title: Temperature dependence of the electrogenic reaction in the QB site of the Rhodobacter sphaeroides photosynthetic reaction center: the QA-QB --> QAQB- transition.
abstract: The temperature dependencies for the kinetics and relative amplitudes of electrogenic reaction(s) coupled with the first reduction of the secondary quinone acceptor QB were measured with dark-adapted chromatophores of Rhodobacter sphaeroides. The kinetics, while acceptably fitted by a single exponent at room temperature, clearly split into two components below 15 degrees C (rise times, 25 micros and 300 micros at pH 7.0 and 10 degrees C) with the slow phase ousting the fast one at pH > 9.0. The activation energies of the fast and slow phases were estimated at pH 7.0 as < 10 kJ/mol and 60-70 kJ/mol, respectively. To explain the kinetic heterogeneity of the QB --> QB- transition, we suggest two possible conformations for the neutral oxidized ubiquinone at the QB site: one with a hydrogen bond between the side chain carboxyl of Glu-L212 and the methoxy oxygen at C3 of the QB ring (QB-H-Glu centers) and the other one, without this bond (QB:Glu- centers). The fast phase is attributed to QA- QB-H-Glu --> QA QB-H-Glu transition, whereas the slow one to the QA- QB:Glu- --> QA- QB-H-Glu --> QA QB(-)-H-Glu transition.
authors.affiliation: A.N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Russia.
authors.forename: O A
authors.initials: OA
authors.lastname: Gopta
authors.forename: D A
authors.initials: DA
authors.lastname: Bloch
authors.forename: D A
authors.initials: DA
authors.lastname: Cherepanov
authors.forename: A Y
authors.initials: AY
authors.lastname: Mulkidjanian
mesh_terms: D020130:Bacterial Chromatophores / Q000378:metabolism; D001692:Biological Transport; D004563:Electrochemistry; D004583:Electrons; D007700:Kinetics; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D011809:Quinones / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones