title: Stabilization of charge separation and cardiolipin confinement in antenna-reaction center complexes purified from Rhodobacter sphaeroides.
abstract: The reaction center-light harvesting complex 1 (RC-LH1) purified from the photosynthetic bacterium Rhodobacter sphaeroides has been studied with respect to the kinetics of charge recombination and to the phospholipid and ubiquinone (UQ) complements tightly associated with it. In the antenna-RC complexes, at 6.5<pH<9.0, P(+)Q(B)(-) recombines with a pH independent average rate constant <k> more than three times smaller than that measured in LH1-deprived RCs. At increasing pH values, for which <k> increases, the deceleration observed in RC-LH1 complexes is reduced, vanishing at pH >11.0. In both systems kinetics are described by a continuous rate distribution, which broadens at pH >9.5, revealing a strong kinetic heterogeneity, more pronounced in the RC-LH1 complex. In the presence of the antenna the Q(A)Q(B)(-) state is stabilized by about 40 meV at 6.5<pH<9.0, while it is destabilized at pH >11. The phospholipid/RC and UQ/RC ratios have been compared in chromatophore membranes, in RC-LH1 complexes and in the isolated peripheral antenna (LH2). The UQ concentration in the lipid phase of the RC-LH1 complexes is about one order of magnitude larger than the average concentration in chromatophores and in LH2 complexes. Following detergent washing RC-LH1 complexes retain 80-90 phospholipid and 10-15 ubiquinone molecules per monomer. The fractional composition of the lipid domain tightly bound to the RC-LH1 (determined by TLC and (31)P-NMR) differs markedly from that of chromatophores and of the peripheral antenna. The content of cardiolipin, close to 10% weight in chromatophores and LH2 complexes, becomes dominant in the RC-LH1 complexes. We propose that the quinone and cardiolipin confinement observed in core complexes reflects the in vivo heterogeneous distributions of these components. Stabilization of the charge separated state in the RC-LH1 complexes is tentatively ascribed to local electrostatic perturbations due to cardiolipin.
authors.affiliation: Dipartimento di Biologia, Laboratorio di Biochimica e Biofisica, Università di Bologna, 40126 Bologna, Italy.
authors.forename: Manuela
authors.initials: M
authors.lastname: Dezi
authors.forename: Francesco
authors.initials: F
authors.lastname: Francia
authors.forename: Antonia
authors.initials: A
authors.lastname: Mallardi
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Colafemmina
authors.forename: Gerardo
authors.initials: G
authors.lastname: Palazzo
authors.forename: Giovanni
authors.initials: G
authors.lastname: Venturoli
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D002308:Cardiolipins / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry / Q000378:metabolism*; D010743:Phospholipids / Q000737:chemistry / Q000378:metabolism; D010788:Photosynthesis / Q000502:physiology; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D014451:Ubiquinone / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D002308:Cardiolipins; D045342:Light-Harvesting Protein Complexes; D010743:Phospholipids; D014451:Ubiquinone