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Stylized illustration of the homocysteine/methionine cycle and biochemical mechanism of action of vitamin B12 in the homocysteine/methionine and the methyl-malonyl-CoA pathway. The complex Kennedy pathway involved in phosphatidylcholine synthesis is not illustrated in detail. PtDEth: phosphatidylethanolamine; PtDCh: phosphatidylcholine; SAM: s-adenosyl-methionine; SAH: s-adenosyl-homocysteine.
