Current analysis of cations substitution in the oxygen-evolving complex of photosystem II.

Water oxidation in photosystem II (PSII) is performed by the oxygen-evolving complex Mn₄CaO₅ which can be extracted from PSII and then reconstructed using exogenous cations Mn(II) and Ca²⁺. The binding efficiency of other cations to the Mn-binding sites in Mn-depleted PSII was investigated without any positive results. At the same time, a study of the Fe cations interaction with Mn-binding sites showed that it binds at a level comparable with the binding of Mn cations. Binding of Fe(II) cations first requires its light-dependent oxidation. In general, the interaction of Fe(II) with Mn-depleted PSII has a number of features similar to the two-quantum model of photoactivation of the complex with the release of oxygen. Interestingly, incubation of Ca-depleted PSII with Fe(II) cations under certain conditions is accompanied by the formation of a chimeric cluster Mn/Fe in the oxygen-evolving complex. PSII with the cluster 2Mn2Fe was found to be capable of water oxidation, but only to the H₂O₂ intermediate. However, the cluster 3Mn1Fe can oxidize water to O₂ with an efficiency about 25% of the original in the absence of extrinsic proteins PsbQ and PsbP. In the presence of these proteins, the efficiency of O₂ evolution can reach 80% of the original when adding exogenous Ca²⁺. In this review, we summarized information on the formation of chimeric Mn-Fe clusters in the oxygen-evolving complex. The data cited may be useful for detailing the mechanism of water oxidation.