Chemical nature of the porphyrin .pi. cation radical in horseradish peroxidase compound I

Abstract
The EPR and Mossbauer properties of native horseradish peroxidase have been compared with those of a synthetic derivative of the enzyme in which a mesohemin residue replaces the natural Fe protoporphyrin IX heme prosthetic group. The oxyferryl .pi. cation radical intermediate, compound I, was formed from both the native and synthetic enzyme, and the magnetic properties of both intermediates were examined. The optical absorption characteristics of compound I prepared from mesoheme-substituted horseradish peroxidase are different from those of the compound I prepared from native enzyme [DiNello et. Dolphin (1981)]. Possibly these optical absorption differences are due to the formation of an A2u and an A1u .pi. cation radical species, respectively. The EPR and Mossbauer properties of the native and synthetic enzyme and of their oxidized intermediates are quite similar, if not identical, and the data favor an A2u radical for both compounds I.

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