Location and function of the high-affinity chloride in the oxygen-evolving complex--implications from comparing studies on Cl(-)/Br(-)/I(-)-substituted photosystem II prepared using two different methods.

Abstract The high-affinity chloride ion (Cl − ) is known to play a key role in water oxidation in photosystem II (PSII). Recent crystallographic studies revealed two Cl − binding sites in PSII. To examine whether these two Cl − ions are correlated to the high-affinity Cl − , we prepared Cl − /Br − /I − -substituted PSII samples from both higher plants and cyanobacteria by using two different protocols: one was the method used in the crystallographic study ( Type 1 ) and the other was a method developed recently to ensure the efficient replacement of Cl − ( Type 2 ). While only minor effects were observed in the Type 1 preparation, efficient Br − /I − substitution by the Type 2 protocol led to significant changes in the EPR properties of the oxygen-evolving complex (OEC) and the Tyr Z , as well as in oxygen-evolving activities. These results are discussed in terms of the binding site of the high-affinity Cl − relative to the two Cl − ions revealed by the recent X-ray structural data.