STUDIES OF THE SLOWLY EXCHANGING CHLORIDE IN PHOTOSYSTEM-II OF HIGHER-PLANTS

Cl-36(-) was used to study the slow exchange of chloride at a binding site associated with Photosystem II (PS II). When PS II membranes were labeled with different concentrations of Cl-36(-), saturation of bind ing at about 1 chloride/PS II was observed. The rate of binding showed a clear dependence on the concentration of chloride approaching a lim iting value of about 3.10(-4) s(-1) at high concentrations, similar to the rate of release of chloride from labeled membranes. These rates w ere close to that found earlier for the release of chloride from PS II membranes isolated from spinach grown on Cl-36(-), which suggests tha t we are observing the same site for chloride binding. The similarity between the limiting rate of binding and the rate of release of chlori de suggests that the exchange of chloride with the surrounding medium is controlled by an intramolecular process. The binding of chloride sh owed a pH-dependence with an apparent pK(a) of 7.5 and was very sensit ive to the presence of the extrinsic polypeptides at the PS II donor s ide. The binding of chloride was competitively inhibited by a few othe r anions, notably Br- and NO3- The slowly exchanging Cl- did not show any significant correlation with oxygen evolution rate or yield of EPR signals from the S-2 state. Our studies indicate that removal of the slowly exchanging chloride lowers the stability of PS II as indicated by the loss of oxygen evolution activity and S-2 state EPR signals.