Homocysteine is metabolized by 1 of 2 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 betaine-homocysteine methyltransferase. Vitamin B12 (cobalamin) is the precursor of methylcobalamin, which is cofactor for methionine synthase.
