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phytochelatinsynthase(Alr0975inthemodelcyanobacterium
Nostoc
PCC7120
(
Tsuji et al., 2004
)) to form phytochelatins ((γ-Glu-Cys)
2-11
-Gly), which
chelates metals (
Pandey, Rai, & Rai, 2012
) and protects cells from the
deleterious effect of UV-B (
Bhargava, Srivastava, Urmil, & Rai, 2005
).
2.3. Degradation of Glutathione
Because of its γ-linkage between the amine group of cysteine and the car-
boxyl group of glutamate, GSH cannot be degraded by protease. How-
ever, one peptidase, γ-glutamyl transpeptidase (GGT), widely distributed
among bacteria and eukaryotes, catabolizes GSH (
Masip et al., 2006
). GGT
releases cysteinyl glycine and transfers the glutamyl moiety of GSH to a
variety of acceptor molecules as water, thereby regenerating glutamate, and
some amino acids and dipeptides (
Fig. 5.2
). The mammalian GGT enzyme
preferentially transfers the glutamyl residue from GSH to cysteine, yielding
the γ-glutamyl-cysteine that is rapidly reduced to γ-glutamyl-cysteine and
cysteine (
Zhang & Forman, 2012
). Mammalian GGTs are embedded in
the plasma membrane by an N-terminal transmembrane peptide and are
heterologously glycosylated. By contrast, bacterial GGTs are generally sol-
uble and localized in the periplasmic space by an N-terminal signal peptide
or secreted in the extracellular environment. In
E. coli
and a few other bacte-
ria, another enzyme, the tripeptidase (PepT), operates in GSH degradation,
and both the GGT- and the PepT-encoding genes are dispensable for cell
growth under favourable laboratory conditions (
Lin, Liao, Zhang, Du,
& Chen, 2009
). We found no
pepT
orthologous gene in cyanobacteria
(
Table 5.2
). Most marine cyanobacteria also lack a GGT-encoding gene,
whereas nonmarine strains possess 1-4 presumptive
ggt
genes (
Table 5.2
). In
Synechocystis
PCC 6803, the
ggt
gene
slr1269
is localized between
slr1267
,
which codes for FtsW an important protein involved in cell division
(
Marbouty, Mazouni, Saguez, Cassier-Chauvat, & Chauvat, 2009
), but this
genome organization is not shared by other cyanobacteria.
Considering the importance of GSH, and of its polymer phytochelatin,
for cyanobacteria, it is urgent to investigate not only its synthesis but also
its turnover, and see whether this function involves the presumptive GGT
enzymes and/or other as yet unidentified enzymes.
2.4. Reduction of Glutathione
Upon the detoxification of ROS, GSH is oxidized as glutathione disulfide
(GSSG), which can subsequently be reduced by the NADPH-dependent
glutathione reductase enzyme present in most but not all organisms
