title: A pH-dependent polarity change at the binuclear center of reduced cytochrome c oxidase detected by FTIR difference spectroscopy of the CO adduct.
abstract: A pH-dependent polarity change at the heme-copper binuclear center of the aa3-type cytochrome c oxidase from Rhodobacter sphaeroides has been identified by low-temperature FTIR difference spectroscopy. "Light"-minus-"dark" FTIR difference spectra of the fully reduced CO-enzyme adduct were recorded at a range of pH, and the dominance of different populations of bound CO, alpha and beta, was found to vary with pH. An apparent pKa of about 7.3 for the transition was obtained. The alpha and beta forms are differentiated by different polarities at the heme-copper binuclear center of the enzyme, sensed by the stretching frequencies of CO bound either to the heme alpha 3 Fe or to CuB. Several site-directed mutants in the vicinity of the heme-copper center are shown to favor either the alpha or the beta forms of the enyzme, suggesting that what is being monitored is an equilibrium between two conformations of the reduced form of the oxidase. Recent resonance Raman evidence has been presented demonstrating that the alpha and beta forms of the R. sphaeroides oxidase exist at room temperature; therefore, the pH-dependent change in the polarity in the vicinity of the heme-copper center may be functionally significant.
authors: School of Chemical Sciences, University of Illinois, Urbana 61801, USA.
authors: D M
authors: DM
authors: Mitchell
authors: J P
authors: JP
authors: Shapleigh
authors: A M
authors: AM
authors: Archer
authors: J O
authors: JO
authors: Alben
authors: R B
authors: RB
authors: Gennis
mesh_terms: D000595:Amino Acid Sequence; D002248:Carbon Monoxide / Q000378:metabolism; D003300:Copper / Q000378:metabolism; D003576:Electron Transport Complex IV / Q000737:chemistry / Q000378:metabolism*; D006418:Heme / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D012242:Rhodobacter sphaeroides / Q000201:enzymology*; D017550:Spectroscopy, Fourier Transform Infrared
chemical_list: D006418:Heme; D003300:Copper; D002248:Carbon Monoxide; D007501:Iron; D003576:Electron Transport Complex IV