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Abstract: The redox potential of QA in Photosystem II (PSII) from Thermosynechococcus elongatus was titrated monitoring chlorophyll fluorescence. A high potential form (E m =+60±25mV) was found in the absence of Mn4Ca, the active site for water oxidation. The low potential form (E m =−60±48mV), which is difficult to measure in conventional titration experiments, could be “locked in” by cross-linking the active enzyme. This indicates that the presence of Mn4Ca is relayed to the quinone site by significant structural changes in the protein. The presence of high and low potential forms agrees with what has been seen in plants, algae from our lab and in T. elongatus (Shibamoto et al., Biochemistry 48 (2009) 10682–10684). In the latter work, the potentials of QA were shifted to lower potentials compared to other measurements. The redox potential of QA in Mn-depleted PSII from spinach was titrated in the presence of redox mediators and the midpoint potential was shifted by 80mV towards a more negative value compared to titrations without mediators. The lower values of the midpoint potential of the () redox couple in the literature could be due to a perturbation due to a specific mediator. [Copyright &y& Elsevier]

Abstract: The core complex of photosystem II (PSII) was purified from thermophillic cyanobaterium Thermosynechococcus elongatus grown in Sr2+-containing and Ca2+-free medium. Functional in vivo incorporation of Sr2+ into the oxygen-evolving complex (OEC) was confirmed by EPR analysis of the isolated and highly purified SrPSII complex in agreement with the previous study of Boussac et al. [J. Biol. Chem. 279 (2004) 22809–22819]. Three-dimensional crystals of SrPSII complex were obtained which diffracted to 3.9 Å and belonged to the orthorhombic space group P212121 with unit cell dimensions of a =133.6 Å, b =236.6 Å, c =307.8 Å. Anomalous diffraction data collected at the Sr K-X-ray absorption edge identified a novel Sr2+-binding site which, within the resolution of these data (6.5 Å), is consistent with the positioning of Ca2+ in the recent crystallographic models of PSII [Ferreira et al. Science 303 (2004) 1831–1838, Loll et al. Nature 438 (2005) 1040–1044]. Fluorescence measurements on SrPSII crystals confirmed that crystallized SrPSII was active in transferring electrons from the OEC to the acceptor site of the reaction centre. However, SrPSII showed altered functional properties of its modified OEC in comparison with that of the CaPSII counterpart: slowdown of the QA-to-QB electron transfer and stabilized S2QA − charge recombination. [Copyright &y& Elsevier]