Antioxidants

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  • 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
  • Antioxidants

    1. 1. AntioxidantsPresentation by : Satya Prakash ChaurasiaDepartment of Botany,University of Delhi,satya.prakash101991@gmail.comwww.facebook.com/91satyawww.twitter.com/Satyapr101991h ttp://www.linkedin.com/pub/satya-prakash chaurasia/65/a76/a25Mob: +919654814497
    2. 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 defenceresponses, 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 presentin much lower concentrations than GSH or do not occur at all [ Kranner etal.,2005,2006.]o Similarly, other ROS scavengers, such as tocotrienols, only occur at lowconcentrations, or are even absent in some organisms , whereas GSH isubiquitous.1. second most abundant antioxidant in mostplant tissues.2. signal transduction either throughglutathionylation of proteins or throughthiol bridge redox reactions
    3. 3. NON-STRESSEDGSH Pool completelyreducedBut, detoxification of ROS by the ASC-GSH cycle leads to transient glutathioneoxidation, 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 anincrease in total glutathione, as well as a simultaneous increase in the oxidationstates of the GSH pool [Kranner et al., 2006]. In Arabidopsis, severe salt stress causes a 6-fold decrease in GSH, a small increasein 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 thecellular redox state.
    4. 4. Redox potentialRedox potential, rather than simply the redox state (GSH/GSSG), which is a key control inprogrammed 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 etal., 2004], ultimately leading to cell death .
    5. 5. Proteinaceous thiols• Thioredoxin(Trx),• glutaredoxin(Grx), and• peroxiredoxin(Prx)proteins moderating protein activity via the reversibleoxidation of Cys-Cys bridges. modulation of photosynthetic activity ,and Participation in stress defence.
    6. 6. Thioredoxin(Trx)• Thioredoxins molecular weight of around 12 kDa.• Disulfide oxidoreductase , due to a redox-active disulfide bridge within its activesite, characteristically consisting of the sequence Trp-Cys-Gly-Pro-Cys.• Reduced Trx (Trx-(SH)2) contains a dithiol group, which becomes a disulfide bridgein the oxidised state(Trx-S2).• receive electrons from aferredoxin-dependentthioredoxin reductaseChloroplastform• NADPH thioredoxinreductase (NTR)In organellesIn turn, the Trx/NTR system supplies electrons to the ribonucleotidereductase, and is therefore directly implicated in the synthesis ofdeoxyribonucleotides and therefore of DNA.
    7. 7. Glutaredoxins• Use GSH as electron donor.• Grx can reduce Trx .Roles:-• Grx have been implicated inassembly of iron-sulphurclusters and in the defenceagainst ROS .• Grx can instigate redoxdependent signalling, forexample, by catalyzingreversible protein S-glutathionylation.Peroxiredoxins (Prx)• neutralise H2O2 molecules.• Grx and Trx as electron donor
    8. 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 mitochondrialelectron 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 oxidisedin the Calvin cycle.• NADPH and NADH are also used in a variety of anabolic reactions, such as lipid and nucleic acidsynthesis 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].

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