Molecular Mechanism of Copper Toxicity on Electron Transfer through Photosystem Ii

Photosystem II (PSII) is the site of action of a large number of commercial herbicides and it is the site, where photoinhibition appears when plants are exposed to high light intensities. It is known that manganese, calcium and chloride ions are essential for optimal electron flow within PSII. Recently we have found that at very low concentrations (1-1.5 Cu 2+ / reaction center) copper improves the efficiency of PSII apparatus. On the other hand the toxicity of high concentrations of copper, which is extensively applied as a fungicide in agricultural practice, has been demonstrated by many authors. However, the binding site(s) of Cu 2+ and the inhibitory mechanism are still matter of debate and discussion. The obtained results suggest that copper inhibits either the donor or the acceptor side in photosystem II. Especially controversial is the potential interaction of Cu with cytochrome b 559 and the non-heme iron. This has been carefully investigated by us in the context of electron transfer within PSII and in protection against photoinhibition. We aimed at the investigation of the mechanism of Cu 2+ action on PSII components using a fast oxygen electrode, studying fluorescence induction kinetics and the effect of copper on a redox state of the HP (high potential) and LP (low potential) forms of cytochrome b 559 , as well as monitoring the binding sites of Cu ions in the vicinity of the heme iron of cytochrome b 559 and the non-heme iron by Mössbauer spectroscopy on 57 Fe. We performed our studies on PSII particles (BBY) isolated from tobacco Nicotiana tabacum cv. John Williams Broadleaf and on thylakoid membranes from a mutant of Chalmydomonas reinhardtii free of photosystem I (strain Xba9). We showed that apart the inhibition of total oxygen evolution, the increased copper concentrations changed the initial S-states distribution of the oxygen evolving complex and decreased the total miss parameter. Copper ions oxidized directly-the-cyt-b 559-LP-format low concentrations -(1-10 μM) and-at-higher-concentrantions (10-100 μM) also the HP form. Inhibition of oxygen evolution and oxidation of cytochrome b 559 by copper ions were found to correlate with the fluorescence quenching of PSII. We suggested the enhanced formation of the chlorophyll CHl Z + radical due to its direct interaction with Cu 2+ ions. This could be responsible for fluorescence quenching and would result in the stimulation of