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Micronutrient Information Center

Homocysteine metabolism

Homocysteine is methylated to form the essential amino acid methionine in two pathways. The reaction of homocysteine remethylation catalyzed by the vitamin B12-dependent methionine synthase captures a methyl group from the folate-dependent one-carbon pool (5-methyl tetrahydrofolate). A second pathway requires betaine (N,N,N-trimethylglycine) as a methyl donor for the methylation of homocysteine catalyzed by betaine homocysteine methyltransferase. The catabolic pathway of homocysteine, known as transsulfuration pathway, converts homocysteine to the amino acid cysteine via two vitamin B6 (PLP)-dependent enzymes: cystathionine beta synthase catalyzes the condensation of homocysteine with serine to form cystathionine, and cystathionine is then converted to cysteine, alpha-ketobutyrate, and ammonia by cystathionine gamma lyase.