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We have recently shown that binding affinities of benzoquinones can be estimated by two methods in photosystem (PS) II particles (K. Satoh et al., Biochim. Biophys. Acta 1102, 45-52 (1992)). Using these methods we calculated the binding affinity of thymoquinone (2-methyl-5-isopropyl-p-benzoquinone) to the Q B  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 of thymoqui­ none to the Q B  site determined by several methods was around 0.33 mᴍ . (2) At low thymoquinone concentrations the quinone was supposed to accept electrons via Q B -plastoquinone, whereas at high concentrations the quinone seemed to bind to the Q B  site and accept an electron directly from Q -  A . Lower rates of photoreduction of the quinone at high concentrations were attributed to a slower turnover rate of the quinone at the Q B  site than that of endogenous plastoquinone. (3) A model for the function of plastoquinone at the Q B  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.

zeitschrift für naturforschung c

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