256
(two O 2
•− are neutralized by the addition of two hydrogen ions) to H 2 O 2 and O 2 ,
thereby decreasing the risk of OH
•
formation via Haber-Weiss-type reaction (Gill and
Tuteja 2010). CAT is indispensable for ROS detoxification during stressed conditions
(Garg and Manchanda 2009). It contains porphyrin haem active sites that directly
dismutate H 2 O 2 into H 2 O and O 2 . In 1 min, one molecule of CAT can convert approximately six million molecules of H 2 O 2 into H 2 O and O 2 (Gill and Tuteja 2010) . APX is
one of the peroxidases (POX) involved in the scavenging of H 2 O 2 utilizing ascorbate
as the electron donor. APX has higher affinity to H 2 O 2 (μM range) than that of CAT
(mM range) (Gill and Tuteja 2010). Besides these, several other enzymatic scavengers,
viz. GR, glutathione S-transferases (GST), guaiacol peroxidase, glutathione peroxidase, dehydro- and monodehydroascorbate reductase, etc. also play an essential role in
the cellular defence against ROS.
Various non-enzymatic antioxidants also play an important role in the mitigation of oxidative stress-induced cellular injuries. Carotenoids, the lipid soluble
pigments, alleviate oxidative toxicity by direct quenching of
1
O 2 or by dissipating the excess excitation energy from chlorophyll by direct transfer or via xanthophyll cycle and suppressing lipid peroxidation (Telfer 2005; Kumar et al.
2011a). Tocopherols, another lipid-soluble
1
O 2 quencher, prevent the chain propagation step in lipid autoxidation (Ahmad et al. 2010). Proline, an osmoprotectant, is now considered an inhibitor of lipid peroxidation and the scavenger of
OH• and
1
O 2 (Ashraf and Foolad 2007; Trovato et al. 2008). Polyphenol is a
powerful non-enzymatic ROS scavenger, which acts as substrate for the H 2 O 2 -
scavenging enzyme POX and prevents the diffusion of ROS by altering the peroxidation kinetics and reducing the fluidity of the cell membrane (Parida and Jha
2013). Ascorbic acid, the most abundant water-soluble antioxidant, protects the
membranes by directly scavenging O 2
•− and OH
•
radicals and regenerating
α-tocopherol from tocopheroxyl radicals (Gill and Tuteja 2010). Glutathione is
the potential scavenger of H 2 O 2 and
1
O 2 and the most dangerous OH• radicals. It
also regenerates another water-soluble antioxidant ascorbate via ascorbate-glutathione cycle (Foyer and Halliwell 1976). Phytochelatins are low molecular
weight compounds synthesized from glutathione by phytochelatin synthase.
Their high sulphur content chelates heavy metals and provides short- term defence
against toxic metals (Cirulis et al. 2013).
Whether ROS will act as damaging, protective or signalling factor depends on
the delicate equilibrium between their production and scavenging at proper site and
time (Gratão et al. 2005). It has been suggested that ROS exhibit dual role
(Vanderauwera et al. 2009; Luis 2015) (Fig. 13.2), and the concept of ‘oxidative
stress’, which strictly suggests a state to be avoided, was re-evaluated and the term
‘oxidative signalling’ or ‘redox signalling’ was created (Foyer and Noctor 2005).
The connection between accumulation of various ROS and microalgal cellular
metabolites might improve our understanding about microalgal cell physiology and
cellular responses to various environmental stress conditions.
K. Chokshi et al.
(two O 2
•− are neutralized by the addition of two hydrogen ions) to H 2 O 2 and O 2 ,
thereby decreasing the risk of OH
•
formation via Haber-Weiss-type reaction (Gill and
Tuteja 2010). CAT is indispensable for ROS detoxification during stressed conditions
(Garg and Manchanda 2009). It contains porphyrin haem active sites that directly
dismutate H 2 O 2 into H 2 O and O 2 . In 1 min, one molecule of CAT can convert approximately six million molecules of H 2 O 2 into H 2 O and O 2 (Gill and Tuteja 2010) . APX is
one of the peroxidases (POX) involved in the scavenging of H 2 O 2 utilizing ascorbate
as the electron donor. APX has higher affinity to H 2 O 2 (μM range) than that of CAT
(mM range) (Gill and Tuteja 2010). Besides these, several other enzymatic scavengers,
viz. GR, glutathione S-transferases (GST), guaiacol peroxidase, glutathione peroxidase, dehydro- and monodehydroascorbate reductase, etc. also play an essential role in
the cellular defence against ROS.
Various non-enzymatic antioxidants also play an important role in the mitigation of oxidative stress-induced cellular injuries. Carotenoids, the lipid soluble
pigments, alleviate oxidative toxicity by direct quenching of
1
O 2 or by dissipating the excess excitation energy from chlorophyll by direct transfer or via xanthophyll cycle and suppressing lipid peroxidation (Telfer 2005; Kumar et al.
2011a). Tocopherols, another lipid-soluble
1
O 2 quencher, prevent the chain propagation step in lipid autoxidation (Ahmad et al. 2010). Proline, an osmoprotectant, is now considered an inhibitor of lipid peroxidation and the scavenger of
OH• and
1
O 2 (Ashraf and Foolad 2007; Trovato et al. 2008). Polyphenol is a
powerful non-enzymatic ROS scavenger, which acts as substrate for the H 2 O 2 -
scavenging enzyme POX and prevents the diffusion of ROS by altering the peroxidation kinetics and reducing the fluidity of the cell membrane (Parida and Jha
2013). Ascorbic acid, the most abundant water-soluble antioxidant, protects the
membranes by directly scavenging O 2
•− and OH
•
radicals and regenerating
α-tocopherol from tocopheroxyl radicals (Gill and Tuteja 2010). Glutathione is
the potential scavenger of H 2 O 2 and
1
O 2 and the most dangerous OH• radicals. It
also regenerates another water-soluble antioxidant ascorbate via ascorbate-glutathione cycle (Foyer and Halliwell 1976). Phytochelatins are low molecular
weight compounds synthesized from glutathione by phytochelatin synthase.
Their high sulphur content chelates heavy metals and provides short- term defence
against toxic metals (Cirulis et al. 2013).
Whether ROS will act as damaging, protective or signalling factor depends on
the delicate equilibrium between their production and scavenging at proper site and
time (Gratão et al. 2005). It has been suggested that ROS exhibit dual role
(Vanderauwera et al. 2009; Luis 2015) (Fig. 13.2), and the concept of ‘oxidative
stress’, which strictly suggests a state to be avoided, was re-evaluated and the term
‘oxidative signalling’ or ‘redox signalling’ was created (Foyer and Noctor 2005).
The connection between accumulation of various ROS and microalgal cellular
metabolites might improve our understanding about microalgal cell physiology and
cellular responses to various environmental stress conditions.
K. Chokshi et al.