Pathways of homocysteine metabolism

Homocysteine is metabolized by one of two divergent pathways: transsulfuration and remethylation. The transsulfuration of homocysteine to cysteine is catalyzed by CBS, a process that requires pyridoxal phosphate (vitamin B6) as a cofactor. Remethylation of homocysteine produces methionine. This reaction is catalyzed either by methionine synthase or by betainehomocysteine methyltransferase. Vitamin B12 (cobalamin) is the precursor of methylcobalamin, which is cofactor for methionine synthase.