cysteine

A polar sulphur-containing amino acid with the formula HSCH₂CH (NH₂)COOH. It is synthesized from the amino acids methionine and serine. Breakdown is to pyruvate and may occur by several routes.
Cysteine is involved in the synthesis of the cofactor biotin. Sulphur-substituted derivatives of cysteine are found in the storage organs of many plants and the sulphide thus stored can be utilized in periods of rapid growth. See also cystine.