title: Light-harvesting complex 1 stabilizes P+QB- charge separation in reaction centers of Rhodobacter sphaeroides.
abstract: The kinetics of charge recombination following photoexcitation by a laser pulse have been analyzed in the reaction center-light harvesting complex 1 (RC-LH1) purified from the photosynthetic bacterium Rhodobacter sphaeroides. In RC-LH1 core complexes isolated from photosynthetically grown cells P(+)Q(B)(-) recombines with an average rate constant, k approximately 0.3 s(-1), more than three times smaller than that measured in RC deprived of the LH1 (k approximately 1 s(-1)). A comparable, slowed recombination kinetics is observed in RC-LH1 complexes purified from a pufX-deleted strain. Slowing of the charge recombination kinetics is even more pronounced in RC-LH1 complexes isolated from wild-type semiaerobically grown cells (k approximately 0.2 s(-1)). Since the kinetics of P(+)Q(A)(-) recombination is unaffected by the presence of the antenna, the P(+)Q(B)(-) state appears to be energetically stabilized in core complexes. Determinations of the ubiquinone-10 (UQ(10)) complement associated with the purified RC-LH1 complexes always yield UQ(10)/RC ratios larger than 10. These quinone molecules are functionally coupled to the RC-LH1 complex, as judged from the extent of exogenous cytochrome c(2) rapidly oxidized under continuous light excitation. Analysis of P(+)Q(B)(-) recombination, based on a kinetic model which considers fast quinone equilibrium at the Q(B) binding site, indicates that the slowing down of charge recombination kinetics observed in RC-LH1 complexes cannot be explained solely by a quinone concentration effect and suggests that stabilization of the light-induced charge separation is predominantly due to interaction of the Q(B) site with the LH1 complex. The high UQ(10) complements detected in RC-LH1 core complexes, but not in purified light-harvesting complex 2 and in RC, are proposed to reflect an in vivo heterogeneity in the distribution of the quinone pool within the chromatophore bilayer.
authors.affiliation: Dipartimento di Biologia, Laboratorio di Biochimica e Biofisica, Università di Bologna, 40126 Bologna, Italy. francia@alma.unibo.it
authors.forename: Francesco
authors.initials: F
authors.lastname: Francia
authors.forename: Manuela
authors.initials: M
authors.lastname: Dezi
authors.forename: Alberto
authors.initials: A
authors.lastname: Rebecchi
authors.forename: Antonia
authors.initials: A
authors.lastname: Mallardi
authors.forename: Gerardo
authors.initials: G
authors.lastname: Palazzo
authors.forename: Bruno Andrea
authors.initials: BA
authors.lastname: Melandri
authors.forename: Giovanni
authors.initials: G
authors.lastname: Venturoli
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D004563:Electrochemistry; D004579:Electron Transport; D005609:Free Radicals; D007700:Kinetics; D007834:Lasers; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism; D010782:Photolysis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism; D012242:Rhodobacter sphaeroides / Q000737:chemistry* / Q000254:growth & development / Q000378:metabolism; D014451:Ubiquinone / Q000737:chemistry* / Q000302:isolation & purification / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D005609:Free Radicals; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D014451:Ubiquinone; C025204:Ubiquinone Q2