title: Hydrogen bonding to the cysteine ligand of superoxide reductase: acid-base control of the reaction intermediates.
abstract: Superoxide reductase (SOR) is a non-heme iron metalloenzyme that detoxifies superoxide radical in microorganisms. Its active site consists of an unusual non-heme Fe(2+) center in a [His4Cys1] square pyramidal pentacoordination, with the axial cysteine ligand proposed to be an essential feature in catalysis. Two NH peptide groups from isoleucine 118 and histidine 119 establish hydrogen bonds involving the sulfur ligand (Desulfoarculus baarsii SOR numbering). To investigate the catalytic role of these hydrogen bonds, the isoleucine 118 residue of the SOR from Desulfoarculus baarsii was mutated into alanine, aspartate, or serine residues. Resonance Raman spectroscopy showed that the mutations specifically induced an increase of the strength of the Fe(3+)-S(Cys) and S-Cβ(Cys) bonds as well as a change in conformation of the cysteinyl side chain, which was associated with the alteration of the NH hydrogen bonding involving the sulfur ligand. The effects of the isoleucine mutations on the reactivity of SOR with O2 (•-) were investigated by pulse radiolysis. These studies showed that the mutations induced a specific increase of the pK a of the first reaction intermediate, recently proposed to be an Fe(2+)-O2 (•-) species. These data were supported by density functional theory calculations conducted on three models of the Fe(2+)-O2 (•-) intermediate, with one, two, or no hydrogen bonds involving the sulfur ligand. Our results demonstrated that the hydrogen bonds between the NH (peptide) and the cysteine ligand tightly control the rate of protonation of the Fe(2+)-O2 (•-) reaction intermediate to form an Fe(3+)-OOH species.
authors.affiliation: Laboratoire de Chimie et Biologie des Métaux, CEA, iRTSV, 17 avenue des Martyrs, 38054, Grenoble, France.
authors.forename: Emilie
authors.initials: E
authors.lastname: Tremey
authors.forename: Florence
authors.initials: F
authors.lastname: Bonnot
authors.forename: Yohann
authors.initials: Y
authors.lastname: Moreau
authors.forename: Catherine
authors.initials: C
authors.lastname: Berthomieu
authors.forename: Alain
authors.initials: A
authors.lastname: Desbois
authors.forename: Vincent
authors.initials: V
authors.lastname: Favaudon
authors.forename: Geneviève
authors.initials: G
authors.lastname: Blondin
authors.forename: Chantal
authors.initials: C
authors.lastname: Houée-Levin
authors.forename: Vincent
authors.initials: V
authors.lastname: Nivière
mesh_terms: D020134:Catalytic Domain; D003545:Cysteine; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D020560:Proteobacteria / Q000201:enzymology; D011789:Quantum Theory
chemical_list: D008024:Ligands; D010088:Oxidoreductases; C121849:superoxide reductase; D003545:Cysteine
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