1. Antioxidants
Presentation by : Satya Prakash Chaurasia
Department of Botany,
University of Delhi,
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2. Glutathione(GSH/GSSG)
Roles:-
• direct scavenging of ROS
• re-reduction of ASC
• Sulphur metabolism
• cell growth control
• detoxification of heavy metals
• gene regulation in defence
responses, and
• signal transduction
‘universal redox buffer’??
o Other redox-related components and ROS scavengers, such as tocopherols,
NADP(H), thioredoxins, peroxiredoxins, and glutaredoxins, are often present
in much lower concentrations than GSH or do not occur at all [ Kranner et
al.,2005,2006.]
o Similarly, other ROS scavengers, such as tocotrienols, only occur at low
concentrations, or are even absent in some organisms , whereas GSH is
ubiquitous.
1. second most abundant antioxidant in most
plant tissues.
2. signal transduction either through
glutathionylation of proteins or through
thiol bridge redox reactions

3. NON-
STRESSED
GSH Pool completely
reduced
But, detoxification of ROS by the ASC-GSH cycle leads to transient glutathione
oxidation, impacting the cellular redox environment through the glutathione redox state [
Foyer et al., 2005].
 acclimation of plants under a relatively high, acute UV-B dose resulted in an
increase in total glutathione, as well as a simultaneous increase in the oxidation
states of the GSH pool [Kranner et al., 2006].
 In Arabidopsis, severe salt stress causes a 6-fold decrease in GSH, a small increase
in GSSG, and a 10-fold decrease in the GSH/GSSG ratio [Borsani et al., 2006]
Kranner et al., 2006, proposed glutathione redox state as a a biomarker for the
cellular redox state.

4. Redox potential
Redox potential, rather than simply the redox state (GSH/GSSG), which is a key control in
programmed cell death, and a marker for oxidative stress.
The buffering capacity of the glutathione system is lost above a redox potential of -160 mV.
At that point, GSH will lose the capacity to protect macromolecules from ROS [ Apel et
al., 2004], ultimately leading to cell death .

5. Proteinaceous thiols
• Thioredoxin(Trx),
• glutaredoxin
(Grx), and
• peroxiredoxin(Prx)
proteins
 moderating protein activity via the reversible
oxidation of Cys-Cys bridges.
 modulation of photosynthetic activity ,and
 Participation in stress defence.

6. Thioredoxin(Trx)
• Thioredoxins molecular weight of around 12 kDa.
• Disulfide oxidoreductase , due to a redox-active disulfide bridge within its active
site, characteristically consisting of the sequence Trp-Cys-Gly-Pro-Cys.
• Reduced Trx (Trx-(SH)2) contains a dithiol group, which becomes a disulfide bridge
in the oxidised state(Trx-S2).
• receive electrons from a
ferredoxin-dependent
thioredoxin reductase
Chloroplast
form
• NADPH thioredoxin
reductase (NTR)In organelles
In turn, the Trx/NTR system supplies electrons to the ribonucleotide
reductase, and is therefore directly implicated in the synthesis of
deoxyribonucleotides and therefore of DNA.

7. Glutaredoxins
• Use GSH as electron donor.
• Grx can reduce Trx .
Roles:-
• Grx have been implicated in
assembly of iron-sulphur
clusters and in the defence
against ROS .
• Grx can instigate redox
dependent signalling, for
example, by catalyzing
reversible protein S-
glutathionylation.
Peroxiredoxins (Prx)
• neutralise H2O2 molecules.
• Grx and Trx as electron donor

8. NAD(P)H
• quartet of pyridine nucleotides NADH and NADPH and their oxidised counterparts NAD+ and NADP+
• NADH is best known for its role as electron shuttle between the Krebs cycle and the mitochondrial
electron transport chain.
• NADPH plays a key role in the chloroplast, where NADP+ is reduced by ferredoxin- NADP+
reductase as the last step of the light-driven electron chain. The NADPH produced is then oxidised
in the Calvin cycle.
• NADPH and NADH are also used in a variety of anabolic reactions, such as lipid and nucleic acid
synthesis or nitrate reduction.
• NAD(P)H are used to regenerate other antioxidants.
– MDHA reductase uses electrons originating in NADH [Berczi et al., 1998].
– GSH and Trx are re-reduced by a NADPH-dependent GSH reductase and NADPH-dependent Trx reductase respectively
[Potters et al., 2002].