title: A Ferric-Peroxo Intermediate in the Oxidation of Heme by IsdI.
abstract: The canonical heme oxygenases (HOs) catalyze heme oxidation via a heme-bound hydroperoxo intermediate that is stabilized by a water cluster at the active site of the enzyme. In contrast, the hydrophobic active site of IsdI, a heme-degrading enzyme from Staphylococcus aureus, lacks a water cluster and is expected to oxidize heme by an alternative mechanism. Reaction of the IsdI-heme complex with either H2O2 or m-chloroperoxybenzoic acid fails to produce a specific oxidized heme iron intermediate, suggesting that ferric-hydroperoxo or ferryl derivatives of IsdI are not involved in the catalytic mechanism of this enzyme. IsdI lacks a proton-donating group in the distal heme pocket, so the possible involvement of a ferric-peroxo intermediate has been evaluated. Density functional theory (DFT) calculations indicate that heme oxidation involving a ferric-peroxo intermediate is energetically accessible, whereas the energy barrier for a reaction involving a ferric-hydroperoxo intermediate is too great in the absence of a proton donor. We propose that IsdI catalyzes heme oxidation through nucleophilic attack by the heme-bound peroxo species. This proposal is consistent with our previous demonstration by nuclear magnetic resonance spectroscopy that heme ruffling increases the susceptibility of the meso-carbon of heme to nucleophilic attack.
authors: †Department of Microbiology and Immunology, ‡Department of Biochemistry and Molecular Biology, and §UBC Centre for Blood Research, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
authors: Shin-Ichi J
authors: SJ
authors: Takayama
authors: †Department of Microbiology and Immunology, ‡Department of Biochemistry and Molecular Biology, and §UBC Centre for Blood Research, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
authors: Slade A
authors: SA
authors: Loutet
authors: †Department of Microbiology and Immunology, ‡Department of Biochemistry and Molecular Biology, and §UBC Centre for Blood Research, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
authors: A Grant
authors: AG
authors: Mauk
authors: †Department of Microbiology and Immunology, ‡Department of Biochemistry and Molecular Biology, and §UBC Centre for Blood Research, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada.
authors: Michael E P
authors: ME
authors: Murphy
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D006418:Heme / Q000737:chemistry* / Q000378:metabolism; D006419:Heme Oxygenase (Decyclizing) / Q000737:chemistry* / Q000378:metabolism; D006861:Hydrogen Peroxide / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D007501:Iron / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction; D013211:Staphylococcus aureus / Q000201:enzymology*
chemical_list: D001426:Bacterial Proteins; D006418:Heme; D006861:Hydrogen Peroxide; D007501:Iron; D006419:Heme Oxygenase (Decyclizing)