Reverse Ordered Sequential Mechanism for Lactoperoxidase with Inhibition by Hydrogen Peroxide

Lactoperoxidase (LPO, Fe^III in its resting state in the absence of substrates)-an enzyme secreted from human mammary, salivary, and other mucosal glands-catalyzes the oxidation of thiocyanate (SCN^-) by hydrogen peroxide (H₂O₂) to produce hypothiocyanite (OSCN^-), which functions as an antimicrobial agent. The accepted catalytic mechanism, called the halogen cycle, comprises a two-electron oxidation of LPO by H₂O₂ to produce oxoiron(IV) radicals, followed by O-atom transfer to SCN^-. However, the mechanism does not explain biphasic kinetics and inhibition by H₂O₂ at low concentration of reducing substrate, conditions that may be biologically relevant. We propose an ordered sequential mechanism in which the order of substrate binding is reversed, first SCN^- and then H₂O₂. The sequence of substrate binding that is described by the halogen cycle mechanism is actually inhibitory.