Figure.

Homocysteine metabolism is at the intersection of two main pathways: remethylation and trans-sulfuration. When the methionine level is low, homocysteine is remethylated into methionine; a process which requires vitamin B₁₂ and folic acid as cofactors. Methionine then forms S-adenosyl-methionine (SAM), which serves as the major methyl group donor in the cell. After demethylation, SAM generates S-adenosyl-homocysteine (SAH) and eventually is0 hydrolyzed back to homocysteine for a new cycle. When methionine levels are high, homocysteine, through the trans-sulfuration pathway, condenses with serine to form cystathionine, and subsequently cysteine in an irreversible reaction.