title: Electron transfer from cytochrome c2 to the primary donor of Rhodobacter sphaeroides reaction centers. A temperature dependence study.
abstract: Kinetics of flash-induced electron transfer from the soluble cytochrome c2 to the primary donor (P) of the reaction center purified from the purple bacterium Rhodobacter sphaeroides R-26 were investigated by time-resolved absorption spectroscopy. Re-reduction of P+ induced by a laser pulse was measured at 1283 nm both in isolated reaction centers and in reconstituted proteoliposomes reproducing the lipid composition of the native membrane. The effects of temperature (230-300 K) and of the cytochrome c2/reaction center stoichiometry were examined. At room temperature, over a wide range of cytochrome c2 to reaction center molar ratios, the biphasic kinetics of cytochrome c2 oxidation in the microsecond-to-millisecond time scale could be accurately described by a minimum reaction scheme which includes a second-order collisional process (k = 1.4 x 10(9) M-1 s-1 and k = 2.4 x 10(9) M-1 s-1 in isolated and reconstituted reaction centers, respectively) and a first-order intracomplex electron donation (t1/2 = 590 +/- 110 ns in isolated reaction centers; t1/2 = 930 +/- 140 ns in proteoliposomes). At cytochrome c2 to reaction center molar ratios exceeding 5, the monomolecular process almost completely accounts for P+ re-reduction. At lower stoichiometries, the relative contribution of the two parallel reaction pathways is modulated by a single binding equilibrium between cytochrome c2 and reaction centers, yielding a binding constant of 3.5 x 10(5) M-1 in both systems.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Dipartimento di Biologia, Università di Bologna, Italy.
authors.forename: G
authors.initials: G
authors.lastname: Venturoli
authors.forename: A
authors.initials: A
authors.lastname: Mallardi
authors.forename: P
authors.initials: P
authors.lastname: Mathis
mesh_terms: D003574:Cytochrome c Group / Q000378:metabolism*; D045362:Cytochromes c2; D004579:Electron Transport; D066298:In Vitro Techniques; D007700:Kinetics; D010084:Oxidation-Reduction; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D011510:Proteolipids; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D003574:Cytochrome c Group; D045322:Photosynthetic Reaction Center Complex Proteins; D011510:Proteolipids; C020485:proteoliposomes; D045362:Cytochromes c2