title: Respiratory conservation of energy with dioxygen: cytochrome C oxidase.
abstract: Cytochrome c oxidase (CcO) is the terminal oxidase of cell respiration which reduces molecular oxygen (O₂) to H2O coupled with the proton pump. For elucidation of the mechanism of CcO, the three-dimensional location and chemical reactivity of each atom composing the functional sites have been extensively studied by various techniques, such as crystallography, vibrational and time-resolved electronic spectroscopy, since the X-ray structures (2.8 Å resolution) of bovine and bacterial CcO have been published in 1995.X-ray structures of bovine CcO in different oxidation and ligand binding states showed that the O₂reduction site, which is composed of Fe (heme a 3) and Cu (CuB), drives a non-sequential four-electron transfer for reduction of O₂to water without releasing any reactive oxygen species. These data provide the crucial structural basis to solve a long-standing problem, the mechanism of the O₂reduction.Time-resolved resonance Raman and charge translocation analyses revealed the mechanism for coupling between O₂reduction and the proton pump: O₂is received by the O₂reduction site where both metals are in the reduced state (R-intermediate), giving the O₂-bound form (A-intermediate). This is spontaneously converted to the P-intermediate, with the bound O₂fully reduced to 2 O²⁻. Hereafter the P-intermediate receives four electron equivalents from the second Fe site (heme a), one at a time, to form the three intermediates, F, O, and E to regenerate the R-intermediate. Each electron transfer step from heme a to the O₂reduction site is coupled with the proton pump.X-ray structural and mutational analyses of bovine CcO show three possible proton transfer pathways which can transfer pump protons (H) and chemical (water-forming) protons (K and D). The structure of the H-pathway of bovine CcO indicates that the driving force of the proton pump is the electrostatic repulsion between the protons on the H-pathway and positive charges of heme a, created upon oxidation to donate electrons to the O₂reduction site. On the other hand, mutational and time-resolved electrometric findings for the bacterial CcO strongly suggest that the D-pathway transfers both pump and chemical protons. However, the structure for the proton-gating system in the D-pathway has not been experimentally identified. The structural and functional diversities in CcO from various species suggest a basic proton pumping mechanism in which heme a pumps protons while heme a 3 reduces O₂as proposed in 1978.
authors.affiliation: Picobiology Institute, Graduate School of Life Science, University of Hyogo, Kamigohri Akoh Hyogo, 678-1297, Japan, yoshi@sci.u-hyogo.ac.jp.
authors.forename: Shinya
authors.initials: S
authors.lastname: Yoshikawa
authors.forename: Atsuhiro
authors.initials: A
authors.lastname: Shimada
authors.forename: Kyoko
authors.initials: K
authors.lastname: Shinzawa-Itoh
mesh_terms: D000818:Animals; D002417:Cattle; D018360:Crystallography, X-Ray; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D006861:Hydrogen Peroxide / Q000737:chemistry / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry* / Q000378:metabolism*; D010101:Oxygen Consumption / Q000502:physiology; D013329:Structure-Activity Relationship
chemical_list: D006861:Hydrogen Peroxide; D003576:Electron Transport Complex IV; D010100:Oxygen