Fig. 1: In the left cycle, folate takes methyl groups from serine to form glycine and then uses them to methylate homocysteine, forming methionine in the right cycle. Methionine is the precursor of SAMe, a methyl donor that is involved in many methylation reactions. THF = tetrahydrofolate, Me = methyl group, SAMe = S-adenosylmethionine, SAH = S-adenosylhomocysteine, R = any molecule that can be methylated by SAMe. The enzymes involved in the metabolic pathways are (1) serine hydroxymethyltransferase (a pyridoxal phosphate–dependent enzyme), (2) methylenetetrahydrofolate reductase, (3) methionine synthase, also known as 5-methyltetrahydrofolate homocysteine methyltransferase (a cobalamin-dependent enzyme), (4) methionine adenosyltransferase, (5) a variety of SAMe-dependent methyltransferase enzymes, (6) adenosylhomocysteine hydrolase. Photo by: Lianne Friesen and Nicholas Woolridge