Light-dependent water oxidation and evolution of molecular oxygen in the oxygenic organisms is performed by oxygen-evolving complex (OEC) of photosystem II (PSII). Catalytic center of the OEC is the Mn4CaO5 cluster the structure of which is determined now. The presence of calcium cation in the OEC is necessary for OEC function and extraction of this cation from the OEC without depletion of Mn cations inhibits the oxygen evolution reaction. However, the mechanism of Ca2+ participation in the water splitting by OEC is unclear now. It is known that the ions of rare-earth metals have similar physical and chemical properties with Ca2+ ions and interact with Ca2+-binding proteins. Possibly such interaction could either inhibit the protein function in the absence of Ca2+ or could partially reduce it function in the absence of Ca2+. In the present study it was investigated the effect of the one of rare-earth metals - terbium - on the electron-transport rate in the membrane preparations of PSII. The focus of attention in our study was the correlation between the terbium effect on the oxygen evolution reaction and the electron transport to artificial electron acceptor. We found that Tb3+ inhibits the oxygen evolution more effectively then reduction of the artificial electron acceptor 2,6-dichlorophenolindophenol on the acceptor side of PSII that is terbium action provides the decoupling of these processes. Similar effect is observed in Ca-depleted PSII membranes [Semin et al. Photosynth. Res. 98 (2008) 235]. Taking these results into account we suppose that terbium inhibition effect is determined by its substitution of Ca2+ cation in the OEC and/or extraction of extrinsic proteins PsbP and PsbQ.
photosystem II, terbium, calcium, oxygen-evolving complex
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