title: Proton translocation in cytochrome c oxidase: insights from proton exchange kinetics and vibrational spectroscopy.
abstract: Cytochrome c oxidase is the terminal enzyme in the electron transfer chain. It reduces oxygen to water and harnesses the released energy to translocate protons across the inner mitochondrial membrane. The mechanism by which the oxygen chemistry is coupled to proton translocation is not yet resolved owing to the difficulty of monitoring dynamic proton transfer events. Here we summarize several postulated mechanisms for proton translocation, which have been supported by a variety of vibrational spectroscopic studies. We recently proposed a proton translocation model involving proton accessibility to the regions near the propionate groups of the heme a and heme a3 redox centers of the enzyme based by hydrogen/deuterium (H/D) exchange Raman scattering studies (Egawa et al., PLoS ONE 2013). To advance our understanding of this model and to refine the proton accessibility to the hemes, the H/D exchange dependence of the heme propionate group vibrational modes on temperature and pH was measured. The H/D exchange detected at the propionate groups of heme a3 takes place within a few seconds under all conditions. In contrast, that detected at the heme a propionates occurs in the oxidized but not the reduced enzyme and the H/D exchange is pH-dependent with a pKa of ~8.0 (faster at high pH). Analysis of the thermodynamic parameters revealed that, as the pH is varied, entropy/enthalpy compensation held the free energy of activation in a narrow range. The redox dependence of the possible proton pathways to the heme groups is discussed. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
authors.forename: Izumi
authors.initials: I
authors.lastname: Ishigami
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
authors.forename: Masahide
authors.initials: M
authors.lastname: Hikita
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Egawa
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
authors.forename: Syun-Ru
authors.initials: SR
authors.lastname: Yeh
authors.affiliation: Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461, USA. Electronic address: rousseau@aecom.yu.edu.
authors.forename: Denis L
authors.initials: DL
authors.lastname: Rousseau
mesh_terms: D003576:Electron Transport Complex IV / Q000737:chemistry* / Q000378:metabolism*; D006418:Heme / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008958:Models, Molecular; D010084:Oxidation-Reduction; D011522:Protons; D013057:Spectrum Analysis / Q000379:methods; D014732:Vibration
chemical_list: D011522:Protons; C027728:heme a; D006418:Heme; D003576:Electron Transport Complex IV
keywords: Cytochrome oxidase; Heme; Proton translocation; Raman spectroscopy; Vibrational spectroscopy