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

摘要

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.