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 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.