Rapid kinetics investigations of peracid oxidation of ferric cytochrome P450cam: Nature and possible function of compound ES

Previously, we reported spectroscopic properties of cytochrome P450cam compound I, (ferryl iron plus a porphyrin pi-cation radical (Fe-IV = O/Por(.+))), as well as compound ES (Fe-IV = O/Tyr(.)) in reactions of substrate-free ferric enzyme with m-chloroperbenzoic acid [T. Spolitak, J.H. Dawson, D.P. Ballou, J. Biol. Chem. 280 (2005) 20300-9]. Compound ES arises by intramolecular electron transfer from nearby tyrosines to the porphyrin pi-cation radical of Compound I, and has been characterized by rapid-freeze-quench-Mossbauer/EPR spectroscopy; the tyrosyl radical was assigned to Tyr96 for wild type or to Tyr75 for the Tyr96Phe variant [V. Schunemann, F. Lendzian, C. Jung, J. Contzen, A.L. Barra, S.G. Sligar, A.X. Trautwein, J. Biol. Chem. 279 (2004) 10919-10930]. Here we report rapid-scanning stopped-flow studies of the reactions of peracids with substrate-free ferric Y75F, Y96F, and Y96F/Y75F P450cam variants, showing how these active site changes influence electron transfer from nearby tyrosines and affect formation of intermediates. Curiously, rates of generation of Compounds I and ES for both single mutants were not very different from wild type. Contrasting with the earlier EPR results, the Y96F/Y75F variant was also shown to form an ES-like species, but more slowly. When substrate is not present, or is improperly bound, compound I rapidly converts to compound ES, which can be reduced to form H2O and ferric P450, thus avoiding the modification of nearby protein groups or release of reactive oxygen species. (c) 2006 Elsevier Inc. All rights reserved.