methionine synthase

me·thi·o·nine syn·thase (mə-thiґo-nēn synґthās) [EC 2.1.1.13] an enzyme of the transferase class that catalyzes the remethylation of homocysteine to form methionine, using 5-methyltetrahydrofolate as a methyl donor and requiring methylcobalamin as a coenzyme; the reaction also regenerates tetrahydrofolate. Deficiency of enzyme activity can result from any of several defects, including deficiency of the apoenzyme, inability to convert cobalamin specifically to methylcobalamin or, more generally, to either of the cobalamin-containing coenzymes, deficiency of 5,10-methylenetetrahydrofolate reductase activity, familial megaloblastic anemia or other defect in absorption or transport of vitamin B₁₂, or nutritional deficiency of vitamin B₁₂ or folate; it results in homocystinuria, with developmental delay and neurologic abnormalities, and hypomethioninemia. Defects in cobalamin metabolism are characterized additionally by hematologic abnormalities and some also by methylmalonicacidemia (q.v.).