5,10-Methylene tetrahydrofolate reductase regulation

Methylene-tetrahydrofolate reductase is inhibited by S-adenosylmethi-onine, which inhibits reduction of the flavin prosthetic group by NADPH. S-Adenosylhomocysteine overcomes this inhibition to some extent, as might be expected for an enzyme that is indirectly involved in the regulation of methionine and S-adenosyhnethionine concentrations in the cell. 

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]. 

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