Creation of a 3Mn/1Fe cluster in the oxygen-evolving complex of photosystem II and investigation of its functional activity

Extraction of Mn cations from the oxygen-evolving complex (OEC) of Ca-depleted PSII membranes (PSII [-Ca,4Mn]) by reductants like hydroquinone (H(2)Q) occurs with lower efficiency at acidic pH (2Mn/reaction center [RC] are extracted at pH 5.7) than at neutral pH (3Mn/RC are extracted at pH 6.5) [Semin et al. Photosynth. Res. 125 (2015) 95]. Fe(II) also extracts Mn cations from PSII(-Ca,4Mn), but only 2Mn/RC at pH 6.5, forming a heteronuclear 2Mn/2Fe cluster [Semin and Seibert, J. Bioenerg. Biomembr. 48 (2016) 227]. Here we investigated the efficiency of Mn extraction by Fe(II) at acidic pH and found that Fe(II) cations can extract only 1Mn/RC from PSII(-Ca,4Mn) membranes at pH 5.7, forming a 3Mn/1Fe cluster. Also we found that the presence of Fe cations in a heteronuclear cluster (2Mn/2Fe) increases the resistance of the remaining Mn cations to H(2)Q action, since H(2)Q can extract Mn cations from homonuclear Mn clusters of PSII(-Ca,4Mn) and PSII (-Ca,2Mn) membranes but not from the heteronuclear cluster in PSII(-Ca,2Mn,2Fe) membranes. H(2)Q also cannot extract Mn from PSII membranes obtained by incubation of PSII(-Ca,4Mn) membranes with Fe(II) cations at pH 5.7, which suggests the formation of a heteronuclear 3Mn/1Fe cluster in the OEC. Functional activity of PSII with a 3Mn/1Fe cluster was investigated. PSII preparations with a 3Mn/1Fe cluster in the OEC are able to photoreduce the exogenous electron acceptor 2,6-dichlorophenolindophenol, possibly due to incomplete oxidation of water molecules as is the case with PSII(-Ca,2Mn,2Fe) samples. However, in the contrast to PSII (-Ca,2Mn,2Fe) samples PSII(-Ca,3Mn,1Fe) membranes can evolve O-2 at a low rate in the presence of exogenous Ca2+ (at about 27% of the rate of O-2 evolution in native PSII membranes). The explanation for this phenomenon (either water splitting and production of molecular O-2 by the 3Mn/1Fe cluster or apparent O-2 evolution due to minor contamination of PSII(3Mn,1Fe) samples with PSII(-Ca,4Mn) membranes) is discussed.