Cysteine (abbreviated to Cys or C) is one of 20 amino acids and has 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 which is also one of the interactions in the tertiary structure of the protein . 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 e.g immunoglobulins in which the loops in the light and heavy chains are stabilised by the disulphide bridges. 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.
- ↑ Berg et al., Biochemistry, 6th Edition, 2007: 31.
- ↑ Jacek Leluk, Institute of Biochemistry and Molecular Biology, why cysteine is special link : http://www.cryst.bbk.ac.uk/pps97/assignments/projects/leluk/project.htm