Disulphide bridges
A disulphide bridge can also be called a disulphide bond. It is the name of the covalent bond that is formed when two thiol groups, SH groups, undergo oxidation and a bond forms between the two sulphur atoms. In the formation of this type of bond, two protons and two electrons are produced as biproducts.
Disulphide bonds are often present in tertiary protein structure between cysteine residues, as they help to stabilise the protein. The S-S bonds are formed as part of the folding of the protein into its final 3D structure in the Endoplasmic reticulum[1].
The disulphide bonds only occur on the extracellular domains of a protein, as in the Cytosol (a reducing environment) the bonds become unstable and break.