Current status of the role of Cl− ion in the oxygen-evolving complex

This minireview summarizes the current state of knowledge concerning the role of Cl− in the oxygen-evolving complex (OEC) of photosystem II (PSII). The model that proposes that Cl− is a Mn ligand is discussed in light of more recent work. Studies of Cl− specificity, stoichiometry, kinetics, and retention by extrinsic polypeptides are discussed, as are the results that fail to detect Cl− ligation to Mn and results that show a lack of a requirement for Cl− in PSII-catalyzed H2O oxidation. Mutagenesis experiments in cyanobacteria and higher plants that produce evidence for a correlation between Cl− retention and stable interactions among intrinsic and extrinsic polypeptides are summarized, and spectroscopic data on the interaction between PSII and Cl− are discussed. Lastly, the question of the site of Cl− action in PSII is discussed in connection with the current crystal structures of the enzyme.