title: Proton exit channels in bovine cytochrome c oxidase.
abstract: Cytochrome c oxidase (CcO) is the terminal transmembrane enzyme of the respiratory electron transport chain in aerobic cells. It catalyzes the reduction of oxygen to water and utilizes the free energy of the reduction reaction for proton pumping, a process which results in a membrane electrochemical proton gradient. Although the structure of the enzyme has been solved for several organisms, the molecular mechanism of proton pumping and proton exit pathways remain unknown. In our previous work, the continuum electrostatic calculations were employed to evaluate the electrostatic potential, energies, and protonation state of bovine cytochrome c oxidase for different redox states of the enzyme. A possible mechanism of oxygen reduction and proton pumping via His291 was proposed. In this paper, using electrostatic calculations, we examine the proton exit pathways in the enzyme. By monitoring the changes of the protonation states, proton affinities, and energies of electrostatic interactions between the titratable groups in different redox states of CcO, we identified the clusters of strongly interacting residues. Using these data, we detected four possible proton exit points on the periplasmic side of the membrane (Lys171B/Asp173B, His24B/Asp25B, Asp51, and Asp300). We then were able to trace the proton exit pathways and to evaluate the energy profiles along the paths. On the basis of energetic considerations and the conservation of the residues in a protein sequence, the most likely exit pathway is one via the Lys171B/Asp173B site. The obtained results are fully consistent with our His291 model of proton pumping, and provide a rationale for the absence of proton leaking in CcO between the pumping strokes.
authors.affiliation: Department of Chemistry, University of California, One Shields Avenue, Davis, California 95616, USA.
authors.forename: Dragan M
authors.initials: DM
authors.lastname: Popović
authors.forename: Alexei A
authors.initials: AA
authors.lastname: Stuchebrukhov
mesh_terms: D000818:Animals; D002384:Catalysis; D002417:Cattle; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000737:chemistry*; D008239:Lysine / Q000737:chemistry; D008958:Models, Molecular; D009206:Myocardium / Q000378:metabolism; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D011487:Protein Conformation; D011522:Protons; D055672:Static Electricity; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
chemical_list: D011522:Protons; D014867:Water; D003576:Electron Transport Complex IV; D008239:Lysine; D010100:Oxygen