an inhibitory site at the donor side of photosystem II but they indicated its location near the hydroxy-
lamine electron donation site [52]. A decrease in the concentration of active photosystem II centers (
type) was also observed [52].
Cadmium was supposed to replace the Mn ions in the water-oxidizing system [60], or alternatively,
an alteration of the lipid environment around the photosystem may be responsible for the inhibition [50].
The partial removal of the 16- and 24-kDa extrinsic polypeptides and the complete removal of the 33-kDa
polypeptide from the oxygen-evolving complex of photosystem II during cadmium treatment also indi-
cated inhibition at the level of the water-oxidizing system [31,61]. Cadmium has been shown to interact
directly with calcium metabolism and the toxic effects of cadmium are similar in many ways to calcium
deficit symptoms [62]. Thus, it can be postulated that this metal may interact with the site where calcium
plays its cofactor role in the oxygen-evolving complex. Accordingly, calcium was shown to relieve the
negative effect of cadmium on the primary photochemistry of bean plants [63]. Cadmium also causes ef-
fects similar to those of Fe deficiency, and an increase in Fe supply relieves the negative effects of cad-
mium on photosynthetic pigment accumulation and on the light phase of photosynthesis [64].
Cadmium was also shown to reduce the turnover rate of the D1 protein of the reaction center of pho-
tosystem II [48]. This action was proposed to originate from the modification of SH groups by cadmium
resulting in the inhibition of messenger RNA (mRNA)-binding protein complex formation involved in
D1 synthesis [48]. However, it was shown by Fourier transform infrared spectroscopy in isolated photo-
system II submembrane fractions that this metal cation forms metal-protein complexes ligating CBO and
CMN groups of amino acids but not SH groups [65].
C. Zinc
Zinc is an important micronutrient associated with several enzymatic activities in all photosynthetic or-
ganisms [66]. However, this metal can inhibit CO 2 assimilation at relatively low concentrations. Higher
concentrations initiate the loss of photosynthetic pigments and a decline in the chlorophyll a/bratio [67]
and also result in inhibition of photosynthetic electron transport and photophosphorylation [68].
Zinc has been shown to affect the water-oxidizing complex by releasing the manganese ions in-
volved in the oxygen evolution mechanism [69]. The Mn atoms released from the oxygen-evolving
complex by millimolar concentrations of zinc are, however, sequestered within the thylakoid interior
[70,71]. The extrinsic polypeptides of 16 and 24 kDa associated with the oxygen-evolving complex
were significantly dissociated from photosystem II submembrane fractions treated with zinc but not the
33-kDa polypeptide [72]. Miller [73] has shown that the four Mn atoms were depleted from the thy-
lakoid membranes as zinc concentration was increased with the concomitant release of the 16- and 24-
kDa extrinsic polypeptides. The inhibitory action is thus due to the loss of manganese atoms rather than
to the release of polypeptides. Although zinc noncompetitively inhibited Ca^2 and Mn^2 binding [74],
Ca^2 did not prevent the release of the extrinsic polypeptides [72]. Zinc has also been proposed to bind
photosystem II on the acceptor side near the QBbinding site in Rhodobacter sphaeroides. This binding
at a site distinct from the nonheme iron was shown to reduce significantly the rate of electron transfer
between QAand QB[75].
D. Mercury
Mercury is an environmental contaminant that is highly toxic to photosynthetic organisms at micromolar
concentrations. Several sites of inhibition have been described in the photosynthetic electron transport
chain, and both photosystems are affected [76–78]. Mercury was proposed to bind proteins through mod-
ification of SH groups [79]. Fourier transform infrared spectroscopy experiments indicated a strong in-
teraction between photosystem II submembrane fractions and mercury causing some alteration of the pro-
tein structure that originated from the formation of metal-protein binding through peptide SH, CBO, and
CMN groups [65]. The binding of mercury to thylakoid membrane proteins was also illustrated by the
quenching of the fluorescence emission associated with aromatic amino acids [80].
In photosystem I, the inhibition was reported at the donor side beyond the cytochrome b/ƒ complex
[15]. More precisely, mercury was shown to react directly with plastocyanine, replacing copper
[77,78,81]. An inhibition was also reported at the acceptor side of photosystem I, where it was proposed
to alter the enzyme ferredoxin:NADP-reductase in which SH groups may be modified by mercury
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