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Metabolism regulates methylation. Methionine donates a methyl group to generate S-adenosylmethionine (SAM). Methyltransferases use SAM, resulting in the generation of S-adenosylhomocysteine (SAH), which is converted to homocysteine. Carbons derived from either choline- or folate-dependent reactions convert homocysteine back to methionine. DHF, dihydrofolate; THF, tetrahydrofolate; 5,10-MTHF, 5,10-methylene THF; CH3, methyl. (Adapted from Kaelin and McKnight 2013.)
