Removal of Ca2+ from the Oxygen-Evolving Complex in Photosystem II Has Minimal Effect on the Mn4O5 Core Structure: A Polarized Mn X-ray Absorption Spectroscopy Study

摘要

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 S1 state. 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 S1 and S2 states, 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).