Lawrence Berkeley National Laboratory

© 2015 American Chemical Society. Ca2+-depleted and Ca2+-reconstituted spinach photosystem II was studied using polarized X-ray absorption spectroscopy of oriented PS II preparations to investigate the structural and functional role of the Ca2+ion in the Mn4O5Ca cluster of the oxygen-evolving complex (OEC). Samples were prepared by low pH/citrate treatment as one-dimensionally ordered membrane layers and poised in the Ca2+-depleted S1(S1′) and S2(S2′) states, the S2′YZ•state, at which point the catalytic cycle of water oxidation is inhibited, and the Ca2+-reconstituted S1state. Polarized Mn K-edge XANES and EXAFS spectra exhibit pronounced dichroism. Polarized EXAFS data of all states of Ca2+-depleted PS II investigated show only minor changes in distances and orientations of the Mn-Mn vectors compared to the Ca2+-containing OEC, which may be attributed to some loss of rigidity of the core structure. Thus, removal of the Ca2+ion does not lead to fundamental distortion or rearrangement of the tetranuclear Mn cluster, which indicates that the Ca2+ion in the OEC is not critical for structural maintenance of the cluster, at least in the S1and S2states, but fulfills a crucial catalytic function in the mechanism of the water oxidation reaction. On the basis of this structural information, reasons for the inhibitory effect of Ca2+removal are discussed, attributing to the Ca2+ion a fundamental role in organizing the surrounding (substrate) water framework and in proton-coupled electron transfer to YZ•(D1-Tyr161).