Mechanisms of cleavage of allosteric disulfide bonds. Disulfide bond reduction occurs via a second-order nucleophilic substitution (SN2)-type reaction mechanism in which the 3 sulfur atoms involved must form an ∼180° angle. (A) For oxidoreductase cleavage, the active site sulfur ion nucleophile of the oxidoreductase (green) attacks 1 of the sulfur atoms of the allosteric disulfide bond (gray). The mixed disulfide that forms then spontaneously decomposes, releasing oxidized oxidoreductase and the substrate protein containing a reduced allosteric disulfide. (B) For thiol-disulfide exchange, the protein contains a sulfur ion nucleophile that is unreactive until a conformational change brings the sulfur ion in line with the allosteric bond, where it attacks 1 of the sulfur atoms of the disulfide, cleaving the bond. The conformational change can be mediated by ligand binding or by mechanical shear of the protein. Intramolecular cleavage is shown, but this can also occur intermolecularly.
