monomeric form of 2-Cys PRX (Kirchsteiger et al. 2009). In contrast, mutant plants
devoid of TRXx showed levels of oxidized and reduced 2-Cys PRX indistinguishable of the wild type, indicating that NTRC is more relevant reductant of 2-Cys PRX
than TRXx (Pulido et al. 2010).
In the presence of excess hydrogen peroxide, 2-Cys PRX may be inactivated by
over-oxidation of the sulfenic acid intermediate of S P (–SOH) to sulfinic acid
(–SO 2 H), which can be reverted by sulfiredoxin (SRX) (Biteau et al. 2003), or
even to sulfonic acid (–SO 3 H), which is irreversible. Interestingly, 2-Cys PRX
from eukaryotes are more sensitive to over-oxidation than those from prokaryotes
due to structural constraints imposed by the presence of two motifs, GGLC and YF,
in the enzyme from eukaryotes (Wood et al. 2003). Since over-oxidation causes the
inactivation of 2-Cys PRX, it was proposed that this feature is a gain-of-function of
the enzymes from eukaryotes that allows the accumulation of hydrogen peroxide
and, thus, its signaling function, the so-called floodgate hypothesis (Wood et al.
2003). Chloroplast 2-Cys PRX are sensitive to over-oxidation, hence behaving as
eukaryotic type enzymes (Kirchsteiger et al. 2009). However, since plant chloroplasts have prokaryotic origin similar to modern cyanobacteria (Gould et al. 2008)
the question arising was whether or not 2-Cys PRX from cyanobacteria are sensitive
to over-oxidation. The finding that the enzyme from Anabaena is sensitive to overoxidation while the one from Synechocystis is not (Pascual et al. 2010) showed the
existence of sensitive 2-Cys PRX in prokaryotes and revealed that cyanobacteria
have developed two strategies to cope with hydrogen peroxide, the strategy of
Anabaena being remarkably similar to the one in chloroplasts.
The level of over-oxidation of chloroplast 2-Cys PRX and, thus, the balance of
antioxidant and signaling activities of these enzymes depends on the reductants
(NTRC and TRX of this organelle), but also on SRX, which catalyzes the reversion
of the over-oxidized to the reduced form of the enzyme (Puerto-Galan et al. 2013).
However, a double mutant of Arabidopsis devoid of NTRC and SRX showed the
phenotype of the ntrc mutant (Puerto-Galan et al. 2015). Altogether, the results of
the biochemical analyses of NTRC in conjunction with the genetic interaction of
NTRC with TRXx and SRX indicate that a central function of NTRC is the control
of the redox state of the 2-Cys PRX, which is important for the balance of the
harmful and signaling activities of hydrogen peroxide.
2.2.4 NTRC Participates in the Redox Regulation of Light Energy
Utilization and Several Chloroplast Biosynthetic Pathways
To address the issue of the function of NTRC the use of Arabidopsis mutants has
been a useful tool. The Arabidopsis knock-out mutant ntrc shows pale green leaves
with a lower content of chlorophylls and retarded growth, as compared to wild type
plants. Moreover, the retarded growth phenotype of the ntrc mutant is dependent on
the photoperiod, short-day conditions aggravating the phenotype (Serrato et al.
2004; Lepistö et al. 2009). More in-depth analyses revealed that the ntrc mutant
displays lower CO 2 fixation rates, especially at low light intensities, and abnormal
On the Elaborate Network of Thioredoxins in Higher Plants
235
devoid of TRXx showed levels of oxidized and reduced 2-Cys PRX indistinguishable of the wild type, indicating that NTRC is more relevant reductant of 2-Cys PRX
than TRXx (Pulido et al. 2010).
In the presence of excess hydrogen peroxide, 2-Cys PRX may be inactivated by
over-oxidation of the sulfenic acid intermediate of S P (–SOH) to sulfinic acid
(–SO 2 H), which can be reverted by sulfiredoxin (SRX) (Biteau et al. 2003), or
even to sulfonic acid (–SO 3 H), which is irreversible. Interestingly, 2-Cys PRX
from eukaryotes are more sensitive to over-oxidation than those from prokaryotes
due to structural constraints imposed by the presence of two motifs, GGLC and YF,
in the enzyme from eukaryotes (Wood et al. 2003). Since over-oxidation causes the
inactivation of 2-Cys PRX, it was proposed that this feature is a gain-of-function of
the enzymes from eukaryotes that allows the accumulation of hydrogen peroxide
and, thus, its signaling function, the so-called floodgate hypothesis (Wood et al.
2003). Chloroplast 2-Cys PRX are sensitive to over-oxidation, hence behaving as
eukaryotic type enzymes (Kirchsteiger et al. 2009). However, since plant chloroplasts have prokaryotic origin similar to modern cyanobacteria (Gould et al. 2008)
the question arising was whether or not 2-Cys PRX from cyanobacteria are sensitive
to over-oxidation. The finding that the enzyme from Anabaena is sensitive to overoxidation while the one from Synechocystis is not (Pascual et al. 2010) showed the
existence of sensitive 2-Cys PRX in prokaryotes and revealed that cyanobacteria
have developed two strategies to cope with hydrogen peroxide, the strategy of
Anabaena being remarkably similar to the one in chloroplasts.
The level of over-oxidation of chloroplast 2-Cys PRX and, thus, the balance of
antioxidant and signaling activities of these enzymes depends on the reductants
(NTRC and TRX of this organelle), but also on SRX, which catalyzes the reversion
of the over-oxidized to the reduced form of the enzyme (Puerto-Galan et al. 2013).
However, a double mutant of Arabidopsis devoid of NTRC and SRX showed the
phenotype of the ntrc mutant (Puerto-Galan et al. 2015). Altogether, the results of
the biochemical analyses of NTRC in conjunction with the genetic interaction of
NTRC with TRXx and SRX indicate that a central function of NTRC is the control
of the redox state of the 2-Cys PRX, which is important for the balance of the
harmful and signaling activities of hydrogen peroxide.
2.2.4 NTRC Participates in the Redox Regulation of Light Energy
Utilization and Several Chloroplast Biosynthetic Pathways
To address the issue of the function of NTRC the use of Arabidopsis mutants has
been a useful tool. The Arabidopsis knock-out mutant ntrc shows pale green leaves
with a lower content of chlorophylls and retarded growth, as compared to wild type
plants. Moreover, the retarded growth phenotype of the ntrc mutant is dependent on
the photoperiod, short-day conditions aggravating the phenotype (Serrato et al.
2004; Lepistö et al. 2009). More in-depth analyses revealed that the ntrc mutant
displays lower CO 2 fixation rates, especially at low light intensities, and abnormal
On the Elaborate Network of Thioredoxins in Higher Plants
235