Mechanism of oxidoreductive cleavage by thiol isomerases. The reduced thiol isomerase aligns with a substrate that contains a disulfide bond such that the free thiolate forms an 180° angle with the vector of the disulfide. A second-order nucleophilic substitution S_N2-type reaction then occurs in which the active site sulfur ion nucleophile of the thiol isomerase attacks the adjacent sulfur atoms of the disulfide bond.⁷⁴  This nucleophilic substitution results in a transient mixed disulfide (blue). The mixed disulfide spontaneously decomposes, resulting in the formation of a disulfide bond in the thiol isomerase and reduction of the disulfide bond in the substrate.