A One-Site, Two-State Model for the Binding of Anions in Photosystem II

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Abstract
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 to evolve oxygen at about 35% of the control rate when measured in Cl--free medium. The Cl--depleted membranes were immediately (- (Kd = 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 36Cl-, occurred over several minutes. The stable binding was further characterized by a Kd of 20 μM and a t1/2 for dissociation of about 1h [Lindberg et al. (1993) Photosynth. Res. 38, 401−408]. The effects on S2 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 (--binding responsible for high oxygen-evolution activity and normal EPR properties of the S2 state may occur either as high affinity (Kd = 20 μM) and slowly exchanging (t1/2 = 1 h), or as low affinity (Kd = 0.5 mM) and rapidly exchanging (t1/2 < 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.