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].
Notes de l'éditeur
A thiol compound glutathione Second most abundant antioxident in most plant tissues Maintenance of a stable redox environment is critical for appropriate functioning of cellular processes and cell survival.Many cellular processes like photo and reduction of oxygen through electron transport chain in mitochondria involves…
Reflects severity of stress conditions and can be used as biomarker …, ROS detoxification is carried out bya network of reactions involving enzymes and metabolites with redox properties. The ASC-glutathione(GSH) cycle is a key part of this network (Noctor andFoyer, 1998). In this cycle ASC is oxidized directly byROS or enzymatically by APX. The first product of itsoxidation, ASC free radical (AFR; also known asmonodehydroascorbate) is partly reduced back by aNAD(P)H-dependent reductase and partly undergoes spontaneous dismutation producing dehydroascorbate (DHA), the final oxidation product ofASC. DHA can be reduced by DHA reductase, anenzyme that, using GSH as electron donor, cooperates with AFR reductase in the recycling of the oxidized ASC.
The redox state of GSH can also be analysed in terms of its redox potential, which is expressed as follows…redox value of standard half reactions
the maintenance of the cellular energyhomeostasis in times of severe stress