Biology Reference
In-Depth Information
disulfide, also termed glutathionylation). Many proteomics studies have
shown that a large number of proteins from bacteria, cyanobacteria (Marteyn,
Sakr, Chauvat and Cassier-Chauvat unpublished results), plants and human
are glutathionylated under artificially imposed oxidative stress (
Dalle-Donne
et al., 2009
;
Zaffagnini, Bedhomme, Lemaire, & Trost, 2012
). These proteins
are involved in diverse processes, including photosynthesis, oxidative stress
responses, protein folding, amino acid biosynthesis, lipid metabolism, trans-
lation, ATP metabolism, and cytoskeletal arrangements. In
Chlamydomonas
reinhardtii
and/or plants, these glutathionylated proteins are, namely, gluta-
thione S-transferase (GST); cytosolic triose phosphate isomerase and chlo-
roplastic fructose-1,6-bisphosphate aldolase; PRX2 the thiol-dependent
peroxidases; GAPDH; Trxf; ICL (iso-citrate lyase) (
Zaffagnini, Bedhomme,
Marchand et al., 2012
). The glutathionylation process can protect the pro-
tein cysteines against irreversible oxidation, allowing them to be reduced
back to their native states by Grxs when the organism has escaped from the
oxidative environment (
Zaffagnini, Bedhomme, Marchand et al., 2012
). In
addition, the glutathionylation/deglutathionylation process can also mediate
the redox regulation of enzyme activity (activation or deactivation) (
Dalle-
Donne et al., 2009
;
Zaffagnini, Bedhomme, Lemaire et al., 2012
).
The disulfide structures can be repaired (
Fig. 5.3
) by the small enzymes
thioredoxins (Trxs) and glutaredoxins (Grxs), which share a similar three-
dimensional structure known as the thioredoxin fold (
Berndt et al., 2008
).
In the thioredoxin system, NADPH transfers its electrons to the thioredoxin
reductase enzyme (NTR, a flavoenzyme) to reduceTrxs, which use two redox-
active cysteines in a conserved CxxC active site motif to reduce protein-S-S-
protein disulfides (
Berndt et al., 2008
;
Lillig et al., 2008
). In the glutaredoxin
system, which preferentially reduces protein-S-SG mixed disulfides (
Lillig
et al., 2008
;
Rouhier et al., 2008
;
Zaffagnini, Bedhomme, Marchand et al.,
2012
), NADPH transfers its electrons to the glutathione reductase enzyme
(GR, a flavoenzyme) present in many, but not all (
Marteyn et al., 2009
), organ-
isms (
Table 5.2
) to reduce GSH and subsequently Grxs. In addition, the Grx
enzymes can also be reduced by NTR (
Marteyn et al., 2009
;
Couturier et al.,
2009
), or by the ferredoxin-dependent thioredoxin reductase (FTR) enzyme
that receives its electrons from photosynthesis (
Zaffagnini, Bedhomme, Lemaire
et al., 2012
,
Zaffagnini, Bedhomme, Marchand et al., 2012
).
The analysis of the specificity and redundancy of the Grx andTrx enzymes
is difficult in plants because they possess multiple Grxs and Trxs, respectively,
31 and 19 in
A. thaliana
(
Michelet et al., 2006
). By contrast, cyanobacteria
possess a smaller number of
trx
and
grx
genes (
Table 5.2
). For instance, the
model strain
Synechocystis
PCC 6803 has only four Trxs and three Grxs.
