Abstract
Water oxidation in photosystem II (PSII) is performed by the oxygen-evolving complex Mn(4)CaO(5) which can be extracted from PSII and then reconstructed using exogenous cations Mn(II) and Ca(2+). 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 H(2)O(2) intermediate. However, the cluster 3Mn1Fe can oxidize water to O(2) 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 O(2) evolution can reach 80% of the original when adding exogenous Ca(2+). 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.