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Updated: Jul 17, 2026

Monitoring ER/SR Calcium Release with the Targeted Ca2+ Sensor CatchER+
Published on: May 19, 2017
The PSII calcium site revisited
M Miqyass1, H J van Gorkom, C F Yocum
1Department of Biophysics, Huygens Laboratory, Leiden University, P.O. Box 9504, Leiden, RA 2300, The Netherlands.
Abstract:
Oxidation of H2O by photosystem II is a unique redox reaction in that it requires Ca2+ as well as Cl- as obligatory activators/cofactors of the reaction, which is catalyzed by Mn atoms. The properties of the binding site for Ca2+ in this reaction resemble those of other Ca2+ binding proteins, and recent X-ray structural data confirm that the metal is in fact ligated at least in part by amino acid side chain oxo anions. Removal of Ca2+ blocks water oxidation chemistry at an early stage in the cycle of redox reactions that result in O-O bond formation, and the intimate involvement of Ca2+ in this reaction that is implied by this result is confirmed by an ever-improving set of crystal structures of the cyanobacterial enzyme. Here, we revisit the photosystem II Ca2+ site, in part to discuss the additional information that has appeared since our earlier review of this subject (van Gorkom HJ, Yocum CF In: Wydrzynski TJ, Satoh K (eds) Photosystem II: the light-driven water:plastoquinone oxidoreductase), and also to reexamine earlier data, which lead us to conclude that all S-state transitions require Ca2+.
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