Structural Characterization of Horseradish Peroxidase Using EXAFS Spectroscopy. Evidence for Fe=0 Ligation in Compounds I and II

Extended X-ray absorption fine structure spectroscopy has 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. In a preliminary communication, we suggested that a short ca. 1.6 A Fe-0 bond is present in the high valent forms of both the enzyme and the synthetic porphyrins. The present work demonstrates unambiguously that a short, ca. 1.64 A, Fe-O bond length is present both in HRP compounds I and II and in their synthetic analogues. This structure is consistent only with an oxo-ferryl (Fe=0) complex as the active oxygen species in horseradish peroxidase. The structural details, their implications for heme protein mediated oxygen activation, and the difference between our results and those recently published by other workers (Chance, B.; Powers, L.; Ching, Y.; Poulos, T.; Schonbaum, G. R.; Yamazaki, I.; Paul, K. G. Arch. Biochem. Biophys. 1984, 235, 596–611) are discussed.