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 to 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- (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 microM 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 (< 30 s) restored. We propose that Cl(-)-binding responsible for high oxygen-evolution activity and normal EPR properties of the S2 state may occur either as high affinity (Kd = 20 microM) 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.
Department of Biochemistry and Biophysics, Göteborg University, Sweden.
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Lindberg
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Andréasson
D000838:Anions / Q000378:metabolism*; D001667:Binding, Competitive; D001965:Bromides / Q000737:chemistry / Q000378:metabolism; D002474:Cell-Free System; D002712:Chlorides / Q000737:chemistry / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D005459:Fluorides / Q000737:chemistry / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D045332:Photosystem II Protein Complex; D011485:Protein Binding
D000838:Anions; D001965:Bromides; D002712:Chlorides; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D005459:Fluorides; D010100:Oxygen