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

Abstract

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 (Q₀) at different temperatures. The dependence of the maximum bleaching amplitude after a light flash on Q₀ concentration and temperature leads to the determination of the enthalpy and entropy changes of binding to the Q_A site (ΔH_QA^°=−67±5 kJ mol⁻¹, ΔS_QA^°=−156±15 J K⁻¹ mol⁻¹). Deconvolution of P⁺ decay shows that, in reverse micellar solutions, Q₀ 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_AQ_B⁻ electron transfer. The enthalpy and entropy changes obtained for the binding at the Q_B site (ΔH_QB^°=−39±3 kJ mol⁻¹, ΔS_QB^°=−72±9 J K⁻¹ mol⁻¹) and for the P⁺Q_A⁻Q_B⇄P⁺Q_AQ_B⁻ equilibrium (ΔH_AB^°=−21.6±1.5 kJ mol⁻¹, ΔS_AB^°=−55±6 J K⁻¹ mol⁻¹) are, evaluated. A comparison of the thermodynamic parameters determined for Q₀ with those previously found for the native ubiquinone-10 (Q₁₀) indicates that the isoprenyl tail of Q₁₀ 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.