Rapid freeze-quench ENDOR study of chloroperoxidase compound I: the site of the radical |
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Authors: | Kim Sun Hee Perera Roshan Hager Lowell P Dawson John H Hoffman Brian M |
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Institution: | Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA. |
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Abstract: | The classical heme-monooxygenase active intermediate, compound I (Cpd-I), incorporates a heme which is oxidized by two equivalents above the resting ferric state, one equivalent associated with a ferryl center, Fe=O]2+ (FeS = 1), and the other with an active-site radical (RS = 1/2). Theoretical calculations on models of a Cpd-I with a thiolato axial ligand have presented divergent views about its electronic structure. In one picture, the radical is on the porphyrin; in the other, it is on the sulfur. In this report, ENDOR spectroscopy answers the question, does Cpd-I of the enzyme chloroperoxidase contain a porphyrin pi-cation radical or an iron-bound cysteinyl radical: the radical is predominantly on the porphyrin, with spin density on sulfur having an upper bound, rhoS = rhoSmax approximately 0.23. We further suggest that the same answer applies to Cpd-I of cytochromes P450. |
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