Location of chloride and its possible functions in oxygen-evolving photosystem II revealed by X-ray crystallography

The chloride ion, Cl-, is an essential cofactor for oxygen evolution of photosystem II (PSII) and is closely associated with the Mn4Ca cluster. Its detailed location and function have not been identified, however. We substituted Cl- with a bromide ion (Br-) or an iodide ion (I-) in PSII and analyzed the crystal structures of PSII with Br- and I- substitutions. Substitution of Cl- with Br- did not inhibit oxygen evolution, whereas substitution of Cl- with I- completely inhibited oxygen evolution, indicating the efficient replacement of Cl- by I-. PSII with Br- and I- substitutions were crystallized, and their structures were analyzed. The results showed that there are 2 anion-binding sites in each PSII monomer; they are located on 2 sides of the Mn4Ca cluster at equal distances from the metal cluster. Anion-binding site 1 is close to the main chain of D1-Glu-333, and site 2 is close to the main chain of CP43-Glu-354; these 2 residues are coordinated directly with the Mn4Ca cluster. In addition, site 1 is located in the entrance of a proton exit channel. These results indicate that these 2 Cl- anions are required to maintain the coordination structure of the Mn4Ca cluster as well as the proposed proton channel, thereby keeping the oxygen-evolving complex fully active.