Reverse Ordered Sequential Mechanism for Lactoperoxidase with Inhibition by Hydrogen Peroxide
Lactoperoxidase (LPO, Fe<sup>III</sup> 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<sup>−</sup>) by hydrogen peroxide (H<sub>2</sub>O<sub...
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| Format: | article |
| Langue: | EN |
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MDPI AG
2021
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| Accès en ligne: | https://doaj.org/article/b6693e4545d24db69cbb3807a9b74354 |
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| Résumé: | Lactoperoxidase (LPO, Fe<sup>III</sup> 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<sup>−</sup>) by hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) to produce hypothiocyanite (OSCN<sup>−</sup>), which functions as an antimicrobial agent. The accepted catalytic mechanism, called the halogen cycle, comprises a two-electron oxidation of LPO by H<sub>2</sub>O<sub>2</sub> to produce oxoiron(IV) radicals, followed by O-atom transfer to SCN<sup>−</sup>. However, the mechanism does not explain biphasic kinetics and inhibition by H<sub>2</sub>O<sub>2</sub> 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<sup>−</sup> and then H<sub>2</sub>O<sub>2</sub>. The sequence of substrate binding that is described by the halogen cycle mechanism is actually inhibitory. |
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