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Updated: Jul 17, 2026
12:30
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.
Photosynthesis Research
|January 20, 2007
Summary
Calcium (Ca2+) and chloride (Cl-) ions are essential for water oxidation by photosystem II. New structural data confirm Ca2+ is crucial for all S-state transitions in this vital redox reaction.
Area of Science:
- Biochemistry
- Photosynthesis research
- Enzyme kinetics
Background:
- Photosystem II (PSII) catalyzes the light-driven oxidation of water (H2O) to O2.
- This unique redox reaction requires Ca2+ and Cl- as essential cofactors.
- The Mn cluster within PSII is the catalytic site for water oxidation.
Purpose of the Study:
- To review recent findings on the Ca2+ binding site in PSII.
- To re-examine existing data on the role of Ca2+ in water oxidation.
- To investigate the necessity of Ca2+ for all S-state transitions.
Main Methods:
- Analysis of X-ray structural data of the cyanobacterial enzyme.
- Review of published experimental data on Ca2+ and Cl- requirements.
- Re-evaluation of S-state transition mechanisms.
Main Results:
- Ca2+ binding site properties resemble those in other Ca2+-binding proteins.
- X-ray structures confirm Ca2+ ligation by amino acid side chain oxo anions.
- Ca2+ removal inhibits water oxidation at an early stage, impacting O-O bond formation.
- All S-state transitions in the catalytic cycle require Ca2+.
Conclusions:
- Ca2+ is indispensable for the entire catalytic cycle of water oxidation in PSII.
- Structural and biochemical evidence strongly supports the critical role of Ca2+.
- Ca2+ is required for all S-state transitions, highlighting its fundamental importance.
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