Modulation of the free energy of the primary quinone acceptor (Q A) in reaction centers from Rhodobacter sphaeroides: contributions from the protein and protein–lipid(cardiolipin) interactions

Abstract

The redox midpoint potential ( E m) of Q A, the primary quinone of bacterial reaction centers, is substantially modulated by the protein environment. Quite subtle mutations in the Q A binding site, e.g., at residues M218, M252 and M265, cause significant increases in the equilibrium constant for electron transfer to Q B, which indicate relative lowering of the E m of Q A. However, reports of functional linkage between the Q A and Q B sites make it difficult to partition such effects between Q A and Q B from purely relative changes. We report here measurements on the yield of delayed fluorescence emission from the primary donor (P) accompanying the thermally activated charge recombination of P +Q A − to form the excited singlet state of the primary donor, P*. The results show that for mutations of the Q A site residues, Met M218 and Ile M265, essentially all the substantial thermodynamic effect is localized at Q A, with no evidence for a significant effect of these residues on the properties of Q B or the mutual influence (linkage) of Q A and Q B. We also report a significant lowering of the E m of Q A by the native lipid, cardiolipin, which brings the E m in isolated reaction centers more in line with that seen in native membrane vesicles (chromatophores). Possible origins of this effect are discussed in the context of the Q A binding site structure.