Binding and functions of the two chloride ions in the oxygen-evolving center of photosystem II.

Light-driven water oxidation in photosynthesis occurs at the oxygen-evolving center (OEC) of photosystem II (PSII). Chloride ions (Cl) are essential for oxygen evolution by PSII, and two Cl ions have been found to specifically bind near the MnCaO cluster in the OEC. The retention of these Cl ions within the OEC is critically supported by some of the membrane-extrinsic subunits of PSII. The functions of these two Cl ions and the mechanisms of their retention both remain to be fully elucidated. However, intensive studies performed recently have advanced our understanding of the functions of these Cl ions, and PSII structures from various species have been reported, aiding the interpretation of previous findings regarding Cl retention by extrinsic subunits. In this review, we summarize the findings to date on the roles of the two Cl ions bound within the OEC. Additionally, together with a short summary of the functions of PSII membrane-extrinsic subunits, we discuss the mechanisms of Cl retention by these extrinsic subunits.