Classification of Protein Disulphide Bonds.

Protein disulphide bonds are the links between pairs of cysteine residues in the polypeptide chain. These bonds are classified based on the sign of the five dihedral angles that define the cystine residue. Twenty disulphide conformations are possible using this convention and all 20 are represented in protein structures. Force distribution analysis of the pairwise forces between the cysteine residues of the different conformations identified 2 of the 20 as having significant strain: the -RHstaple and -/+RHhook disulphide bonds. These two disulphide conformations are associated with allosteric function in proteins. An online tool is available that provides a comprehensive analysis of disulphide bonds in protein structures, including conformation, strain energy, solvent accessibility and secondary structures that the disulphide links.