diarylpropane model compounds, but the gene has not been identified. The product
obtained is lignostilbene, and the enzyme (lignostilbenedioxygenase) responsible for
its degradation into vanillin has been reported (Bugg et al. 2011b). The degradation
of phenylcoumarane and pinoresinol has been studied with model compounds in S.
paucimobilis SYK-6, but the genes responsible for degradation are still not clear. It
was proposed that degradation of these heterocyclic lignin components is initiated by
a hydroxylation (Bugg et al. 2011b).
4.4.2 Central Pathways for Lignin Degradation in Bacteria
4.4.2.1 Oxidative Cleavage of Aromatic Rings
The process of aromatic ring cleavage is predominantly aerobic, but anaerobic
process exists in nature (Fuchs et al. 2011). Funneling pathways for lignin degradation results into formation of derivatives such as vanillic acid, vanillin, syringate, or
guaiacol, and these are further converted into few common central intermediates
such as protocatechuate (PCA), catechol, and gallic acid. Protocatechuic acid is the
most common intermediate formed during funneling pathways of lignin degradation.
This central intermediate acts as substrate for ring-cleaving dioxygenases. The
pathway for cleavage of aromatic ring can be divided into ortho cleavage and
meta cleavage on the basis of position of hydroxyl group and their fission in aromatic
ring (Bugg et al. 2011b). Ortho (intradiol) cleavage takes place between two
hydroxyl groups catalyzed by Fe
3+ -dependent dioxygenase, and meta (extradiol)
cleavage occurs adjacent to one of the hydroxyl group catalyzed by Fe
2+ -dependent
dioxygenase (Masai et al. 2007; Bugg et al. 2011b). PCA is catalyzed by PCA 2,3dioxygenase (2,3-PCD), PCA 3,4-dioxygenase (3,4-PCD; intradiol), and PCA 4,5dioxygenase (4,5-PCD; extradiol). 4,5-PCD pathway has been well studied in S.
paucimobilis SYK-6. Meta cleavage of catechol gives acetaldehyde and pyruvate
while PCA yields pyruvate. Pathways for oxidative aromatic ring cleavage have
been shown in Fig. 4.4. 3,4-PCD is most widely distributed among proteobacteria
and actinobacteria, but 4,5-PCD is only found in proteobacteria. The β and γ
proteobacteria genus Burkholderia, Pseudomonas, Xanthomonas, Klebsiella, and
Ralstonia contains either 3,4-PCD, 4,5-PCD, or both the enzymes (Masai et al. 2007;
Bugg et al. 2011b).The pathways for oxidative cleavage of aromatic rings and βketoadipate pathway have been shown in Fig. 4.4.
4.4.2.2 b-Ketoadipate Pathway
β-ketoadipate pathway is a highly conserved metabolic route and has been extensively characterized in Pseudomonas putida, Acinetobacter calcoaceticus, and
Agrobacterium tumefaciens (Masai et al. 2007). Ortho cleavage of catechol and
PCA leads to β-ketoadipate followed by succinate and acetyl-CoA formation (Fuchs
et al. 2011; Abdelaziz et al. 2016). β-KA pathway links the lignin-derived upper
funneling pathway with TCA cycle through PCA or catechol intermediates
(Harwood and Parales 1996; Pérez-Pantoja et al. 2010). There are several enzymes
involved in degradation of PCA and catechol to β-KA, and the enzymes are tightly
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M. Kumar et al.
obtained is lignostilbene, and the enzyme (lignostilbenedioxygenase) responsible for
its degradation into vanillin has been reported (Bugg et al. 2011b). The degradation
of phenylcoumarane and pinoresinol has been studied with model compounds in S.
paucimobilis SYK-6, but the genes responsible for degradation are still not clear. It
was proposed that degradation of these heterocyclic lignin components is initiated by
a hydroxylation (Bugg et al. 2011b).
4.4.2 Central Pathways for Lignin Degradation in Bacteria
4.4.2.1 Oxidative Cleavage of Aromatic Rings
The process of aromatic ring cleavage is predominantly aerobic, but anaerobic
process exists in nature (Fuchs et al. 2011). Funneling pathways for lignin degradation results into formation of derivatives such as vanillic acid, vanillin, syringate, or
guaiacol, and these are further converted into few common central intermediates
such as protocatechuate (PCA), catechol, and gallic acid. Protocatechuic acid is the
most common intermediate formed during funneling pathways of lignin degradation.
This central intermediate acts as substrate for ring-cleaving dioxygenases. The
pathway for cleavage of aromatic ring can be divided into ortho cleavage and
meta cleavage on the basis of position of hydroxyl group and their fission in aromatic
ring (Bugg et al. 2011b). Ortho (intradiol) cleavage takes place between two
hydroxyl groups catalyzed by Fe
3+ -dependent dioxygenase, and meta (extradiol)
cleavage occurs adjacent to one of the hydroxyl group catalyzed by Fe
2+ -dependent
dioxygenase (Masai et al. 2007; Bugg et al. 2011b). PCA is catalyzed by PCA 2,3dioxygenase (2,3-PCD), PCA 3,4-dioxygenase (3,4-PCD; intradiol), and PCA 4,5dioxygenase (4,5-PCD; extradiol). 4,5-PCD pathway has been well studied in S.
paucimobilis SYK-6. Meta cleavage of catechol gives acetaldehyde and pyruvate
while PCA yields pyruvate. Pathways for oxidative aromatic ring cleavage have
been shown in Fig. 4.4. 3,4-PCD is most widely distributed among proteobacteria
and actinobacteria, but 4,5-PCD is only found in proteobacteria. The β and γ
proteobacteria genus Burkholderia, Pseudomonas, Xanthomonas, Klebsiella, and
Ralstonia contains either 3,4-PCD, 4,5-PCD, or both the enzymes (Masai et al. 2007;
Bugg et al. 2011b).The pathways for oxidative cleavage of aromatic rings and βketoadipate pathway have been shown in Fig. 4.4.
4.4.2.2 b-Ketoadipate Pathway
β-ketoadipate pathway is a highly conserved metabolic route and has been extensively characterized in Pseudomonas putida, Acinetobacter calcoaceticus, and
Agrobacterium tumefaciens (Masai et al. 2007). Ortho cleavage of catechol and
PCA leads to β-ketoadipate followed by succinate and acetyl-CoA formation (Fuchs
et al. 2011; Abdelaziz et al. 2016). β-KA pathway links the lignin-derived upper
funneling pathway with TCA cycle through PCA or catechol intermediates
(Harwood and Parales 1996; Pérez-Pantoja et al. 2010). There are several enzymes
involved in degradation of PCA and catechol to β-KA, and the enzymes are tightly
92
M. Kumar et al.