Environmental Engineering Reference
In-Depth Information
The cyanobacterium,
Synechococcus
PCC 7942, synthesizes a 56 amino acid
cysteine-rich protein, SmtA, that is strongly activated during exposure to cad-
mium, copper, and zinc [59]. Similar proteins have also been identified in the
algae
Chlorella
and
Euglena
[60]. The transcription of
smtA
tends to be maxi-
mally induced by the presence of zinc in
Synechococcus
with copper and cadmium
exhibiting lower inductive capabilities [59]. Mutants that lack functional
smtA
have
a several fold decrease in their zinc tolerance [61]. Other prokaryotic species [62]
as well as algae [60] also possess similar metallothioneins.
3.4.2 Class III Metallothioneins
Some species and ecotypes of algae have adapted to live in the presence of toxic
metal concentrations that are normally lethal. Perales-Vela and colleagues [57] listed
ten divisions and 24 genera of algae that possess metal-MtIII complexes as their
main contingency for heavy metal stabilization. MtIII production appears to play
an major role in the adaptive ability of these species to cope with the heavy metals.
Gekeler et al. [60] first determined that phytochelatin (MtIII) synthesis is ubiquitous
to algae and preferentially induced by high concentrations of Cd(II) and Cu(II).
These metallothioneins, also known as phytochelatins, are enzymatically synthe-
sized and composed of short chain polypeptides rich in cysteine. The most potent
activator of their production is Cd(II), followed by Pb(II), Zn(II), and Cu(II) [57].
The metalloids, As and Se, may act as weak activators. Steffens et al. [26] deter-
mined that the promotion of a class III metallothionein synthesis seems only to be
linked with heavy metal and metalloid presence.
The gamma bond present between glutamate and cysteine in phytochelatins can-
not be formed during protein translation. Instead, the bond is made by phytochelatin
synthase, an amylcysteine dipeptidyl
transpeptidase [63, 64]. This enzyme
has the general mechanism of [
Glu-Cys]
n+1
-
Gly+Gly 136
[60, 65]. Respective genes have been isolated from
Schizosaccharomyces pombe
[66],
Arabidopsis thaliana
[67], and
Triticum aestivum
[68]. Thus far, however,
regulatory mechanisms governing the induction of phytochelatins remain unclear.
Mutants of
Arabidopsis thaliana
that are unable to make phytochelatins show
increased sensitivity to Cd(II) [69]. Therefore, MtIII peptides play a particularly
important role in stabilizing intracellular heavy metals.
γ
Glu-Cys]
n
-
Gly
→
[
γ
3.4.3 Labile and Non-labile Phases of Metals
Metals such as Cd(II), Pb(II), Zn(II), Cu(II) and Co(II) within cells form labile
and non-labile phases [70]. Cadmium may be present in both phases within algal
species. Labile Cd(II), bound to phytochelatins, is capable of being mobilized and
exported. Non-labile phase metals that are bound to cytoplasmic proteins and mem-
branes are relatively unavailable for export. Lee [70] observed in the marine diatom,
