title: The protonation state of a heme propionate controls electron transfer in cytochrome c oxidase.
abstract: In cytochrome c oxidase (CcO), exergonic electron transfer reactions from cytochrome c to oxygen drive proton pumping across the membrane. Elucidation of the proton pumping mechanism requires identification of the molecular components involved in the proton transfer reactions and investigation of the coupling between internal electron and proton transfer reactions in CcO. While the proton-input trajectory in CcO is relatively well characterized, the components of the output pathway have not been identified in detail. In this study, we have investigated the pH dependence of electron transfer reactions that are linked to proton translocation in a structural variant of CcO in which Arg481, which interacts with the heme D-ring propionates in a proposed proton output pathway, was replaced with Lys (RK481 CcO). The results show that in RK481 CcO the midpoint potentials of hemes a and a(3) were lowered by approximately 40 and approximately 15 mV, respectively, which stabilizes the reduced state of Cu(A) during reaction of the reduced CcO with O(2). In addition, while the pH dependence of the F --> O rate in wild-type CcO is determined by the protonation state of two protonatable groups with pK(a) values of 6.3 and 9.4, only the high-pK(a) group influences this rate in RK481 CcO. The results indicate that the protonation state of the Arg481 heme a(3) D-ring propionate cluster having a pK(a) of approximately 6.3 modulates the rate of internal electron transfer and may act as an acceptor of pumped protons.
authors.affiliation: Department of Biochemistry and Biophysics, The Arrhenius Laboratories for Natural Sciences, Stockholm University, SE-106 91 Stockholm, Sweden.
authors.forename: Gisela
authors.initials: G
authors.lastname: Brändén
authors.forename: Magnus
authors.initials: M
authors.lastname: Brändén
authors.forename: Bryan
authors.initials: B
authors.lastname: Schmidt
authors.forename: Denise A
authors.initials: DA
authors.lastname: Mills
authors.forename: Shelagh
authors.initials: S
authors.lastname: Ferguson-Miller
authors.forename: Peter
authors.initials: P
authors.lastname: Brzezinski
mesh_terms: D000818:Animals; D020134:Catalytic Domain; D004579:Electron Transport; D003576:Electron Transport Complex IV / Q000737:chemistry* / Q000378:metabolism*; D006418:Heme / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D006736:Horses; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010782:Photolysis; D011422:Propionates / Q000737:chemistry* / Q000378:metabolism; D017494:Proton Pumps / Q000737:chemistry* / Q000378:metabolism; D011522:Protons; D012242:Rhodobacter sphaeroides; D012996:Solutions; D013816:Thermodynamics
chemical_list: D011422:Propionates; D017494:Proton Pumps; D011522:Protons; D012996:Solutions; C027728:heme a; D006418:Heme; D003576:Electron Transport Complex IV