title: Crystal structure of rat heme oxygenase-1 in complex with heme bound to azide. Implication for regiospecific hydroxylation of heme at the alpha-meso carbon.
abstract: Heme oxygenase (HO) catalyzes physiological heme degradation consisting of three sequential oxidation steps that use dioxygen molecules and reducing equivalents. We determined the crystal structure of rat HO-1 in complex with heme and azide (HO-heme-N(3)(-)) at 1.9-A resolution. The azide, whose terminal nitrogen atom is coordinated to the ferric heme iron, is situated nearly parallel to the heme plane, and its other end is directed toward the alpha-meso position of the heme. Based on resonance Raman spectroscopic analysis of HO-heme bound to dioxygen, this parallel coordination mode suggests that the azide is an analog of dioxygen. The azide is surrounded by residues of the distal F-helix with only the direction to the alpha-meso carbon being open. This indicates that regiospecific oxygenation of the heme is primarily caused by the steric constraint between the dioxygen bound to heme and the F-helix. The azide interacts with Asp-140, Arg-136, and Thr-135 through a hydrogen bond network involving five water molecules on the distal side of the heme. This network, also present in HO-heme, may function in dioxygen activation in the first hydroxylation step. From the orientation of azide in HO-heme-N(3)(-), the dioxygen or hydroperoxide bound to HO-heme, the active oxygen species of the first reaction, is inferred to have a similar orientation suitable for a direct attack on the alpha-meso carbon.
authors.affiliation: Department of Biology, Graduate School of Science, Osaka University, Toyonaka, Japan.
authors.forename: Masakazu
authors.initials: M
authors.lastname: Sugishima
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Sakamoto
authors.forename: Yuichiro
authors.initials: Y
authors.lastname: Higashimoto
authors.forename: Yoshiaki
authors.initials: Y
authors.lastname: Omata
authors.forename: Shunsuke
authors.initials: S
authors.lastname: Hayashi
authors.forename: Masato
authors.initials: M
authors.lastname: Noguchi
authors.forename: Keiichi
authors.initials: K
authors.lastname: Fukuyama
mesh_terms: D000818:Animals; D001386:Azides / Q000737:chemistry* / Q000378:metabolism; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D006418:Heme / Q000737:chemistry / Q000378:metabolism; D006419:Heme Oxygenase (Decyclizing) / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D051547:Heme Oxygenase-1; D006860:Hydrogen Bonding; D008958:Models, Molecular; D015394:Molecular Structure; D010100:Oxygen / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D051381:Rats
chemical_list: D001386:Azides; D006418:Heme; D006419:Heme Oxygenase (Decyclizing); D051547:Heme Oxygenase-1; D010100:Oxygen