Calcium in the oxygen-evolving complex: Structural and mechanistic role determined by X-ray spectroscopy

This review describes the results from X-ray Absorption Spectroscopy studies that have contributed to an understanding of the role of Ca in the photosynthetic water-oxidation reaction. The results include the first Mn, Ca and Sr X-ray spectroscopy studies using Ca or Sr-substituted PS II samples that established the presence of a MnCa heteronuclear structure and its orientation, and the most recent Sr X-ray spectroscopy study using biosynthetically prepared Sr-containing PS II in the various S-states that provide important insights into the requirement for Ca in the mechanism of the Mn4Ca catalytic center.

► X-ray spectroscopy confirms the presence of a heteronuclear Mn4Ca cluster.
► Sr XAS is used to determine the orientation of Ca/Sr–Mn vectors in the membrane.
► XAS determines the Mn4Ca cluster structural changes in all the intermediate states.
► The mechanistic requirement for Ca for the water-oxidation reaction is discussed.