Sepharose insoluble support has been prepared and applied to the purification of iV -methyltetrahydrofolate-homocysteine cobalamin methyltransferase from E. coll The scheme for the synthesis of the solid support is summarized in Fig. 6.14. 

A relatively large number of agents have been utilized to treat this intractable disorder folinic acid (5-formyl-tetrahydrofolic acid), folic acid, methyltetrahydrofolic acid, betaine, methionine, pyridoxine, cobalamin and carnitine. Betaine, which provides methyl groups to the beta i ne ho mocystei ne methyltransferase reaction, is a safe treatment that lowers blood homocysteine and increases methionine. 

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... 

This cobalamin-dependent enzyme [EC 2.1.1.13], also known as methionine synthase and tetrahydropteroyl-glutamate methyltransferase, catalyzes the reaction of 5-methyltetrahydrofolate with L-homocysteine to produce tetrahydrofolate and L-methionine. Interestingly, the bacterial enzyme is reported to require 5-adenosyl-L-methionine and FADH2. See also Tetrahydropteroyl-triglutamate Methyltransferase... 

In contrast to coenzyme Bi2, where the alkyl moiety serves purely in a catalytic role, the alkyl group of methyl cobamides (MeCba s) is utilized as a reagent by MeCba-dependent enzymes it is only the cobamide portion of the coenzyme that is catalytic. The cobamide-dependent methyl transferases have been reviewed [11,24-27,165], Three cobamide-dependent methyl transferases have been studied in some cases, more than one protein is required. The Bi2 proteins include methionine synthase (officially called 5-methyltetrahydrofolate-L-homocysteine-S-methyltransferase [HCM] EC 2.1.1.13) MeCba-dependent enzyme from Meth-anosarcina barkeri (MT 0 and the corrinoid/Fe-S protein from Clostridium ther-moaceticum. 

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