Structural organization of the oxidizing side of photosystem II. Exogenous reductants reduce and destroy the Mn-complex in photosystems II membranes depleted of the 17 and 23 kDa polypeptides

Abstract

Removal of 23 and 17 kDa water-soluble polypeptides from PS II membranes causes a marked decrease in oxygen-evolution activity, exposes the oxidizing side of PS II to exogenous reductants (Ghanotakis, D.F., Babcock, G.T. and Yocum, C.F. (1984) Biochim. Biophys. Acta 765, 388–398) and alters a high-affinity binding site for Ca 2+ in the oxygen-evolving complex (Ghanotakis, D.F., Topper, J.N., Babcock, G.T. and Yocum, C.F. (1984) FEBS Lett. 170, 169–173). We have examined further the state of the functional Mn complex in PS II membranes from which the 17 and 23 kDa species have been removed by high-salt treatment. These membranes contain a structurally altered Mn complex which is sensitive to destruction by low concentrations of NH 2OH which cannot, in native PS II membranes, cause extraction of functional Mn. In addition to NH 2OH, a wide range of other small (H 2O 2, NH 2NH 2, Fe 2+) and bulky (benzidine, hydroquinone) electron donors extract Mn (up to 80%) from the polypeptide-depleted PS II preparations. This extraction is due to reduction of the functional Mn complex since light, which would generate higher oxidation states within the Mn complex, prevents Mn release by reductants. Release of Mn by reductants does not extract the 33 kDa water-soluble protein implicated in Mn binding to the oxidizing side of PS II, although the protein can be partially or totally extracted from Mn-depleted preparations by exposure to high ionic strength or to high (0.8 M) concentrations of Tris. We view our results as evidence for a shield around the Mn complex of the oxygen-evolving complex comprised of the 33 kDa polypeptide along with the 23 and 17 kDa proteins and tightly bound Ca 2+.