X-ray crystallography identifies two chloride binding sites in the oxygen evolving centre of Photosystem II

Bromide anomalous X-ray diffraction analyses have been used to locate chloride binding sites in the vicinity of the water splitting/oxygen evolving centre (OEC) of Photosystem II. Three-dimensional crystals of PSII from Thermosynechococcus elongatus were grown from (i) isolated PSII crystals infiltrated with bromide or (ii) PSII obtained from cells cultured in a medium in which the chloride content was totally replaced by bromide. In either case, the anomalous diffraction yielded the same result, the existence of two bromide binding sites in the vicinity of the OEC. Neither are in the first coordination sphere of the Mn and Ca ions which form the catalytic centre of the OEC, being about 6 to 7 Å from the metal-cluster. Site 1 is located close to the side chain nitrogen of D2-K317 and the backbone nitrogen of D1-Glu333 while Site 2 is adjacent to backbone nitrogens of CP43-Glu354 and D1-Asn338. Their positioning close to postulated hydrophilic channels may suggest a role in proton removal from, or substrate access to, the OEC.