STRUCTURAL CHARACTERIZATION OF HIGH VALENT INTERMEDIATES IN HORSERADISH PEROXIDASE

Extended X-ray absorption fine structure and X-ray absorption near edge structure spectroscopies have been utilized to determine the structural environment of the heme iron sites in horseradish peroxidase compounds I and II. For comparison, analogous studies have been undertaken on putative ferryl, Fe (IV) =0, porphyrin model compounds and on crystallographically characterized Cr (IV) =0 and Cr (V) =N porphyrins. The present work demonstrates unambiguously that a short, ca. 1.64 Å, Fe-O bond length is present both in HRP Compounds I and II and in their synthetic analogues. Edge spectra show the oxidation state of these high-valent species to be Fe (IV) or greater. These structures are consistent only with an oxoferryl (Fe=O) complex as the active oxygen species of horseradish peroxidase. The structural details and their implications for heme protein mediated oxygen activation are discussed