Rate of exchange of Cl⁻ between the aqueous phase and its action site in the O₂ evolving reaction of photosystem II studied by rapid, ionic‐jump‐induced Cl⁻ depletion

Abstract

Rates of release and binding of Cl-from/to its action site in the 0, evolving reaction in photosystem II particles derived from spinach chloroplasts were estimated by measuring the suppression of 0, evolution by salt addition (ionic-jump) and its recovery by the readdition of Cl-. It was estimated that depletion and rebinding of Cl-were completed within a few seconds. These results suggest that the Cl--action site is located in a space which is almost freely accessible to various ions in the outer medium, with no barrier to ion movements. These results can be explained by electrostatic attraction of Cll to its action site, as was proposed in a study of anion effects on 0, evolution [(1986) Plant Cell Physiol. 27, in press]. Cl-effectOxygen evolution Photosystem II Salt effect PS Uparticle

Chloride binding proteins: mechanistic implications for the oxygen-evolving complex of Photosystem II

“…Since the OEC polypeptides are not enclosed within the lumen, they are more susceptible to detachment at high pH (Chapman and Barber 1987). For this reason, pH-or anion-jump treatments are often employed (Homann 1985, Itoh andIwaki 1986). Homann (1985) mesaured the apparent dissociation constant for C1-as a function ofpH in PS II membranes, and was able to show that C1-binding required protonation of a group having a pKa = 6.0.…”