Abstract
The tetra-manganese complex of photosystem II is the site of the production of most of the atmospheric dioxygen. To understand its function, an atomic-resolution structural model is required. X-ray absorption spectroscopy and protein crystallography yielded models for the arrangement of the four Mn atoms. The location of the functionally essential calcium cofactor within the complex, however, is still debated. To dwell on this point, XAS was performed at the Ca K-edge. Simulations of Ca EXAFS spectra reveal that at least two Mn atoms are located at a distance of ∼3.3 Å to one Ca ion. Ca bound to the complex seems not to be fully hydrated. On basis of the complementary structural information derived from XAS at the Mn and Ca K-edge, a model of the Mn4/Ca complex in its S1-state is proposed which predicts intimate links between the Ca ion and Mn, possibly by bridging substrate-water molecules.
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