Mechanism of Electron Flow through the Q_B Site in Photosystem II. 4. Reaction Mechanism of Plastoquinone Derivatives at the Q_B Site in Spinach Photosystem II Membrane Fragments

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Interactions of externally added plastoquinone (PQ) derivatives (PQ_0-PQ_3) with the photosystem II (PSII) acceptor side were investigated in PSII membrane fragments prepared from spinach by measuring the photoreduction rates of PQ derivatives at various PQ concentrations, and the following results were obtained. From the kinetic analysis, all the PQ derivatives (PQ_0-PQ_2) except PQ_3 were shown to accept electrons at two sites (the Q_B site and the PQ site) as in the case of Synechococcus vulcanus PSII particles with benzoquinone derivatives [Satoh et al. (1995) Plant Cell Physiol. 36: 597]. Affinities of PQ derivatives at the Q_B site increased as the length of the isoprene side chain got longer, while those at the PQ site were not very much different for all the PQ derivatives tested in this study. The inhibitory effect of DCMU was noncompetitive, and, therefore, the affinity of PQ_3 for the PQ site was deter-mined while that for the Q_B Site could not be estimated presumably due to its fairly high affinity to the site. Based on the results obtained using PQ derivatives, the mechanism of interaction of an authentic PQ, PQ_9, at the Q_B Site is discussed.
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