Binding affinities of oxidized and reduced forms of tetrahalogenated benzoquinones to the QB site in oxygen-evolving photosystem II particles from Syncechococcus elongatus
1994
Satoh, K. | Katoh, S. | Donner, A. Donner, A. | Oettmeier, W. Oettmeier, W.
Binding affinities of the QB site for four tetrahalogenated benzoquinones (THBQs) were investigated by measuring their ability to serve as electron acceptors or act as inhibitors of oxygen evolution in Synechococcus photosystem II particles. Iodanil, bromanil and chloranil but not fluoranil induced a rapid oxidation of QA- and doubled the area over the fluorescence induction curve, indicating dark oxidation of Q400. Analyses of these two THBQ-induced reactions and inhibition of the accelerated QA- oxidation by DCMU yielded binding constants of the quinones comparable to those determined from measurements of oxygen evolution. Generally, THBQs bound tightly to the QB site. However, the binding affinity varied in a wide range with THBQs. The QB site bound iodanil with an extremely high affinity but fluoranil relatively weakly. The hydroquinone forms of the THBQs also bound to the QB site and inhibited QA- oxidation by QB. The concentrations of the hydroquinones required for 50% inhibition of QA- oxidation suggest that the QB site binds the hydroquinones more weakly than the corresponding quinones except for fluoranil, which binds to the QB site more tightly in its reduced form than in oxidized one. The abilities of THBQs to function as electron acceptors or inhibitors of oxygen evolution, and as oxidants of Q400 in the dark, are discussed in relation to the binding affinities of the quinones to the QB site.
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