Binding Sites of Cadmium Ions Within Photosystem II

Abstract

We investigated the inhibitory action of cadmium ions on the energy and electron transfer within PSII using fluorescence spectroscopy. We have examined thylakoid membranes isolated from a wild type (WT) and a mutant (M) of tobacco with a point mutation in the β-chain of cytochrome b₅₅₉. Variable fluorescence measurements were made of PSII samples treated with various concentrations of cadmium salts. From these experiments we were able to determine the impact of the cadmium cations on the efficiency of photochemical processes within PSII and the activity of its acceptor side. We observed that cadmium ions act differently on the WT and M in the presence of chloride anions. Sulfate ions enhanced the inhibitory effect of Cd²⁺ but at low concentrations, CdSO₄ diminished some characteristic transitions monitored for CdCl₂. At the lowest applied concentrations, the salts had no influence on the Kautsky effect in the mutant, whereas a strong effect is observed in the WT. This phenomenon is related to the point mutation of cytochrome b₅₅₉ which is supposed to modify the Q_B binding site on the acceptor side of PSII (Burda et al., these Proceedings). At higher applied concentrations of Cd salts, one has to consider additional Cd²⁺ interactions with chlorophylls from the light harvesting antenna.