Protein/lipid interaction in bacterial photosynthetic reaction center: The role of phosphatidylcholine and phosphatidylglycerol in charge stabilization

The role of characteristic phospholipids of native membranes, phosphatidylcholine, PC, phosphatidylglycerol, PG, and cardiolipin, CL, was studied in the energetics of the acceptor quinone side in photosynthetic reaction centers of Rhodobacter sphaeroides. The rates of the first, kAB(1), and the second, kAB(2), electron transfer and that of the charge recombination, kBP, the free energy levels of QA-QB and QAQB- states, and the changes of charge compensating protein relaxation were determined in RCs incorporated into artificial lipid bilayer membranes. In RCs embedded in PC vesicle, kAB(1) and kAB(2) increased (3100 s-1 to 4100 s-1 and 740 s-1 to 3300 s-1, respectively), kBP decreased (0.77 s-1 to 0.39 s-1) compared to that measured in detergent at pH 7. In PG, kAB(1) and kBP decreased (to values of 710 s-1, and 0.26 s-1 respectively), while kAB(2) increased to 1500 s-1 at pH 7. The free energy between the QA-QB and QAQB- states decreased in PC and PG (G0QA-QB->QAQB- = -76.9 meV and -88.5 meV, respectively) compared to that of measured in detergent (-61.8 meV). The changes of the QA/QA- redox potential measured by delayed luminescence showed: 1) differential effect of lipids whether incorporated in micelles or vesicules; 2) altered binding interaction between anionic lipids and RC; 3) direct influence of PC and PG on the free energy levels of the primary and secondary quinones probably through the intraprotein hydrogen bonding network; 4) larger increase of the QA/QA- free energy in PG than in PC both in detergent micelles or in single component vesicles. Based on recent structural data possible interactions between lipids and the electron transfer components will be discussed.