The functional role of calcium in photosystem II

Abstract

The oxygen we breathe, like the fuel we burn with it, is produced by photosynthesis. A series of electron transfer processes, powered by sunlight, ultimately leads to oxidation of water and reduction of carbon dioxide. How plants oxidize water to oxygen is not clear. The active site contains 4 manganese, 1 calcium, and 1 chloride ion as essential cofactors. The manganese is involved in accumulating the oxidative power generated by 4 successive photoreactions, which is subsequently released in a concerted 4-electron reaction oxidizing 2 H2O molecules to 1 O2 molecule. The chloride may tune the relative redox potentials of the Mn ions. The essential role of the calcium ion, discovered 25 years ago and addressed in hundreds of publications, remains unknown. Current models suggest its requirement for O-O bond formation in the final step, but without calcium the highest oxidation state of the complex cannot be reached. Re-interpretation of the literature has now led to a different view that is confirmed by measurements showing that calcium is required for each step in the redox cycle. Probably each oxidation requires the removal of a proton via calcium-bound water, creating bases whose protonation during water oxidation makes the reaction energetically possible.