lignin-degrading microbes in lignocellulosic biorefinery. Some prominent pathways
have been discussed.
4.4.1 b-Aryl Ether Degradation Pathway
β-aryl ether linkage is the most predominant linkages (50–70%) in lignin; therefore,
cleavage of β-aryl ether bond is crucial for lignin biodegradation. The cleavage of
ether bond leads to formation of various industrially important aromatic compounds.
Sphingobium sp. SYK-6 has been extensively studied on various lignin model
compounds for the degradation of β-aryl ether bond. Lig EFG gene cluster enzymes,
lignin peroxidase, and β-aryl-OH elimination followed by decarboxylation, vanillate
dehydrogenase, and demethylation mechanism has been reported for β-aryl ether
metabolism (Masai et al. 2007; Bugg et al. 2011b). Lig EFG has been discussed in
the enzyme section. The degradation of β-aryl ether bond has also been studied in
Rhodococcus jostii RHA1, Pseudomonas acidovorans, Pseudomonas putida, Pseudomonas sp. HR199, Novosphingobium, etc. (Masai et al. 2007; Bugg et al. 2011b;
Chen and Wan 2017).
4.4.1.1 Biphenyl Degradation Pathways
Biphenyl linkage is the second most abundant linkage (10%) found in lignin after βaryl ether. Biphenyl is a major environmental pollutant and affects human health.
The degradation pathway has been extensively studied in bacteria. Degradation of
biphenyls has been studied in genus Pseudomonas, Ralstonia, Burkholderia,
Comamonas, Achromobacter, Rhodococcus, Acinetobacter, and Bacillus. The reaction is initiated by a biphenyl 2, 3-dioxygenase of Rieske nonheme iron oxygenases
family. Study on model compounds by S. paucimobilis SYK-6 suggested Odemthylation reaction followed by extradiol ring cleavage by dioxygenase and
finally degraded by β-KAP pathway into acetyl-coA and succinyl-CoA (Masai et
al. 2007).
4.4.1.2 Ferulate, Diarylpropane, Phenylcoumarane, and Pinoresinol
Catabolic Pathways
Ferulic acid is attached by ester linkage to hemicellulose, and their degradation is
carried out by esterases. Ferulate esterases have been identified in several bacteria.
There are two types of pathway reported for degradation of ferulate. In one pathway,
side chain cleavage occurs to eliminate two carbons from ferulate by two enzymes (a
feruloyl-CoA synthetase, feruloyl-CoA hydratase/lyase FerB). The enzymes have
been reported in P. putida WCS358, Amycolatopsis sp. HR167, Pseudomonas sp.
HR199, Pseudomonas fluorescens, Pseudomonas putida, etc. Another pathway is
the release of one carbon by nonoxidative decarboxylation of ferulate side chain.
This pathway has been identified in Bacillus sp. BP-7 and Enterobacter sp. Px6-4
(Masai et al. 2007; Bugg et al. 2011b). This degradation of diarylpropane has been
studied in Pseudomonas paucimobilis TMY1009. The enzyme responsible for
diarylpropane degradation has been characterized while growing this strain on
4 Bacterial-Mediated Depolymerization and Degradation of Lignin
91
have been discussed.
4.4.1 b-Aryl Ether Degradation Pathway
β-aryl ether linkage is the most predominant linkages (50–70%) in lignin; therefore,
cleavage of β-aryl ether bond is crucial for lignin biodegradation. The cleavage of
ether bond leads to formation of various industrially important aromatic compounds.
Sphingobium sp. SYK-6 has been extensively studied on various lignin model
compounds for the degradation of β-aryl ether bond. Lig EFG gene cluster enzymes,
lignin peroxidase, and β-aryl-OH elimination followed by decarboxylation, vanillate
dehydrogenase, and demethylation mechanism has been reported for β-aryl ether
metabolism (Masai et al. 2007; Bugg et al. 2011b). Lig EFG has been discussed in
the enzyme section. The degradation of β-aryl ether bond has also been studied in
Rhodococcus jostii RHA1, Pseudomonas acidovorans, Pseudomonas putida, Pseudomonas sp. HR199, Novosphingobium, etc. (Masai et al. 2007; Bugg et al. 2011b;
Chen and Wan 2017).
4.4.1.1 Biphenyl Degradation Pathways
Biphenyl linkage is the second most abundant linkage (10%) found in lignin after βaryl ether. Biphenyl is a major environmental pollutant and affects human health.
The degradation pathway has been extensively studied in bacteria. Degradation of
biphenyls has been studied in genus Pseudomonas, Ralstonia, Burkholderia,
Comamonas, Achromobacter, Rhodococcus, Acinetobacter, and Bacillus. The reaction is initiated by a biphenyl 2, 3-dioxygenase of Rieske nonheme iron oxygenases
family. Study on model compounds by S. paucimobilis SYK-6 suggested Odemthylation reaction followed by extradiol ring cleavage by dioxygenase and
finally degraded by β-KAP pathway into acetyl-coA and succinyl-CoA (Masai et
al. 2007).
4.4.1.2 Ferulate, Diarylpropane, Phenylcoumarane, and Pinoresinol
Catabolic Pathways
Ferulic acid is attached by ester linkage to hemicellulose, and their degradation is
carried out by esterases. Ferulate esterases have been identified in several bacteria.
There are two types of pathway reported for degradation of ferulate. In one pathway,
side chain cleavage occurs to eliminate two carbons from ferulate by two enzymes (a
feruloyl-CoA synthetase, feruloyl-CoA hydratase/lyase FerB). The enzymes have
been reported in P. putida WCS358, Amycolatopsis sp. HR167, Pseudomonas sp.
HR199, Pseudomonas fluorescens, Pseudomonas putida, etc. Another pathway is
the release of one carbon by nonoxidative decarboxylation of ferulate side chain.
This pathway has been identified in Bacillus sp. BP-7 and Enterobacter sp. Px6-4
(Masai et al. 2007; Bugg et al. 2011b). This degradation of diarylpropane has been
studied in Pseudomonas paucimobilis TMY1009. The enzyme responsible for
diarylpropane degradation has been characterized while growing this strain on
4 Bacterial-Mediated Depolymerization and Degradation of Lignin
91