 The pair of cysteines may be oxidized (forming an internal disulfide) or reduced (as free thiols), and thus allows for oxidoreductase activity by serving as an electron pair donor or acceptor, depending on oxidation state.This reaction generally proceeds through a mixed-disulfide intermediate, in which a cysteine from the enzyme forms a bond to a cysteine on the substrate.DsbA is responsible for introducing disulfide bonds into nascent proteins.