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Location of chloride and its possible functions in oxygen-evolving photosystem II revealed by X-ray crystallography
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Location of chloride and its possible functions in oxygen-evolving photosystem II revealed by X-ray crystallography
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Journal Article
Location of chloride and its possible functions in oxygen-evolving photosystem II revealed by X-ray crystallography
2009
Overview
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.
Publisher
National Academy of Sciences,National Acad Sciences
