Cysteine

Cysteine (abbreviated to Cys or C) is an amino acid with the side chain -CH₂SH.  Cysteine, like Methionine, contains sulphur. It is a non-polar amino acid that contains a very reactive thiol or sulfhydryl group (-SH). The presence of this group allows cysteine to form strong disulphide bridges when paired with another cysteine. Cysteine therfore plays an important structural role in proteins.