dehydrogenase, β-etherase, and glutathione lyase) from Sphingobium sp. SYK6
known for the β-O-4-aryl ether linkage has been expressed in E. coli and has
shown to convert complex lignin structure from hardwood and softwood into
monomers (Reiter et al. 2013). Dehydrogenases act on toxic aldehyde (generated
during degradation) and convert them into their acids that are less toxic to cells and
hence play important part in catabolic funneling (Pérez-Pantoja et al. 2010;
Abdelaziz et al. 2016).
4.5.7 O-Demethylases
The enzymes of demethylase system removes methyl group from methoxysubstituted lignin-derived aromatic compounds such as syringate, vanillate, or
guaiacol in the presence of cofactors. There are two types of demethylation system.
Type one demethylase comprises of an oxygenase (iron-binding site and a Rieske
type [2Fe-2S] cluster) and a reductase (a flavin and a [2Fe-2S] redox center)
predominant in aerobic microbes such as Pseudomonas and Acinetobacter (Masai
et al. 2007; Bugg et al. 2011b; Abdelaziz et al. 2016). Type two demethylase system
catalyzes tetrahydrofolate-dependent demethylation of lignin-derived intermediates
mainly reported in anaerobic microbes such as Acetobacterium dehalogenans and
Acetobacterium woodii. After demethylation, the products converge at a few common intermediates (protocatechuic acid, catechol, or gallic acid), which undergo ring
cleavage (intra- or extradiol) and are metabolized further by β-ketoadipate pathway
(Masai et al. 2007; Bugg et al. 2011b; Abdelaziz et al. 2016).
4.5.8 Dioxygenases
Streptomyces sp. SirexAA-E was found to secrete a fusion enzyme SACTE_2871
containing aromatic ring dioxygenase (intradiol) domain and lignin-binding domain
while growing on lignocellulosic biomass. This enzyme contains Fe
3+ active site and
performs oxygen-dependent cleavage of catechol; however, no activity on their
methylated derivatives was observed. The secretion is carried out by Tat translocation pathway (Bianchetti et al. 2013). An extradiol dioxygenase was also discovered
in the Sphingomonas paucimobilis SYK-6 responsible for lignin degradation. This
dioxygenase was identified while studying degradation of biphenyl compound as
substrate by this strain (Sonoki et al. 2009). These evidences clearly indicate the role
of dioxygenase in lignin degradation.
4.5.9 Aromatic Alcohol Oxidase (EC 1.1.3.7)
This enzyme belongs to the class oxidoreductases and attacks on the CH–OH group
of the donor, with oxygen as acceptor. It generally attacks the primary aromatic
alcohol and converts it into aldehyde. It is a monomeric enzyme mostly found in
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M. Kumar et al.
known for the β-O-4-aryl ether linkage has been expressed in E. coli and has
shown to convert complex lignin structure from hardwood and softwood into
monomers (Reiter et al. 2013). Dehydrogenases act on toxic aldehyde (generated
during degradation) and convert them into their acids that are less toxic to cells and
hence play important part in catabolic funneling (Pérez-Pantoja et al. 2010;
Abdelaziz et al. 2016).
4.5.7 O-Demethylases
The enzymes of demethylase system removes methyl group from methoxysubstituted lignin-derived aromatic compounds such as syringate, vanillate, or
guaiacol in the presence of cofactors. There are two types of demethylation system.
Type one demethylase comprises of an oxygenase (iron-binding site and a Rieske
type [2Fe-2S] cluster) and a reductase (a flavin and a [2Fe-2S] redox center)
predominant in aerobic microbes such as Pseudomonas and Acinetobacter (Masai
et al. 2007; Bugg et al. 2011b; Abdelaziz et al. 2016). Type two demethylase system
catalyzes tetrahydrofolate-dependent demethylation of lignin-derived intermediates
mainly reported in anaerobic microbes such as Acetobacterium dehalogenans and
Acetobacterium woodii. After demethylation, the products converge at a few common intermediates (protocatechuic acid, catechol, or gallic acid), which undergo ring
cleavage (intra- or extradiol) and are metabolized further by β-ketoadipate pathway
(Masai et al. 2007; Bugg et al. 2011b; Abdelaziz et al. 2016).
4.5.8 Dioxygenases
Streptomyces sp. SirexAA-E was found to secrete a fusion enzyme SACTE_2871
containing aromatic ring dioxygenase (intradiol) domain and lignin-binding domain
while growing on lignocellulosic biomass. This enzyme contains Fe
3+ active site and
performs oxygen-dependent cleavage of catechol; however, no activity on their
methylated derivatives was observed. The secretion is carried out by Tat translocation pathway (Bianchetti et al. 2013). An extradiol dioxygenase was also discovered
in the Sphingomonas paucimobilis SYK-6 responsible for lignin degradation. This
dioxygenase was identified while studying degradation of biphenyl compound as
substrate by this strain (Sonoki et al. 2009). These evidences clearly indicate the role
of dioxygenase in lignin degradation.
4.5.9 Aromatic Alcohol Oxidase (EC 1.1.3.7)
This enzyme belongs to the class oxidoreductases and attacks on the CH–OH group
of the donor, with oxygen as acceptor. It generally attacks the primary aromatic
alcohol and converts it into aldehyde. It is a monomeric enzyme mostly found in
98
M. Kumar et al.