Interaction of Terbium Cations with the Donor Side of Photosystem II in Higher Plants

Аннотация

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.