Halides And The Amine Induced Inhibition Of Photosynthetic Oxygen Evolution (chloride, Hydrogen Peroxide, Manganese).

Abstract

The photosynthetic oxygen evolving complex (OEC) interfaces the four-electron oxidation of two-water molecules to four single electron reductions of the photosystem II reaction center (P680('+)). Oxygen evolving activity is known at present to require the presence of a number of components; Mn, Ca('+2), Cl('-) and three peripheral polypeptides of 17, 24 and 32 kDa. Fairly recent evidence indicates that the Mn is redox active: the functions of the other components are poorly understood at present. Amines have for some time been studied as inhibitors of oxygen evolution. It has in general been concluded that amines inhibit by binding as ligands to OEC Mn. It is interesting to note that one group of Lewis bases (chloride and certain other anions) are required for activity, whereas another group of Lewis bases (the amines) are inhibitors. This suggests that perhaps both chloride and amines bind to the same Lewis acid binding site (probably OEC Mn). Using steady state kinetics and EPR spectroscopy, evidence was collected which supports the following conclusions: (1) Inhibitory binding of amines to the OEC excludes chloride binding as a cofactor. (2) Ammonia has two sites of inhibition; only one of these is the chloride binding site. (3) Fluoride is an inhibitor of oxygen evolution; whose binding excludes the binding of both chloride and amines. (4) The inhibitory effects of both fluoride and amines are enhanced by the presence of a class III electron acceptor (DCBQ). Presumably this phenomenon reflects preferential binding of the inhibitors to the higher S-States. (5) Inhibition induced by ADRY reagents is unaffected by chloride. (6) Hydrogen peroxide oxidation by chloride depleted thylakoid membranes is supported by a free or loosely bound pool of Mn localized in the thylakoid lumen. (7) Hydrogen peroxide can induce the release of OEC bound Mn. These results suggest that chloride functions in the OEC as a ligand to Mn. It would appear that when large, soft anions (Cl('-), Br('-), I('-)) occupy the chloride site, the system is active, whereas when small, hard atoms (amines, hydroxide and F('-)) are bound to the site, the system is inhibited.