RECONSTITUTION OF THE WATER-OXIDIZING COMPLEX IN MANGANESE-DEPLETED PHOTOSYSTEM-II COMPLEXES BY USING SYNTHETIC BINUCLEAR MANGANESE COMPLEXES

The efficiency of synthetic binuclear manganese complexes in reconstit uting PS II electron flow and oxygen-evolution capacity was analyzed i n PS II enriched preparations deprived of their manganese and of the e xtrinsic regulatory Subunits. Measurements of the variable fluorescenc e induced by actinic illumination with continuous light led to the fol lowing results: (a) the synthetic binuclear complexes are more efficie nt than MnCl2 in establishing a PS II electron flow; (b) an almost com plete restoration is achieved at concentrations df these complexes tha t correspond with an overall stoichiometry of two manganese per PS II; and (c) the electron flow restored by the binuclear manganese complex es closely resembles that of normal O-2-evolving PS II preparations in its resistance to addition of 50 mu M EDTA, while that supported by M nCl2 is practically completely suppressed at the same chelator concent ration. The rate of O-2 evolution was used as a measure of the capabil ity to function as manganese Source in reconstitution of the oxygen ev olution capacity. It was found that (i) as in the case of PS II electr on transport, the synthetic binuclear manganese complexes are signific antly more efficient than MnCl2; (ii) with respect to the manganese co ncentration, the maximum effect is achieved with a mu-oxo bridged binu clear Mn(III) complex (symbolized by M-3) at concentrations correspond ing to four manganese per PS II; and (iii) at all concentrations of bi nuclear manganese complex M-3 a significantly higher restoration of th e O-2 evolution rate is achieved if the reconstitution assay contains in addition the extrinsic regulatory 33 kDa protein (PS II-O protein). The implications of these data are discussed in terms of electron don ation and photoligation of the functional tetranuclear cluster of the water oxidase.