title: 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.
authors.affiliation: Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden.
authors.forename: Kajsa G V
authors.initials: KG
authors.lastname: Sigfridsson
authors.forename: Gábor
authors.initials: G
authors.lastname: Bernát
authors.forename: Fikret
authors.initials: F
authors.lastname: Mamedov
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
mesh_terms: D002104:Cadmium / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002462:Cell Membrane / Q000187:drug effects* / Q000378:metabolism* / Q000528:radiation effects; D004305:Dose-Response Relationship, Drug; D004578:Electron Spin Resonance Spectroscopy; D008027:Light; D010084:Oxidation-Reduction; D045332:Photosystem II Protein Complex / Q000187:drug effects* / Q000378:metabolism* / Q000528:radiation effects; D018724:Spinacia oleracea / Q000187:drug effects / Q000378:metabolism / Q000528:radiation effects
chemical_list: D045332:Photosystem II Protein Complex; D002104:Cadmium; D002118:Calcium