oxidases), (2) hydrolytic transition mediated by hydrolytic enzymes (hydrolases)
that cleaves bonds of substrate by adding hydrogen or hydroxyl group from water
molecules, (3) reductive transformation mediated by reductive enzymes
(nitroreductase) by which removal of anion occurs by reduction, and reductive
dehalogenation is mediated by reductive dehalogenase enzyme (Commandeur and
Parsons 1994; Odukkathil and Vasudevan 2013), and (4) synthetic/conjugation
reactions by which an exogenous or endogenous natural compound is added to the
pesticide to facilitate mineralization.
Three enzymes were involved in the first few stages of the degradation of atrazine
by Pseudomonas spp. strain ADP, which used atrazine as the sole carbon source
(De Souza et al. 1996; Wackett et al. 2002). Most of the catabolic genes encoding
these degradative enzymes were located on the plasmid (Nour et al. 2017; Nayak
et al. 2018). Likewise, biodegradation of 2,4-D is regulated by genes carried on the
plasmid (Don and Pemberton 1985). Studies have shown that mineralization and
co-metabolism were the major mechanisms for further degradation of pesticides and
their by-products (Boivin et al. 2005; Arora et al. 2012; Ye et al. 2018). On
ingestion, inhalation, or absorption dermally, chlorpyrifos may be metabolized by
the enzymes of cytochrome P450 that cause derylation (oxidative ester cleavage) of
the chlorpyrifos and formed 3,5,6-trichloro-2 pyridinol (TCP) and
diethylthiophophate (Komori et al. 1990). Chlorpyrifos degradation mainly leads
to TCP, which is then degraded by bacterial enzymatic oxidation and hydrolytic
reactions (Li et al. 2010b). TCP is broken down via the release of three chlorine
molecules during its sequential dechlorination, in which one oxidation and two
hydrolytic steps 3,6-dihydroxypyridine-2,5-dione (Li et al. 2010b; Ramakrishnan
et al. 2011) were formed.
The degradation of 2,4-dichlorophenoxy acetic acid (2,4-D) was shown to have
two different pathways (Amy et al. 1985). These two degradation pathways were
mediated by Pseudomonas spp. and Alcaligenes spp. isolates, respectively (Amy
et al. 1985). In one way, the sixth carbon is oxidized by the addition of the OH group,
yielding 6-OH-2, 4-D, and followed by removal of acetate, resulting in the formation
of 3,5-dichlorocatechol. In the second path, two carbon side chains are removed,
resulting in glyoxylate and 2,4-DCP. The oxygenases synthesized by Pseudomonas
spp. caused degradation of tetrachlorobenzene to trichlorocatechol by removing HCl
from the compound (Sander et al. 1991). Mono- and dioxygenases were actively
involved in the dehalogenation-mediated degradation of halogen-based pesticides
(Braus-Stromeyer et al. 1993). Peroxidases synthesized by fungi and bacteria were
reported to biodegrade pesticides and their derivatives. For example, the compound
3,4-dichloroaniline was converted to 4,4-tetrachloroazobenzene by peroxidases
produced by soil microorganisms (Bordeleau et al. 1972). The peroxidases secreted
by P. chrysosporium added on chlorine to 2,4-di-, trichlorophenol, 2,4,6trichlorophenol and pentachlorophenol at their para positions and formed p-benzoquinone (Hammel and Tardone 1988). These peroxidases mineralized 2,4,5-TCP
rapidly. Different reactions and enzymes involved in pesticides degradation are
illustrated in Fig. 2.4.
2 Bioremediation of Pesticides: An Eco-Friendly Approach for Environment. . .
49
that cleaves bonds of substrate by adding hydrogen or hydroxyl group from water
molecules, (3) reductive transformation mediated by reductive enzymes
(nitroreductase) by which removal of anion occurs by reduction, and reductive
dehalogenation is mediated by reductive dehalogenase enzyme (Commandeur and
Parsons 1994; Odukkathil and Vasudevan 2013), and (4) synthetic/conjugation
reactions by which an exogenous or endogenous natural compound is added to the
pesticide to facilitate mineralization.
Three enzymes were involved in the first few stages of the degradation of atrazine
by Pseudomonas spp. strain ADP, which used atrazine as the sole carbon source
(De Souza et al. 1996; Wackett et al. 2002). Most of the catabolic genes encoding
these degradative enzymes were located on the plasmid (Nour et al. 2017; Nayak
et al. 2018). Likewise, biodegradation of 2,4-D is regulated by genes carried on the
plasmid (Don and Pemberton 1985). Studies have shown that mineralization and
co-metabolism were the major mechanisms for further degradation of pesticides and
their by-products (Boivin et al. 2005; Arora et al. 2012; Ye et al. 2018). On
ingestion, inhalation, or absorption dermally, chlorpyrifos may be metabolized by
the enzymes of cytochrome P450 that cause derylation (oxidative ester cleavage) of
the chlorpyrifos and formed 3,5,6-trichloro-2 pyridinol (TCP) and
diethylthiophophate (Komori et al. 1990). Chlorpyrifos degradation mainly leads
to TCP, which is then degraded by bacterial enzymatic oxidation and hydrolytic
reactions (Li et al. 2010b). TCP is broken down via the release of three chlorine
molecules during its sequential dechlorination, in which one oxidation and two
hydrolytic steps 3,6-dihydroxypyridine-2,5-dione (Li et al. 2010b; Ramakrishnan
et al. 2011) were formed.
The degradation of 2,4-dichlorophenoxy acetic acid (2,4-D) was shown to have
two different pathways (Amy et al. 1985). These two degradation pathways were
mediated by Pseudomonas spp. and Alcaligenes spp. isolates, respectively (Amy
et al. 1985). In one way, the sixth carbon is oxidized by the addition of the OH group,
yielding 6-OH-2, 4-D, and followed by removal of acetate, resulting in the formation
of 3,5-dichlorocatechol. In the second path, two carbon side chains are removed,
resulting in glyoxylate and 2,4-DCP. The oxygenases synthesized by Pseudomonas
spp. caused degradation of tetrachlorobenzene to trichlorocatechol by removing HCl
from the compound (Sander et al. 1991). Mono- and dioxygenases were actively
involved in the dehalogenation-mediated degradation of halogen-based pesticides
(Braus-Stromeyer et al. 1993). Peroxidases synthesized by fungi and bacteria were
reported to biodegrade pesticides and their derivatives. For example, the compound
3,4-dichloroaniline was converted to 4,4-tetrachloroazobenzene by peroxidases
produced by soil microorganisms (Bordeleau et al. 1972). The peroxidases secreted
by P. chrysosporium added on chlorine to 2,4-di-, trichlorophenol, 2,4,6trichlorophenol and pentachlorophenol at their para positions and formed p-benzoquinone (Hammel and Tardone 1988). These peroxidases mineralized 2,4,5-TCP
rapidly. Different reactions and enzymes involved in pesticides degradation are
illustrated in Fig. 2.4.
2 Bioremediation of Pesticides: An Eco-Friendly Approach for Environment. . .
49