78
The bioremediation process can be ex situ and in situ (Hamzah et al. 2013) and
is influenced by many external factors. The deciding factors may include physical,
chemical, physiological and environmental factors such as extent of aeration, solubility of the substrate, toxicity of the substrate, pH, temperature, incubation time,
agitation of the medium and growth rate of the selected organism (Atlas 1995;
Al-Sulaimani et al. 2010; Adams et al. 2014).
3.3
Microbial Enzymes as Bioremediators
The use of microbial enzymes may represent a good alternative for the remediation
of xenobiotic pollutants. Varieties of bacterial, fungal and phytoenzymes can be
catalysts in the degradation of these pollutants of concern. It is observed that such
enzymes have a broad specificity range. Enzymes are catalysts with either narrow or
broad specificity and cause the complete conversion of toxic chemicals to inorganic
end products. They can be applied under extreme conditions and are more mobile
than microorganisms. All these make enzymes eco-friendly catalysts and the mechanisms an environment-friendly process.
Some protein superfamilies are specifically important in biodegradation and biocatalysis. Usually enzymes involved in biodegradation are found to exhibit a broad
range of specificity; for example, naphthaline dioxygenase has 70 known substrates
and toluene dehydrogenases has more than 100 substrates (Seo et al. 2010). The
important classes of enzymes involved in the remediation of pollutants include:
hydrolases, dehalogenases, transferases and oxidoreductases. Phosphotriesterases,
amidases, proteases, cellulases, amylases, lipases, depolymerases, mono- or dioxygenases, reductases, cytochrome P450 monooxygenases, phenoloxidases, laccases,
tyrosinases, lignin and manganese peroxidises are the main classes of enzymes
involved in biodegradation of xenobiotic compounds (Table 3.1).
3.3.1 Oxidoreductases
Monooxygenases form an important group of degradative enzymes belonging to
oxidoreductase. These enzymes are involved in cometabolism and transformation
of xenobiotics. Microbial monooxygenases differ in their susceptibility with respect
to the nature of compounds like aliphatic n-alkynes. Monooxygenases are critical in
the conversion of aromatic compounds like phenol. The destruction of the resonant
structure of benzene ring can be achieved by the enzyme action. As a convergent
point in the decomposition of most aromatic compounds, catechol is formed by
introducing a hydroxyl group to the aromatic ring. Catechol can be further cleaved
by the ortho or meta pathway (Nair et al. 2008). In monoxygenase action, reductive
dechlorination of compounds takes place under less oxygenated situations, and
under high oxygen levels oxidative dehalogenation takes place.
Cytochrome P450 monooxygenase is a well-studied enzyme in bioremediation.
The initial step of cDCE degradation was catalysed bycytochrome P450
I.C. Nair and K. Jayachandran
The bioremediation process can be ex situ and in situ (Hamzah et al. 2013) and
is influenced by many external factors. The deciding factors may include physical,
chemical, physiological and environmental factors such as extent of aeration, solubility of the substrate, toxicity of the substrate, pH, temperature, incubation time,
agitation of the medium and growth rate of the selected organism (Atlas 1995;
Al-Sulaimani et al. 2010; Adams et al. 2014).
3.3
Microbial Enzymes as Bioremediators
The use of microbial enzymes may represent a good alternative for the remediation
of xenobiotic pollutants. Varieties of bacterial, fungal and phytoenzymes can be
catalysts in the degradation of these pollutants of concern. It is observed that such
enzymes have a broad specificity range. Enzymes are catalysts with either narrow or
broad specificity and cause the complete conversion of toxic chemicals to inorganic
end products. They can be applied under extreme conditions and are more mobile
than microorganisms. All these make enzymes eco-friendly catalysts and the mechanisms an environment-friendly process.
Some protein superfamilies are specifically important in biodegradation and biocatalysis. Usually enzymes involved in biodegradation are found to exhibit a broad
range of specificity; for example, naphthaline dioxygenase has 70 known substrates
and toluene dehydrogenases has more than 100 substrates (Seo et al. 2010). The
important classes of enzymes involved in the remediation of pollutants include:
hydrolases, dehalogenases, transferases and oxidoreductases. Phosphotriesterases,
amidases, proteases, cellulases, amylases, lipases, depolymerases, mono- or dioxygenases, reductases, cytochrome P450 monooxygenases, phenoloxidases, laccases,
tyrosinases, lignin and manganese peroxidises are the main classes of enzymes
involved in biodegradation of xenobiotic compounds (Table 3.1).
3.3.1 Oxidoreductases
Monooxygenases form an important group of degradative enzymes belonging to
oxidoreductase. These enzymes are involved in cometabolism and transformation
of xenobiotics. Microbial monooxygenases differ in their susceptibility with respect
to the nature of compounds like aliphatic n-alkynes. Monooxygenases are critical in
the conversion of aromatic compounds like phenol. The destruction of the resonant
structure of benzene ring can be achieved by the enzyme action. As a convergent
point in the decomposition of most aromatic compounds, catechol is formed by
introducing a hydroxyl group to the aromatic ring. Catechol can be further cleaved
by the ortho or meta pathway (Nair et al. 2008). In monoxygenase action, reductive
dechlorination of compounds takes place under less oxygenated situations, and
under high oxygen levels oxidative dehalogenation takes place.
Cytochrome P450 monooxygenase is a well-studied enzyme in bioremediation.
The initial step of cDCE degradation was catalysed bycytochrome P450
I.C. Nair and K. Jayachandran