Light-Harvesting Complex 1 Stabilizes P+QB- Charge Separation in Reaction Centers of Rhodobacter sphaeroides

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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+QB- recombines with an average rate constant, 〈k〉 ≈ 0.3 s-1, more than three times smaller than that measured in RC deprived of the LH1 (〈k〉 ≈ 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〉 ≈ 0.2 s-1). Since the kinetics of P+QA- recombination is unaffected by the presence of the antenna, the P+QB- state appears to be energetically stabilized in core complexes. Determinations of the ubiquinone-10 (UQ10) complement associated with the purified RC−LH1 complexes always yield UQ10/RC ratios larger than 10. These quinone molecules are functionally coupled to the RC−LH1 complex, as judged from the extent of exogenous cytochrome c2 rapidly oxidized under continuous light excitation. Analysis of P+QB- recombination, based on a kinetic model which considers fast quinone equilibrium at the QB 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 QB site with the LH1 complex. The high UQ10 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.

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