Protein/Lipid Interaction in the Bacterial Photosynthetic Reaction Center: Phosphatidylcholine and Phosphatidylglycerol Modify the Free Energy Levels of the Quinones

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, k AB(1), and the second, kAB(2), electron transfer and that of the charge recombination, kBP, the free energy levels of Q A-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 the PC vesicle, kAB(1) and kAB(2) increased (from 3100 to 4100 s-1 and from 740 to 3300 s-1, respectively) and kBP decreased (from 0.77 to 0.39 s-1) compared to those measured in detergent at pH 7. In PG, kAB(1) and kBP decreased (to values of 710 and 0.26 s-1, respectively), while kAB(2) increased to 1506 s-1 at pH 7. The free energy between the QA-QB and Q AQB- states decreased in PC and PG (ΔG°QA-QB→QAQ B- = -76.9 and -88.5 meV, respectively) compared to that measured in detergent (-61.8 meV). The changes of the QA/Q A- redox potential measured by delayed luminescence showed (1) a differential effect of lipids whether RC incorporated in micelles or vesicles, (2) an altered binding interaction between anionic lipids and RC, (3) a direct influence of PC and PG on the free energy levels of the primary and secondary quinones probably through the intraprotein hydrogen-bonding network, and (4) a larger increase of the QA/QA- free energy in PG than in PC both in detergent micelles and in single-component vesicles. On the basis of recent structural data, implications of the binding properties of phospholipids to RC and possible interactions between lipids and electron transfer components will be discussed

ENTHALPY/ENTROPY DRIVEN ACTIVATION OF THE FIRST INTERQUINONE ELECTRON TRANSFER IN BACTERIAL PHOTOSYNTHETIC REACTION CENTERS EMBEDDED IN VESICLES OF PHYSIOLOGICALLY IMPORTANT PHOSPHOLIPIDS

The thermodynamics and kinetics of light-induced electron transfer in bacterial photosynthetic RCs are sensitive to physiologically important lipids (phosphatidylcholine, cardiolipin and phosphatidylglycerol) in the environment. The analysis of the temperature-dependence of the rate of the P+QA-QB→ P+QAQB- interquinone electron transfer revealed high enthalpy change of activation in zwitterionic or neutral micelles and vesicles and low enthalpy change of activation in vesicles constituted of negatively charged phospholipids. The entropy change of activation was compensated by the changes of enthalpy, thus the free energy change of activation (≈ 500 meV) did not show large variation in vesicles of different lipids

Carbon nanotubes quench singlet oxygen generated by photosynthetic reaction centers

Photosensitizers may convert molecular oxygen into reactive oxygen species (ROS) including, e.g., singlet oxygen (O-1(2)), superoxide anion (O-2(-center dot)), and hydroxyl radicals ((OH)-O-center dot), chemicals with extremely high cyto- and potential genotoxicity. Photodynamic ROS reactions are determinative in medical photodynamic therapy (cancer treatment with externally added photosensitizers) and in reactions damaging the photosynthetic apparatus of plants (via internal pigments). The primary events of photosynthesis take place in the chlorophyll containing reaction center protein complex (RC), where the energy of light is converted into chemical potential. O-1(2) is formed by both bacterial bacteriochlorophylls and plant RC triplet chlorophylls in high light and if the quenching of O-1(2) is impaired. In plant physiology, reducing the formation of the ROS and thus lessening photooxidative membrane damage (including the RC protein) and increasing the efficiency of the photochemical energy conversion is of special interest. Carbon nanotubes, in artificial systems, are also known to react with singlet oxygen. To investigate the possibility of O-1(2) quenching by carbon nanotubes in a biological system, we studied the effect of carbon nanotubes on O-1(2) photogenerated by photosynthetic RCs purified from purple bacteria. 1,3-Diphenylisobenzofuran (DPBF), a dye responding to oxidation by O-1(2) with absorption decrease at 420nm was used to measure O-1(2) concentrations. O-1(2) was produced either from a photosensitizer (methylene blue) or from triplet photosynthetic RCs and the antioxidant capacity of carbon nanotubes was assessed. Less O-1(2) was detected by DPBF in the presence of carbon nanotubes, suggesting that these are potential quenchers of this ROS. (C) 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei