(1) The single covalent bond formed from the coupling of thiol groups, especially of cysteine residues.
(2) The linkage formed by the oxidation of two SH groups, each attached to a cysteine, stabilizing the structure of many protein molecules (during protein synthesis).
Disulfide bonds play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium.
Below is an example of a disulfide bond:
Cysteine---SH + HS---Cysteine → Cysteine---S-S---Cysteine + 2H+ + 2e-
Two cysteines linked by a disulfide bond forms a cystine.
Variant: disulphide bond