title: Electronic and stereochemical characterizations of intermediates in the photolysis of ferric cytochrome P450scc nitrosyl complexes. Effects of cholesterol and its analogues on ligand binding structures.
abstract: Low temperature photolysis of nitric oxide from the nitrosyl complexes of ferric cytochrome P450scc was examined by EPR spectroscopy to elucidate the stereochemical interaction between heme-bound ligand and side-chain of cholesterol or its hydroxylated analogues at the substrate-binding site. The photoproducts of the NO complexes trapped at 5 K exhibited new EPR absorptions providing information on the steric crowding of the distal heme moiety. Without substrate, the photoproduct exhibited a broad EPR absorption at g-8 due to magnetic dipole-dipole interaction between the photo-dissociated NO (S = 1/2) and the ferric iron (S = 5/2). This indicates that the photo-dissociated NO can move far away from the heme iron in the less restricted distal heme moiety of the substrate-free cytochrome P450scc. In the presence of substrates, such as cholesterol, 20(S)-hydroxycholesterol, 22(S)-hydroxycholesterol, 22(R)-hydroxycholesterol, and 25-hydroxycholesterol, the EPR spectra of the photoproducts exhibited many variations having broad g-8 absorptions and/or the widespread signals together with zero-field absorption. Among the steroid complexes used, 20(S)-hydroxycholesterol complex exhibited a conspicuously widespread EPR signal with a distinct zero-field absorption due to a spin-coupled interaction between the ferric iron (S = 5/2) and the photolyzed NO (S = 1/2). These results indicate that the 20(S)-hydroxycholesterol complex has restricted substrate-binding structure and that the hydroxylation of the cholesterol side-chain at the 22R position is necessary to proceed the side-chain cleavage reaction properly in cytochrome P450scc.
authors.affiliation: Department of Biophysical Engineering, Faculty of Engineering Science, Osaka University, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Hori
authors.forename: F
authors.initials: F
authors.lastname: Masuya
authors.forename: M
authors.initials: M
authors.lastname: Tsubaki
authors.forename: S
authors.initials: S
authors.lastname: Yoshikawa
authors.forename: Y
authors.initials: Y
authors.lastname: Ichikawa
mesh_terms: D000302:Adrenal Cortex / Q000201:enzymology; D000818:Animals; D002417:Cattle; D002784:Cholesterol / Q000378:metabolism; D002786:Cholesterol Side-Chain Cleavage Enzyme / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D006888:Hydroxycholesterols / Q000378:metabolism; D006900:Hydroxylation; D008024:Ligands; D008928:Mitochondria / Q000201:enzymology; D009569:Nitric Oxide / Q000378:metabolism*; D010782:Photolysis; D013237:Stereoisomerism; D013696:Temperature
chemical_list: D006888:Hydroxycholesterols; D008024:Ligands; D009569:Nitric Oxide; D002784:Cholesterol; D002786:Cholesterol Side-Chain Cleavage Enzyme