Investigation of the functional role of Ca2+ in the oxygen-evolving complex of photosystem II:: A pH-dependence study of the substitution of Ca2+ by Sr2+

The oxygen-evolving complex (OEC) of photosystem II (PSII) is the catalytic site for the oxidation of water to O-2 in photosynthesis. The OEC consists of a tetranuclear manganese (Mn-4) cluster with calcium and chloride ions functioning as essential cofactors. Previous studies have shown that substitution of Ca2+ in the OEC by various divalent and trivalent metal ions results in the loss of oxygen-evolution activity of PSII. Sr2+ is the only ion that has been found to restore the oxygen-evolving activity in PSII. Although several models have been proposed for the mechanism of water oxidation in PSII, the functional role of Ca2+ remains unclear. In order to test the proposal that Ca2+ functions as a Lewis acid in water oxidation chemistry, the pH dependence of the oxygen-evolution activity of intact Ca2+-containing PSII and Sr2+-substituted PSII has been investigated. In both samples, the pH dependence exhibits a bell-shaped curve. Sr2+ substitution shifts the peak of the curve to higher pH in comparison to Ca2+-containing PSII. The pH-dependent O-2-evolving rates of these two samples are fit to a diprotic model in which protonation of an essential basic group at acidic pH inhibits activity (pKa(l)) and deprotonation of an essential acidic group at higher pH inhibits activity (pKa(2)). Sr2+ substitution has no effect on pKa2. This result suggests that the high pH inhibition may be on the electron-acceptor side of PSII. On the other hand, Sr2+ substitution causes pKa(l) to be shifted to higher pH by 1.0 pH unit. This pH-shift indicates a direct role of Ca2+/Sr2+ in the OEC. The functional role of Ca2+ in oxygen evolution in PSII is discussed based on these results.