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

Abstract

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 -7 was enhanced by millimolar concentrations of the divalent anion S042-and also by divalent cations like Mg2" and Ca2"; (b) the dissociation was opposed by relatively low concentrations of monovalent anions with an order of effectiveness Cl-= Br-> N03-> F > C104-; (c) at molar concentrations, S042-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°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.Removal of Cl-from PSII of chloroplast thylakoids causes an aberration and interruption of the normal progress of charge accumulation in and perhaps around the Mn-cluster ofthe water oxidizing complex (for reviews, see Refs. 15, 17, and 21). However, the exact function of Cl-is not understood. Sandusky and Yocum (33) have proposed that an essential step in the mechanism of water oxidation is a ligation of Cl-to the Mn-cluster itself. This concept was developed on the basis of their finding that Cl-, in a competitive fashion, protected the water oxidizing mechanism from the inhibitory action of NH3 and various amines, the action of which was attributed to a binding to the oxidant storing Mn-complex. Taking an alternative view, Govindjee's group (8, 17) and we (19,21) have proposed various mechanistic versions for a role of protein-bound Cl-in the deprotonation events accompanying water oxidation. Such an indirect participation of the anion in the catalytic process was postulated because of precedents in other enzyme systems (8,14) and to account for the reported need of more than one Cl-per ' Supported by grant DMB 8304416 from the National Science Foundation.water oxidizing complex (35), the unique pH dependence of Clbinding (21), the complex pattern of 35Cl-NMR line broadening (2, 10), and the ability of other monovalent anions such as NO3 to serve as rather efficient substitutes for 18,24,28).The Cl-relations of PSII are known to be greatly influenc...

Differential response of chloride binding sites to elevated temperature: a comparative study in spinach thylakoids and PSII-enriched membranes

“…Such a proposition is in good agreement with the earlier reports showing the requirement of bound Cl -for the proper functioning of the 33 kDa Mn stabilizing extrinsic protein (Coleman et al 1987). Also, the 17 and 23 kDa extrinsic polypeptides have been shown to aid in Cl -retention (Homann 1988b) and are required for Cl -induced functional activation of the OEC (Homann 1988a). The same pool of tightly-bound Cl -plays a role in protection during heat treatment.…”

Stabilization of the water oxidizing polypeptide assembly on Photosystem II membranes by osmolytes and other solutes

“…Their interesting finding supplements our own published data on the solute dependence of the association of the water oxidizing site with its extrinsic 17 and 23 kDa polypeptides (Homann 1987(Homann , 1988(Homann , 1989. In the present contribution we summarize relevant additional resuits and explain them within the framework of the concepts developed by Timasheff's group (e.g., Timasheff 1985, andreview by Low 1985) for the interactions of solutes and proteins.…”

“…Since such a stabilizing influence is well known also for non-ionic solutes like sugars and other polyolic compounds (Collins and Washabaugh 1985), we tested what effect some of these would have on PSII membranes under conditions known to cause dissociation of the 17 and 23 kDa extrinsic polypeptides. In our earlier work we had discovered that moderate concentrations of Na2SO4, in contrast to high concentrations of the same salt, strongly labilized the two extrinsic polypeptides when the media were free of added C1-and their pH was raised above 6.5 (Homann 1987(Homann ,1988. In Fig.…”

“…We have previously suggested that the widely exploited ability of moderate concentrations of Na2SO 4 to cause CI-depletion of the water oxidizing complex is a consequence not of a displacement of CI-from its functional site by SO~-, but of its dissociating action on the C1--stabilizing 17 and 23 kDa polypeptides (Homann 1987(Homann , 1988. A supporting argument was the correlation we found between the pH dependence of the SO24 -induced dissociation of the 23 kDa polypeptide and that reported for the suitability of Na2SO 4 to serve as an efficient C 1 -d e p l e t i n g agent (Sandusky et al 1983).…”

Characterization of inhibitory effects of NH2OH and its N-methyl derivatives on the O2-evolving complex of Photosystem II

“…As we show in Table 1, anion-induced retardation of CH3NHOH and (CH3)2NOH inhibition of activity in salt-washed PS II membranes is Ck specific; SO42-and F-do not produce the same protective effect. Although SO t-is able to stabilize PS II structure (Homann 1988), it is not competitive with C1-in the O2-evolving complex (Sandusky and Yocum 1986). If C1-interference with the inhibition of the O2-evolving complex by N-methylated hydroxylamines were due to its ability to exert a purely lyotropic effect on protein structure and stability similar to the effects observed for SO42-in intact PS II preparations (Homann 1989), then the latter anion should also retard inhibitions produced by hydroxylamines in salt-washed PS II membranes.…”