A native electrostatic environment near QB is not sufficient to ensure rapid proton delivery in photosynthetic reaction centers

AbstractFlash-induced absorption spectroscopy has been used to characterize Rhodobacter capsulatus reaction centers mutated in the secondary quinone acceptor site (QB). We compared the wild-type, the L212Glu-L213Asp→Ala-Ala photosynthetically incompetent double mutant (DM), and two photocompetent revertants, the DM+L217Arg→Cys and the DM+M5Asn→Asp strains. The electrostatic environment for QB− is different in the two revertant strains. Only the L217Arg→Cys mutation nearly restores the native electrostatic environment of QB−. However, the level of recovery of the reaction center function, measured by the rates of second electron transfer and cytochrome c turnover, is quite incomplete in both strains. This shows that a wild-type-like electrostatic environment of QB− cannot ensure on its own, rapid and efficient proton transfer to QB−