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 Mn 4 CaO 5 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.