Modulation of the free energy of the primary quinone acceptor (QA) in reaction centers from Rhodobacter sphaeroides: contributions from the protein and protein-lipid(cardiolipin) interactions.

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.