Homocysteine and methionine metabolism (right) and folate metabolism (left).
AdoHcy indicates S-adenosylhomocysteine; AdoMet,S-adenosylmethionine; Cbl, cobalamin; 5,10-CH2-THF, 5,10-methylenetetrahydrofolate; 5-CH3-THF, 5-methyltetrahydrofolate; 10-CHO-THF, 10-formyltetrahydrofolate; Cys-Gly, cysteinylglycine; DHF, dihydrofolate; dTMP, deoxythymidine monophosphate; dUMP, deoxyuridine monophosphate; GSH, glutathione; THF, tetrahydrofolate. Mediating enzymes for reactions are as follows: (1) 5,10-methylenetetrahydrofolate reductase; (2) thymidylate synthase; (3) methionine synthase (requires cobalamin and 5-methyltetrahydrofolate); (4) glycine N-methyltransferase; (5) a variety of cellular methyltransferases; (6) cystathionine β-synthase; (7) γ-glutamylcysteine synthetase, followed by a reaction mediated by glutathione synthetase; (8) γ-glutamyl transpeptidase (occurs extracellularly).
