One-electron oxidations by peroxidases.

1. Peroxidases typically follow the reaction cycle: native enzyme --> compound I --> compound II --> native enzyme, in which the latter two steps involve hydrogen atom transfer from substrate to enzyme. 2. Exceptions involve (1) very facile, rapidly reacting reducing substrates that transfer an electron rather than a hydrogen atom, resulting in formation of a substrate pi-cation radical; (2) two two-electron transfer steps: native enzyme --> compound I --> native enzyme; and (3) compound III and the reduced form of the enzyme containing iron(II). 3. Prostaglandin H synthase is a peroxidase with some of the properties of a P450 in that compound I can abstract the hydrogen atom from a C-H bond. 4. The so-called cyclooxygenase and peroxidase activities of prostaglandin H synthase are intimately connected and, with the above exception, both are part of a conventional peroxidase cycle.