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

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• 作者：Thomas Lohmiller ; Megan L. Shelby ; Xi Long ; Vittal K. Yachandra ; Junko Yano
• 刊名：Journal of Physical Chemistry B
• 出版年：2015
• 出版时间：October 29, 2015
• 年：2015
• 卷：119
• 期：43
• 页码：13742-13754
• 全文大小：702K
• ISSN：1520-5207

文摘

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鈥? and S₂ (S₂鈥? states, the S₂鈥瞃_Z^{鈥?/sup> 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鈥揗n 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 Y_Z鈥?/sup> (D1-Tyr161).}