Abstract
Cl- is one of the essential cofactors indispensable for photosystem II oxygen evolution. Its location in PSII, however, has not been determined in the current crystal structure due to limited resolution as well as the difficulties in detecting the chloride ion. Here we substituted Cl- with Br- in PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus, and crystallized, analyzed the crystal structure of Br-substituted PSII. Difference-Fourier and anomalous difference-Fourier map between native PSII (Cl-PSII) and Br-substituted PSII revealed two sites of binding of Br- surrounding the Mn4Ca-cluster, which were located in each side of the metal cluster. The distances between each of the Br-binding site and Mn atoms are similar with a value of 7.0 Å, and those between Br- and Ca are also similar with a value of 10.0 Å. These two Br-binding sites may represent the Cl-binding sites in PSII that have important functions in oxygen evolution.
