Interaction of terbium cations with the oxidizing side of photosystem II higher plants

Photosystem 2 (PSII) of the higher plants carries out the photoinduced oxidation of water and releases the molecular oxygen into atmosphere as a by-product of this reaction. The oxygen evolving complex (OEC) is located on the donor side of PSII and contains the Mn₄CaO₅ cluster catalyzing water oxidation. A cofactor of this reaction is Ca²⁺ cation. According to some physical and chemical properties (ionic radius, a coordination number) lanthanides ions are similar to Ca²⁺ ion parameters, and in the calcium binding proteins substitution of Ca²⁺ with these cations is possible. Some representatives of this cations group can bind to Ca-binding site in the OEC of PSII. In presented paper we investigated the interaction with the PSII donor side one of the least studied lanthanides — terbium. Results of our experiments showed that the incubation of native PSII preparations with Tb³⁺ cations lead to irreversible inhibition of oxygen evolving function (»75% inhibition at 2 mM of Tb³⁺). At the same time electron transport on the acceptor side of PSII substantially remains. Addition to incubation buffer 30 mM Ca²⁺ reduces the inhibition effect of terbium approximately twice. Obtained results well correspond to the data of measurements of fluorescence induction kinetic in PSII membranes in the presence of exogenous Ca²⁺ and Tb³⁺ and allow to assume that terbium cations displace Ca²⁺ from OEC. Calcium release induced by Tb³⁺ results to incomplete water oxidation producing H₂O₂ instead of molecular oxygen as it was shown for PSII without Ca²⁺ in OEC earlier.

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