title: Rapid freeze-quench ENDOR study of chloroperoxidase compound I: the site of the radical.
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.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA.
authors.forename: Sun Hee
authors.initials: SH
authors.lastname: Kim
authors.forename: Roshan
authors.initials: R
authors.lastname: Perera
authors.forename: Lowell P
authors.initials: LP
authors.lastname: Hager
authors.forename: John H
authors.initials: JH
authors.lastname: Dawson
authors.forename: Brian M
authors.initials: BM
authors.lastname: Hoffman
mesh_terms: D002711:Chloride Peroxidase / Q000737:chemistry*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D005609:Free Radicals; D005615:Freezing; D006860:Hydrogen Bonding; D011487:Protein Conformation
chemical_list: D005609:Free Radicals; D002711:Chloride Peroxidase