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 therefore plays an important structural role in proteins. The ability of cysteine to form disulphide bridges is of great significance for proteins that have to work outside of the cell. This is because the cysteine side chain of these proteins forms disulphide bonds which increases the stability of the proteins. It should be noted, however, that proteins working inside the cell do not form disulphide bonds .