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

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 C1 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 oxygenevolving 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. (C) 2014 Elsevier B.V. All rights reserved.