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+

Abstract

AbstractThe oxygen‐evolving complex (OEC) of photosystem II (PSII) is the catalytic site for the oxidation of water to O2 in photosynthesis. The OEC consists of a tetranuclear manganese (Mn4) 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 O2‐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 (pKa1) and deprotonation of an essential acidic group at higher pH inhibits activity (pKa2). 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 pKa1 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.