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Methylenetetrahydrofolate reductase (MTHFR). 5,10-Methylenetetrahydrofolate is reduced to 5-methyltetrahydrofolate by methylenetetrahydrofolate reductase, a cytoplasmic, NAD(P)-dependent enzyme. 5-methyltetrahydrofolate is utilized by methionine synthase to convert homocysteine to methionine. The enzymatic mechanism is a ping-pong bi-bi mechanism, in which NAD(P)+ release precedes the binding of methylenetetrahydrofolate and the acceptor is free FAD. The family includes the 5,10-methylenetetrahydrofolate reductase EC:1.7.99.5 from prokaryotes and methylenetetrahydrofolate reductase EC: 1.5.1.20 from eukaryotes. The bacterial enzyme is a homotetramer and NADH is the preferred reductant while the eukaryotic enzyme is a homodimer and NADPH is the preferred reductant. In humans, there are several clinically significant mutations in MTHFR that result in hyperhomocysteinemia, which is a risk factor for the development of cardiovascular disease.
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