A

David

Darling

cysteine

cysteine molecule

Ball-and-stick representation of the cysteine molecule. Gray=C, red=O, blue=N, yellow=S, white=H.


Cysteine is a sulfur-containing amino acid (HSCH2CH2COOH) that occurs in animal proteins, especially those in hair, hooves, and the keratin of skin; the HS– group in it makes it the catalytic element in some enzymes. The disulfide (S-S) links between adjacent cysteine molecules in polypeptide chains contribute to the three-dimensional molecular structure of proteins.