Electron acceptors of bacterial photosynthetic reaction centers II. H+ binding coupled to secondary electron transfer in the quinone acceptor complex

摘要: The photoreduction of ubiquinone in the electron acceptor complex (Q1Q11) photosynthetic reaction centers from Rhodopseudomonas sphaeroides, R26, was studied a series short, saturating flashes. specific involvement H+ reduction revealed by pH dependence transfer events and net binding during formation ubiquinol, which requires two turnovers photochemical act. On first flash Q11 receives an via Q1 to form stable ubisemiquinone anion (Q−11); second generates Q−1. At low semiquinones rapidly disproportionate with uptake 2 H+, produce Q11H2. This yields out-of-phase binary oscillations for anionic semiquinone uptake. Above 6 there is progressive increase on equivalent decrease until, at about 9.5, extent same all oscillations, however, are undiminished up 9. It suggested that non-chromophoric, acid-base group undergoes pK shift response appearance this site protonation flash. group, may be center protein, appears subsequently involved leading fully reduced ubiquinol. other proton always taken bound directly Q−11. values above 8.0, it rate limiting disproportionation kinetics, diffusion controlled, properly responsive prevailing pH. Below 8, further step mechanism shown both following