Abstract

Photosystem II (PSII) of higher plants provides photoinduced water oxidation and produces molecular oxygen as a by-product of this reaction followed by its release into the atmosphere. The oxygen-evolving complex (OEC) is located on the donor side of PSII and contains an Mn₄CaO₅ cluster catalyzing water oxidation. A cofactor of this reaction is a Ca²⁺ cation. Lanthanide ions are similar to Ca²⁺ ions in relation to some physical and chemical properties, such as the ionic radius and coordination number. In the case of calcium-binding proteins, Ca²⁺ substitution with these cations is possible. Some representatives of this group of cations are able to bind to the Ca-binding site of PSII. In this study, we investigated an interaction between the donor side of PSII and terbium, one of the least studied lanthanides. According to the obtained results, the incubation of native PSII preparations with Tb³⁺ cations caused an irreversible inhibition (by ~75% for 2 mM of Tb³⁺) of the oxygen evolving function. At the same time, changes in the electron transport at the acceptor side of PSII remained rather insignificant. Supplementation of an incubation buffer with 30 mM Ca²⁺ reduced the inhibiting effect of terbium almost two times. The obtained results agree with the fluorescence induction kinetics measurement in PSII preparations in the presence of exogenous Ca²⁺ and Tb³⁺ and allow for the supposition that terbium cations displace Ca²⁺ ions from OEC. This terbium-induced release of calcium from the catalytic center results in incomplete water oxidation producing H₂O₂ instead of molecular oxygen as it has been shown earlier for PSII in the absence of Ca²⁺ in OEC.

中文翻译：

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Higher阳离子与高等植物光系统II供体的相互作用

摘要

高等植物的光系统II（PSII）提供光诱导的水氧化，并产生分子氧作为该反应的副产物，然后释放到大气中。放氧络合物（OEC）位于PSII的供体侧，并包含Mn ₄ CaO ₅簇，该簇催化水氧化。该反应的辅助因子是Ca ²⁺阳离子。就某些物理和化学性质（例如离子半径和配位数）而言，镧系元素离子与Ca ²⁺离子相似。如果是钙结合蛋白，则Ca ²⁺这些阳离子的取代是可能的。这组阳离子的某些代表能够与PSII的Ca结合位点结合。在这项研究中，我们调查了PSII供体与and之间的相互作用，,是研究最少的镧系元素之一。根据获得的结果，将天然PSII制剂与Tb ³⁺阳离子一起孵育会导致对氧气释放功能的不可逆抑制（对于2 mM Tb ^3+而言，约为75％）。同时，PSII受体侧电子传输的变化仍然很小。用30 mM Ca ²⁺补充孵育缓冲液几乎降低了of的抑制效果两倍。获得的结果与在存在外源Ca ²⁺和Tb ³⁺的情况下PSII制剂中的荧光诱导动力学测量相符，并假设ter阳离子取代了OEC中的Ca ²⁺离子。ter从催化中心释放钙会导致水氧化不完全，从而生成H ₂ O _2，而不是分子氧，正如先前在OEC中不存在Ca ^2+的PSII中所显示的那样。