ChEBI: Homocysteine is a sulfur-containing amino acid consisting of a glycine core with a 2-mercaptoethyl side-chain. It has a role as a fundamental metabolite. It is a sulfur-containing amino acid, a member of homocysteines and a non-proteinogenic alpha-amino acid. It is a conjugate acid of a homocysteinate. It is a tautomer of a homocysteine zwitterion.