Like all amino acids, it has an amino group (-NH2) - blue with 2 white balls - at one end, and a carboxylic acid group (-COOH) - grey, connected to red, and red and white - at the other end. Between these is the
α- (alpha) carbon to which the 'R' group is attached.
Its 'R' group is -CH2SH, a methyl group with a thiol group (-SH) attached - yellow and white.
Cysteine can form disulphide bonds ( -SS- ) with other cysteine residues. These bonds form a strong covalent bridge between sections of the polypeptide chain and so contribute to the protein's tertiary structure.
Cysteine is involved in the formation of keratin in hair and skin, nails and animal horn. These double bonds are disrupted by the chemicals and heat used in the process of perming hair.
Cysteine also plays a part in the formation of gluten in dough.