Resonance Raman Spectroscopy of the Fe(IV)=O Group in Peroxidase Intermediates

Abstract

The heme proteins known as peroxidases and catalases catalyze reactions of peroxides. The reaction sequences of peroxidases and catalases involve two colored intermediates, known as compounds I and II. Horseradish peroxidase is one of the most studied peroxidases because of its universal availability from commercial sources, and because its reactions are typical of a large number of peroxidases. Upon reaction with hydrogen peroxide, horseradish peroxidase forms a green colored intermediate known as compound I, which is two oxidation equivalents above the resting enzyme. A one electron reduction of compound I results in a red colored intermediate known as compound II. Compound II is an Fe(IV) heme, one oxidation equivalent above the resting enzyme. While compound I is formally an Fe(V) heme, it is believed to contain an Fe(IV) with another electron removed from the highest occupied molecular orbital of the porphyrin group, resulting in a porphyrin η-radical cation.