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Homocysteine (left) and methionine (right) are related by demethylation and remethylation. Remethylation is a major step in the conversion of homocysteine to the essential amino acid methionine . The remethylation process involves the enzyme methionine synthase (MS), which requires vitamin B 12 as a cofactor, and also depends indirectly on ...
In the body, homocysteine can be recycled into methionine or converted into cysteine with the aid of vitamin B 6, B 9, and B 12. [ 3 ] High levels of homocysteine in the blood ( hyperhomocysteinemia ) is regarded as a marker of cardiovascular disease, likely working through atherogenesis , which can result in ischemic injury .
The production of homocysteine through transsulfuration allows the conversion of this intermediate to methionine, through a methylation reaction carried out by methionine synthase. The reverse pathway is present in several organisms, including humans, and involves the transfer of the thiol group from homocysteine to cysteine via a similar ...
Methionine synthase catalyzes the final step in the regeneration of methionine (Met) from homocysteine (Hcy). Both the cobalamin-dependent and cobalamin-independent forms of the enzyme carry out the same overall chemical reaction, the transfer of a methyl group from 5-methyltetrahydrofolate (N 5 -MeTHF) to homocysteine, yielding ...
This is hydrolysed to homocysteine and adenosine by S-adenosylhomocysteine hydrolase EC 3.3.1.1 and the homocysteine recycled back to methionine through transfer of a methyl group from 5-methyltetrahydrofolate, by one of the two classes of methionine synthases (i.e. cobalamin-dependent (EC 2.1.1.13) or cobalamin-independent (EC 2.1.1.14)).
Methylenetetrahydrofolate reductase (MTHFR) is the rate-limiting enzyme in the methyl cycle, and it is encoded by the MTHFR gene. [5] Methylenetetrahydrofolate reductase catalyzes the conversion of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, a cosubstrate for homocysteine remethylation to methionine.
Betaine (N,N,N-trimethylglycine) is used to reduce concentrations of homocysteine by promoting the conversion of homocysteine back to methionine, i.e., increasing flux through the re-methylation pathway independent of folate derivatives (which is mainly active in the liver and in the kidneys). The re-formed methionine is then gradually removed ...
Homocysteine is a non-protein amino acid, synthesized from methionine and either recycled back into methionine or converted into cysteine with the aid of the B-group vitamins [citation needed]. About 50% of homocysteine [citation needed] is converted back to methionine by remethylation via the methionine synthase major pathway.