EFFECT OF MANGANESE IONS ON THE PROTON-ACCEPTING PROPERTIES OF PHOTOSYSTEM-2 SUBCHLOROPLAST PREPARATIONS

The proton-accepting properties of Photosystem 2 (PS 2) subchloroplast preparations were studied with the donor side inactivated and after r eactivation. After Tris-MgCl2 treatment, which removed water-soluble p roteins with molecular weights of 18, 24, and 33 kD and more than 95% of the manganese, the accessibility of ionogenic groups of PS 2 to tit rant increased as indicated by increased proton binding during nonequi librium titration. In the absence of the water-soluble proteins, addit ion of catalytically low concentrations of manganese to the PS 2 prepa rations in the dark decreased the accessibility of ionogenic groups to titrant nearly to the control level. The number of ionogenic groups w hose proton-binding parameters changed was similar to 40 per atom of a dded manganese. The removal of the water-soluble proteins (but not the manganese cluster) with 1 M CaCl2 did not increase the accessibility of ionogenic groups of PS 2 to titrant. These data suggest that signif icant structural rearrangements in the PS 2 pigment-protein complex ac company the removal and subsequent addition of Mn2+. The rearrangement s are probably the basis for the high efficiency of Mn2+ binding occur ring Photosystem 2.