A one-site, two-state model for the binding of anions in photosystem II

Photosystem II membranes, dialyzed against a Cl--free buffer to remove bound Cl-, lost about 65% of the control activity. A light-intensity study of the Cl--free membranes showed that all PS II centers were able tn evolve oxygen at about 35% of the control rate when measured in Cl--free medium. The Cl--depleted membranes were immediately (< 15 s) reactivated to 85-90% of the original activity by the addition of fairly high concentrations of Cl- (K-d = 0.5 mM), but both Cl- and the activity were promptly lost when the membranes immediately after reactivation were diluted in a Cl--free medium. However, stabilization of Cl--binding could be accomplished by prolonged incubation in the presence of Cl-. The transition to stable binding, followed using Cl-36(-), occurred over several minutes. The stable binding was further characterized by a K-d Of 20 mu M and a t(1/2) for dissociation of about 1 h [Lindberg et al. (1993) Photosynth. Res. 38, 401-408]. The effects on S-2 signals of removal of Cl- were studied using EPR. The depletion of Cl- was accompanied by a shift in intensity toward the g = 4.1 signal at the expense of the multiline signal. When Cl- or Br- but not F- was added to the depleted PS II membranes, the original distribution of the signals was immediately (< 30 s) restored, We propose that Cl--binding responsible for high oxygen-evolution activity and normal EPR properties of the S-2 State may occur either as high affinity (K-d = 20 mu M) and slowly exchanging (t(1/2) = 1 h), or as low affinity (K-d = 0.5 mM) and rapidly exchanging (t(1/2) < 15 s). Our results suggest that Br- but not F- has a mode of binding similar to that of Cl-. The high-affinity state is the normal state of binding, but once Cl- has been removed, it will first rebind as low-affinity, rapidly exchanging followed by conversion into a high-affinity, slowly exchanging mode of binding.