THE DONOR SIDE OF PHOTOSYSTEM-II AS THE COPPER-INHIBITORY BINDING-SITE - FLUORESCENCE AND POLAROGRAFIC STUDIES

We have measured, under Cu (II) toxicity conditions, the oxygen-evolvi ng capacity of spinach PS II particles in the Hill reactions H2O --> S iMo (in the presence and absence of DCMU) and H2O --> PPBQ, as well as the fluorescence induction curve of Tris-washed spinach PS II particl es. Cu (II) inhibits both Hill reactions and, in the first case, the D CMU-insensitive H2O --> SiMo activity. In addition, the variable fluor escence is lowered by Cu (II). We have interpreted our results in term s of a donor side inhibition close to the reaction center. The same po larographic and fluorescence measurements carried out at different pHs indicate that Cu (II) could bind to amino acid residues that can be p rotonated and deprotonated. In order to reverse the Cu (II) inhibition by a posterior EDTA treatment, in experiments of preincubation of PS II particles with Cu (II) in light we have demonstrated that light is essential for the damage due to Cu (II) and that this furthermore is i rreversible.