Reduction and protonation of the secondary quinone acceptor of Rhodobacter sphaeroides photosynthetic reaction center: kinetic model based on a comparison of wild-type chromatophores with mutants carrying Arg→Ile substitution at sites 207 and 217 in the L-subunit

AbstractAfter the light-induced charge separation in the photosynthetic reaction center (RC) of Rhodobacter sphaeroides, the electron reaches, via the tightly bound ubiquinone QA, the loosely bound ubiquinone QB. After two subsequent flashes of light, QB is reduced to ubiquinol QBH2, with a semiquinone anion QB− formed as an intermediate after the first flash. We studied QBH2 formation in chromatophores from Rb. sphaeroides mutants that carried Arg→Ile substitution at sites 207 and 217 in the L-subunit. While Arg-L207 is 17 Å away from QB, Arg-L217 is closer (9 Å) and contacts the QB-binding pocket. From the pH dependence of the charge recombination in the RC after the first flash, we estimated ΔGAB, the free energy difference between the QA−QB and QAQB− states, and pK212, the apparent pK of Glu-L212, a residue that is only 4 Å away from QB. As expected, the replacement of positively charged arginines by neutral isoleucines destabilized the QB− state in the L217RI mutant to a larger extent than in the L207RI one. Also as expected, pK212 increased by ∼0.4 pH units in the L207RI mutant. The value of pK212 in the L217RI mutant decreased by 0.3 pH units, contrary to expectations. The rate of the QA−QB−→QAQBH2 transition upon the second flash, as monitored by electrometry via the accompanying changes in the membrane potential, was two times faster in the L207RI mutant than in the wild-type, but remained essentially unchanged in the L217RI mutant. To rationalize these findings, we developed and analyzed a kinetic model of the QA−QB−→QAQBH2 transition. The model properly described the available experimental data and provided a set of quantitative kinetic and thermodynamic parameters of the QB turnover. The non-electrostatic, ‘chemical’ affinity of the QB site to protons proved to be as important for the attracting protons from the bulk, as the appropriate electrostatic potential. The mutation-caused changes in the chemical proton affinity could be estimated from the difference between the experimentally established pK212 shifts and the expected changes in the electrostatic potential at Glu-L212, calculable from the X-ray structure of the RC. Based on functional studies, structural data and kinetic modeling, we suggest a mechanistic scheme of the QB turnover. The detachment of the formed ubiquinol from its proximal position next to Glu-L212 is considered as the rate-limiting step of the reaction cycle