Light-harvesting complex 1 stabilizes P+QB- charge separation in reaction centers of Rhodobacter sphaeroides

Biochemistry. 2004 Nov 9;43(44):14199-210. doi: 10.1021/bi048629s.

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.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / chemistry*
  • Bacterial Proteins / metabolism
  • Binding Sites
  • Electrochemistry
  • Electron Transport
  • Free Radicals
  • Kinetics
  • Lasers
  • Light-Harvesting Protein Complexes / chemistry*
  • Light-Harvesting Protein Complexes / isolation & purification
  • Light-Harvesting Protein Complexes / metabolism
  • Photolysis
  • Photosynthetic Reaction Center Complex Proteins / chemistry
  • Photosynthetic Reaction Center Complex Proteins / metabolism
  • Rhodobacter sphaeroides / chemistry*
  • Rhodobacter sphaeroides / growth & development
  • Rhodobacter sphaeroides / metabolism
  • Ubiquinone / chemistry*
  • Ubiquinone / isolation & purification
  • Ubiquinone / metabolism

Substances

  • Bacterial Proteins
  • Free Radicals
  • Light-Harvesting Protein Complexes
  • Photosynthetic Reaction Center Complex Proteins
  • Ubiquinone
  • Ubiquinone Q2