Heterogeneity of the quinone electron acceptor system in bacterial reaction centers

In reaction centers from Rhodopseudomonas viridis, biphasicity of the charge recombination kinetics between P+, the primary electron donor, and QA- and QB-, the primary and secondary quinone electron acceptors, respectively, have been analyzed by the flash-induced absorption change technique. We have studied the effect of quinone environment modifications on the ratio of the two phases for the P+QA- ([A fast/A slow]a) and P+QB- ([A fast/A slow]b) charge recombination processes. In reaction centers from Rps. viridis reconstituted in phosphatidylcholine liposomes a notable influence of the nature of the QB pocket occupancy was observed on (A fast/A slow)a. This ratio is much affected by the presence of ophenanthroline compared to reaction centers with an empty QB pocket or with terbutryn present. Because o-phenanthroline was proposed to hydrogen bind HisL190, whereas terbutryn does not, we suggest that a HisL190-Fe-HisM217 (the equivalent to HisL190 in the QA pocket) "wire" may be involved in the existence of the two conformational states associated with the two phases of charge recombination. In chromatophores from the T1 (SerL223 leads to Ala; ArgL2l7 leads to His) and T4 (TyrL222 leads to Phe) mutants no modification of the (A fast/A slow)a ratio is detected, whereas the (A fast/A slow)b ratios are substantially modified compared to the wild type (WT). In the T3 mutant (PheL216 leads to Ser; Val M263 leads to Phe [4.1 A apart from QA]), (A fast/A slow)a is notably changed compared to the WT. Our data show that any modification in the close protein environment of the quinones and/or of the HisL190 and HisM217 affects the equilibrium between the two reaction center states. This is consistent with the existence of two reaction center states from Rps. viridis, associated with two different conformations of the quinones-histidines-iron system. This "wire" allows both quinone protein pockets to interact over quite long distances.