X-ray absorption studies of myoglobin peroxide show that although it is not identical with compound I or II of horseradish peroxidase [Chance, B., Powers, L., Ching, Y., Poulos, T., Yamazaki, I., & Paul, K. G. (1984) Arch. Biochem. Biophys. 235, 596–611], it has some structural features in common with both. As seen in compound I, the Fe-O distance is short, but the iron—pyrrole nitrogen distance is contracted with a longer iron—histidine distance like compound II. The iron has a higher oxidation state than Fe3+, suggesting an oxyferryl ion type species. Comparison of the structures of various peroxidase and myoglobin compounds points out systematic differences that may explain the catalytic activity of the π cation radical as well as some of the differences between globins and heme enzymes.
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