Interaction of Zn(2+) with the donor side of Photosystem II

The inhibitory effect of Zn(2+) on photosynthetic electron transport was investigated in native and CaCl2-treated (depleted in extrinsic polypeptides) Photosystem II (PS II) submembrane preparations. Inhibition of 2,6-dichlorophenolindophenol photoreduction by Zn(2+) was much stronger in protein-depleted preparations in comparison to the native form. It was found that Ca(2+) significantly reduced the inhibition in the native PS II preparations, as did Mn(2+) in a combination with H2O2 in the protein-depleted counterparts. No other tested monovalent or divalent cations could replace Ca(2+) or Mn(2+) in the respective experiments. Diphenylcarbazide could partially relieve (40-45%) the inhibition in both types of preparations. The above indicates the presence of an active Zn(2+) inhibitory site on the donor side of PS II. However, neither Ca(2+) nor Mn(2+) could completely prevent inhibition by high concentrations of Zn(2+) (>1 mM). We propose that elevated levels of Zn(2+) strongly perturb the conformation of the PS II core complex and might also affect the acceptor side of the photosystem.