Interactions of photosynthetic reaction center with 2,3-dimethoxy-5-methyl-1,4-benzoquinone in reverse micelles

The photosynthetic reaction center from the bacterium Rhodobacter sphaeroides has been depleted of native quinone and solubilized in reverse micelles of phospholipids. The kinetics of the charge recombination from the secondary quinone acceptor (Q(B)) to the bacteriochlorophyll dimer (P) has been investigated, by flash absorption spectroscopy, as a function of the concentration of 2,3-dimethoxy-5-methyl-1,4-benzoquinone (Q(0)) at different temperatures. The dependence of the maximum bleaching amplitude after a light flash on Q(0) concentration and temperature leads to the determination of the enthalpy and entropy changes of binding to the Q(A) site (Delta H-QA(degrees) = -67 +/- 5 kJ mol(-1), Delta S-QA(degrees) = -156 +/- 15 J K-1 mol(-1)). Deconvolution of P+ decay shows that, in reverse micellar solutions, Q(0) molecules are in fast exchange between the Q(B) site of the protein and the organic bulk. Global analysis of the P+ decays allows proper separation of the contribution of the binding at Q(B) from that of the P(+)Q(A)(-)Q(B) --> P(+)Q(A)Q(B)(-) electron transfer. The enthalpy and entropy changes obtained for the binding at the Q(B) site (Delta H-QB(degrees) = -39 +/- 3 kJ mol(-1), Delta S-QB(degrees) = -72 +/- 9 J K-1 mol(-1)) and for the P(+)Q(A)(-)Q(B) reversible arrow P(+)Q(A)Q(B)(-) equilibrium (Delta H-AB(degrees) = -21.6 +/- 1.5 kJ mol(-1), Delta S-AB(degrees) = -55 +/- 6 J K-1 mol(-1)) are, evaluated. A comparison of the thermodynamic parameters determined for Q(0) with those previously found for the native ubiquinone-10 (Q(10)) indicates that the isoprenyl tail of Q(10) is strongly involved in Q(B) binding and function, and that both processes are strongly affected by entropy changes, likely due to tail-protein interaction.