Interpretation of the reactivity of peroxidase compounds I and II with phenols by the Marcus equation

The catalytic cycle of heme peroxidases involves two reactive states, compound I and compound II. Although their reduction potentials at pH 7 are similar, compound I is in general more reactive towards organic substrates than compound II, The different reactivities have until now remained unexplained, In this study, the reactions of compounds I and II of peroxidase from horseradish with phenols were analyzed using the Marcus equation of electron-transfer. Both reactions exhibit similar reorganization energies, and the different reactivities of the two enzyme states can be ascribed to a higher apparent rate of activationless electron-transfer in the compound I reactions, This can be attributed to the shorter electron-tunneling distance on electron-transfer to the porphyrin radical cation in compound I, compared to electron-transfer to the iron ion in compound II. (C) 1997 Federation of European Biochemical Societies.