Molecular interference of Cd with Photosystem II |
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Authors: | Kajsa GV Sigfridsson Gábor Bernát Fikret Mamedov |
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Institution: | Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden |
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Abstract: | Many heavy metals inhibit electron transfer reactions in Photosystem II (PSII). Cd2+ is known to exchange, with high affinity in a slow reaction, for the Ca2+ 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 Cd2+ 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 Cd2+-binding to those sites, we have studied how Cd2+ affects electron transfer reactions in PSII after short incubation times and in sites, which interact with Cd2+ 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 Cd2+ were observed: (i) the amplitude of the flash-induced variable fluorescence was lost indicating that electron transfer from YZ to P680+ was inhibited; (ii) QA− to QB electron transfer was slowed down; (iii) the S2 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 QA−Fe2+ was modified but its amplitude was not sensitive to the presence of Cd2+. In addition, the presence of both Ca2+ and DCMU abolished Cd2+-induced effects partially and in different sites. The number of sites for Cd2+ binding and the possible nature of these sites are discussed. |
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Keywords: | Chl chlorophyll Cyt cytochrome DCMU 3-(3&prime 4&prime -dichloro)-1 1-dimethylurea EPR electron paramagnetic resonance Fmax the maximal fluorescence from QA&minus F0 initial fluorescence before the actinic flash Fv variable fluorescence Fmax&minus F0 MES 4-morpholineethanesulfonic acid ML multiline EPR signal in S2 state OEC oxygen evolving complex PpBQ phenyl-p-benzoquinone PSII Photosystem II P680 primary donor of PSII QA and QB the primary and secondary quinone acceptors in PSII S-states S0&rarr S4 are the different redox states of the Ca/Mn cluster YZ and YD redox-active tyrosine residues D1-161 and D2-160 in PSII |
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