title: A combined quantum chemical and crystallographic study on the oxidized binuclear center of cytochrome c oxidase.
abstract: Cytochrome c oxidase (CcO) is the terminal enzyme of the respiratory chain. By reducing oxygen to water, it generates a proton gradient across the mitochondrial or bacterial membrane. Recently, two independent X-ray crystallographic studies ((Aoyama et al. Proc. Natl. Acad. Sci. USA 106 (2009) 2165-2169) and (Koepke et al. Biochim. Biophys. Acta 1787 (2009) 635-645)), suggested that a peroxide dianion might be bound to the active site of oxidized CcO. We have investigated this hypothesis by combining quantum chemical calculations with a re-refinement of the X-ray crystallographic data and optical spectroscopic measurements. Our data suggest that dianionic peroxide, superoxide, and dioxygen all form a similar superoxide species when inserted into a fully oxidized ferric/cupric binuclear site (BNC). We argue that stable peroxides are unlikely to be confined within the oxidized BNC since that would be expected to lead to bond splitting and formation of the catalytic P intermediate. Somewhat surprisingly, we find that binding of dioxygen to the oxidized binuclear site is weakly exergonic, and hence, the observed structure might have resulted from dioxygen itself or from superoxide generated from O(2) by the X-ray beam. We show that the presence of O(2) is consistent with the X-ray data. We also discuss how other structures, such as a mixture of the aqueous species (H(2)O+OH(-) and H(2)O) and chloride fit the experimental data.
authors.affiliation: Institute of Biotechnology, University of Helsinki, FI-00014 Helsinki, Finland. ville.kaila@nih.gov
authors.forename: Ville R I
authors.initials: VR
authors.lastname: Kaila
authors.forename: Esko
authors.initials: E
authors.lastname: Oksanen
authors.forename: Adrian
authors.initials: A
authors.lastname: Goldman
authors.forename: Dmitry A
authors.initials: DA
authors.lastname: Bloch
authors.forename: Michael I
authors.initials: MI
authors.lastname: Verkhovsky
authors.forename: Dage
authors.initials: D
authors.lastname: Sundholm
authors.forename: Mårten
authors.initials: M
authors.lastname: Wikström
mesh_terms: D000818:Animals; D020134:Catalytic Domain; D002417:Cattle; D018360:Crystallography, X-Ray; D003576:Electron Transport Complex IV / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D010545:Peroxides / Q000737:chemistry; D011487:Protein Conformation; D011789:Quantum Theory; D013481:Superoxides / Q000737:chemistry
chemical_list: D010545:Peroxides; D013481:Superoxides; D003576:Electron Transport Complex IV; D010100:Oxygen