Structural Effects of Cl and Other Anions on the Water Oxidizing Complex of Chloroplast Photosystem II.

To further our understanding of the role of Cl(-) and certain other monovalent anions in the oxygen evolving photosystem II of chloroplasts, dissociating and stabilizing anion effects on the extrinsic 17 and 23 kilodalton polypeptides of the photosynthetic water oxidizing complex were investigated. It was found that (a) the dissociation of the two polypeptides in Cl(-) free media of pH approximately 7 was enhanced by millimolar concentrations of the divalent anion SO(4) (2-) and also by divalent cations like Mg(2+) and Ca(2+); (b) the dissociation was opposed by relatively low concentrations of monovalent anions with an order of effectiveness Cl(-) = Br(-) > NO(3) (-) > F(-) > ClO(4) (-); (c) at molar concentrations, SO(4) (2-) stabilized the binding of the 23 kilodalton polypeptide, while Cl(-) and Br(-) became dissociating agents, in agreement with studies by Blough and Sauer (1984 Biochim Biophys Acta 767: 377-381); (d) the binding of the polypeptides was strengthened at room temperature relative to 0 degrees C, indicating an involvement of hydrophobic forces. It is suggested that a specific binding of Cl(-), or certain substitutes, organizes the protein surfaces and/or the adjacent water layers in the water oxidizing complex in a way that not only stabilizes its assembly, but is essential for the catalytic mechanism as well. Binding of, or charge screening by, divalent ions interferes with this process. At high salt concentrations, all these effects are overridden by "lyotropic" actions of the solutes that affect the integrity of the water oxidizing protein complex by stabilizing or disrupting critical hydrophobic domains.