title: Interactions of iodide ions with isolated photosystem 2 particles.
abstract: The effects of I- ions on O2 evolution by photosystem 2 particles, which were depleted of the 18-kDa and the 23-kDa extrinsic proteins of the O2 evolution complex by NaCl washing (dPS2 particles) were examined. In the absence of Cl- (incompetent dPS2) I- stimulated O2 evolution up to 3-6 mM, depending on the associated cation, and inhibited it at higher concentrations. In the presence of Cl- (competent dPS2), I- was inhibitory at all concentrations. The inhibition was reversible, it occurred at a site preceding Tyrz (Tyr residue mediating electron transfer from H2O to photosystem 2), and it interfered noncompetitively with the reactivation of incompetent dPS2 with Cl-. Furthermore, the organic salts tetrabutyl ammonium iodide and tetraphenyl phosphonium iodide proved to be stronger inhibitors than the inorganic NaI. This is interpreted as an indication of a negatively charged surface, situated behind a hydrophobic permeability barrier. Permeant organic cations, being better compensators of the inner surface charge than Na+, are also more apt in facilitating access of the I- ions to the inhibitory site in the vicinity of Tyrz.
authors.affiliation: National Research Center Demokritos, Institute of Biology, Athens, Greece.
authors.forename: G C
authors.initials: GC
authors.lastname: Papageorgiou
authors.forename: T
authors.initials: T
authors.lastname: Lagoyanni
mesh_terms: D000838:Anions; D002712:Chlorides / Q000494:pharmacology; D002736:Chloroplasts / Q000737:chemistry*; D002794:Choline / Q000494:pharmacology; D007454:Iodides / Q000494:pharmacology*; D009861:Onium Compounds / Q000494:pharmacology; D009943:Organophosphorus Compounds / Q000494:pharmacology; D010100:Oxygen / Q000378:metabolism*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010944:Plants / Q000032:analysis*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D012974:Sodium Iodide / Q000494:pharmacology
chemical_list: D000838:Anions; D002712:Chlorides; D007454:Iodides; D009861:Onium Compounds; D009943:Organophosphorus Compounds; D045322:Photosynthetic Reaction Center Complex Proteins; D000644:Quaternary Ammonium Compounds; C009405:tetrabutylammonium; D012974:Sodium Iodide; D002794:Choline; D010100:Oxygen; C013289:tetraphenylphosphonium