Electron Transport from QA to Thymoquinone in a Synechococcus Oxygen-Evolving Photosystem II Preparation: Role of QB and Binding Affinity of Thymoquinone to the QB Site

We have recently shown that binding affinities o f benzoquinones can be estimated by two methods in photosystem (PS) II particles (K. Satoh et al., Biochim. Biophys. Acta 1 1 0 2 ,4 5 -5 2 (1992)). Using these methods we calculated the binding affinity o f thymoquinone (2-methyl5-isopropyl-/?-benzoquinone) to the QB site and studied how the quinone accepts electrons in oxygen-evolving PS II particles isolated from the thermophilic cyanobacteria, Synechococcus elongatus and S. vulcanus. The results are as follows: (1) The binding constant o f thymoqui­ none to the QB site determined by several methods was around 0.33 m M . (2) At low thymoqui­ none concentrations the quinone was supposed to accept electrons via QB-plastoquinone, whereas at high concentrations the quinone seemed to bind to the QB site and accept an elec­ tron directly from Q~A. Lower rates o f photoreduction o f the quinone at high concentrations were attributed to a slower turnover rate o f the quinone at the QB site than that o f endogenous plastoquinone. (3) A model for the function o f plastoquinone at the QB site, which can explain all the results, was presented. According to this model, the plastoquinone molecule at the Q B site is not replaced by another plastoquinone molecule. Instead, it transfers electrons to pool plastoquinone molecules by turning over its head group but remaining its long side chain bound to the PS II complexes.