Homocysteine 5-methyltetrahydrofolate

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. 

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. 

Methylation of homocysteine by 5-methyltetrahydrofolate-homocysteine methyl reductase depends on an adequate supply of 5-methyltetrahydrofoIate. The unmethylated folate is recycled in a cobalamin-dependent pathway, by remethylation to 5,10-methylene-tetrahydrofolate, and subsequent reduction to 5-methyltetrahydrofolate. The transferase enzyme, also named 5,10-methyltretrahydrofolate reductase catalyzes the whole cycle [3,91]. S-adenosylmethionine and 5-methyltetrahydrofolate are the most important methyl unit donors in biological system. S-adenosylmethionine is reported to regulate methylation and transsulfuration pathways in the homocysteine metabolism [3,91]. 

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... 

N5-Methyltetrahydrofolate homocysteine methyl-transferase (= methionine synthase). This reaction is essential to restore tetrahydrofolate from N5-methyltetrahydrofolate (Fig. 2). 

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. 

This cofactor is involved in the synthesis of acetic acid from C02,869 methionine from homocysteine (catalyzed by IV-methyltetrahydrofolate-homocysteine methyl transferase)870 and in the... 

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. 

Methylcobalamia is iavolved ia a critically important physiological transformation, namely the methylation of homocysteine (8) to methionine (9) (eq. 2) catalyzed by A/ -methyltetrahydrofolate homocysteine methjitransferase. The reaction sequence involves transfer of a methji group first from A/5 -methjltetrahydrofolate to cobalamin (yielding methjicobalamin) and thence to homocysteine. Once again, the intimate details of the reaction are not weU known (31). Demethylation of tetrahydrofolate to tetrahydrofohc acid is a step in the formation of thymidine phosphate, in turn requited for DNA synthesis. In the absence of the enzyme, excess RNA builds up in ted blood cells. 

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. 

Defective Activity of N -Methyltetrahydrofolate Homocysteine Methyl-Transferase and Cobalamin Activation... 

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

Draper et al. (101) described an assay for /Vs-methyltetrahydrofolate-homocysteine transmethylase activity using instant TLC which permits the simultaneous determination of the newly synthesized methionine as well as the residual /Vs-methyltetrahydrofolate. 

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-... 

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. 

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. 

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