X-ray absorption studies of compound ES of cytochrome c peroxidase show a short iron-oxygen distance of 1.67 ± 0.04 Å, an iron-histidine distance of 1.91 ± 0.03 Å, and an iron-pyrrole nitrogen average distance of 2.02 ± 0.02 Å. This is identical within the error with the reported structure of horseradish peroxidase compound I [Chance, B., Powers, L., Ching, Y., Poulos, T., Yamazaki, I., & Paul, K. G. (1984) Arch. Biochem. Biophys. 235, 596-611]. Comparisons of the structures of myoglobin peroxide [Chance, M., Powers, L., Kumar, C., & Chance, B. (1986) Biochemistry (preceding paper in this issue)], compound ES, and the intermediates of horseradish peroxidase reveal the possible mechanisms for the stabilization of the free radical species generated during catalysis. The proximal histidine regulates the structure and function of the pyrrole nitrogens and the heme, allowing for the formation and maintenance of the characteristic intermediates.
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