Molecular interference of Cd 2+ with Photosystem II

Abstract

Many heavy metals inhibit electron transfer reactions in Photosystem II (PSII). Cd 2+ is known to exchange, with high affinity in a slow reaction, for the Ca 2+ cofactor in the Ca/Mn cluster that constitutes the oxygen-evolving center. This results in inhibition of photosynthetic oxygen evolution. There are also indications that Cd 2+ binds to other sites in PSII, potentially to proton channels in analogy to heavy metal binding in photosynthetic reaction centers from purple bacteria. In search for the effects of Cd 2+-binding to those sites, we have studied how Cd 2+ affects electron transfer reactions in PSII after short incubation times and in sites, which interact with Cd 2+ with low affinity. Overall electron transfer and partial electron transfer were studied by a combination of EPR spectroscopy of individual redox components, flash-induced variable fluorescence and steady state oxygen evolution measurements. Several effects of Cd 2+ were observed: (i) the amplitude of the flash-induced variable fluorescence was lost indicating that electron transfer from Y Z to P 680 + was inhibited; (ii) Q A − to Q B electron transfer was slowed down; (iii) the S 2 state multiline EPR signal was not observable; (iv) steady state oxygen evolution was inhibited in both a high-affinity and a low-affinity site; (v) the spectral shape of the EPR signal from Q A −Fe 2+ was modified but its amplitude was not sensitive to the presence of Cd 2+. In addition, the presence of both Ca 2+ and DCMU abolished Cd 2+-induced effects partially and in different sites. The number of sites for Cd 2+ binding and the possible nature of these sites are discussed.