A possible substrate reduction reaction mechanism for BC3987 based on Model II involving two active site thiolates.

(A) The reaction is initiated by a nucleophilic attack on the substrate disulfide bridge by the Cys12 thiolate and a mixed disulfide intermediate is formed. (B) The buried Cys15 Sγ-atom that is stabilized in its thiolate form by hydrogen bonding toThr8 and Gln9 perform a second nucleophilic attack on the Cys12 Sγ-atom. (C) The reduced substrate is released and the two active site cysteines in BC3987 form a disulfide bridge.