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. Exce ptions involve (1) very facile, rapidly reacting reducing substrates t hat transfer an electron rather than a hydrogen atom, resulting in for mation of a substrate pi-cation radical; (2) two two-electron transfer steps: native enzyme --> compound I --> native enzyme; and (3) compou nd III and the reduced form of the enzyme containing iron(II). 3. Pros taglandin 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 prostag landin H synthase are intimately connected and, with the above excepti on, both are part of a conventional peroxidase cycle.