Location and function of the high-affinity chloride in the oxygen-evolving complex--implications from comparing...

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 TyrZ, 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.