Molecular mechanism of action of Pb2+ and Zn2+ on water oxidizing complex of photosystem II

Pb2+ and Zn2+ inhibition of photosystem II (PSII) activity was reported to be mediated via displacement of native inorganic cofactors (Cl−, Ca2+ and Mn2+) from the oxygen evolving complex, OEC [Rashid and Popovic (1990) FEBS Lett. 271, 181–184; Rashid et al. (1991) Photosynth. Res. 30, 123–130]. Since the binding sites of these cofactors are protected by a shield of three extrinsic polypeptides (17, 23 and 33 kDa), we investigated whether these metal ions affect the extrinsic polypeptide shield of OEC. By immunoblotting with antibodies recognizing the 23 and 33 kDa polypeptides, we showed that both the metal ions significantly dissociated the 23 kDa (+17 kDa) polypeptide, and partially dissociated the 33 kDa. Ca2+, one of the important inorganic cofactors of oxygen evolution, strongly prevented the dissociating action of Pb2+ but did not prevent the action of Zn2+. The probable molecular mechanism of action of Pb2+ and Zn2+ on PSII OEC is discussed.