Nitrogen ligation to the manganese cluster of Photosystem II in the absence of the extrinsic proteins and as a function of pH

Three extrinsic proteins (PsbO, PsbP and PsbQ), with apparent molecular wei ghts of 33, 23 and 17 kDa, bind to the lumenal side of Photosystem II (PS I I) and stabilize the manganese, calcium and chloride cofactors of the oxyge n evolving complex (OEC). The effect of these proteins on the structure of the tetramanganese cluster, especially their possible involvement in mangan ese ligation, is investigated in this study by measuring the reported histi dine-manganese coupling [Tang et al. (1994) Proc Natl Acad Sci USA 91: 704- 708] of PS II membranes depleted of none, two or three of these proteins us ing ESEEM (electron spin echo envelope modulation) spectroscopy. The result s show that neither of the three proteins influence the histidine ligation of manganese. From this, the conserved histidine of the 23 kDa protein can be ruled out as a manganese ligand. Whereas the 33 and 17 kDa proteins lack conserved histidines, the existence of a 33 kDa protein-derived carboxylat e ligand has been posited; our results show no evidence for a change of the manganese co-ordination upon removal of this protein. Studies of the pH-de pendence of the histidine-manganese coupling show that the histidine ligati on is present in PS II centers showing the S-2 multiline EPR signal in the pH-range 4.2-9.5.