Endo-acting ß-mannanases are hydrolases that impact the inner glycosidic links
of the mannan backbone chain by releasing the short ß-1,4 mannooligosaccharides
(Shallom and Shoham 2003), and they are located in the GH families of 5, 26, and
113 (Yeoman et al. 2010).
On the other hand, exo-acting mannosidases are hydrolases that release the
mannose away from oligosaccharides by attacking its terminal links on the
non-reducing ends in addition to transforming mannobiose into mannose units that
are included in the GH family 1, 2, and 5 (Moreira 2008; Van Zyl et al. 2010;
Yeoman et al. 2010).
With the help of ß-mannases, ß-glucosidases release the 1,4 glucopyranose units
at the non-reducing end of the oligomers that are formed as a result of the degradation of glucomannan and galactomannan, and they are in GH families of 1 and
3 (Mccleary and Matheson 1987; Bhatia et al. 2002; Cairns and Esen 2010).
Moreover, to remove the side groups from galactomannan, glucomannan, and
galactoglucomannan, auxiliary enzymes are required, specifically α-galactosidase
and acetyl mannan esterase (Tenkanen et al. 1995; Ganter et al. 2001; Moreira 2008).
9.3.4 Lignin and Ligninolytic Enzymes
Lignin, which is the most abundant source of raw material in nature after cellulose
and consists of phenylpropanoid units linked through covalent bonds, is the main
structural component of the plant cell wall (Li et al. 2009; Sriharti et al. 2017). In
addition to having an important mechanical role for the plants, it also helps the plants
to gain resistance against microbial attacks (Vance et al. 1980; Li et al. 2009).
The enzymes that participate in the degradation of lignin are laccases and
peroxidases (lignin peroxidase and manganese peroxidase) (Kuhad et al. 1997;
Plácido and Capareda 2015; Sindhu et al. 2016).
1. Laccases (EC 1.10.3.2) are extracellular glycoproteins that contain copper
(Mayer and Staples 2002; Alcalde 2007). Due to their low substrate specificities,
they enable the deterioration of phenolic structured compounds (Zouari-Mechichi
et al. 2006). The potential of the laccases to degrade lignocelluloses depends on
the phenolic compounds (e.g., 3-hydroxyanthranilic acid; 2,2
0 -azino-bis(3 ethylbenzothiazoline-6-sulfonic acid; vanilin)) that function as redox mediators (Camarero et al. 2004). When the mediators are absent, the activity of the
laccases is limited (Saloheimo et al. 2002; Fillat and Roncero 2009; Plácido and
Capareda 2015). Laccase enzyme does not require additional compounds such as
manganese or hydrogen peroxide for its activity; therefore, it is the most interesting among the environmental applications. Moreover, in hypersaline environments, they synchronously carry out actions such as degradation and color
removal of lignin (Molitoris et al. 2000).
Various microorganisms catalyze the enzymatic break down of lignin are
shown in Fig. 9.5 (Althuri et al. 2017; Siroosi et al. 2018).
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T. Karaytuğ et al.
of the mannan backbone chain by releasing the short ß-1,4 mannooligosaccharides
(Shallom and Shoham 2003), and they are located in the GH families of 5, 26, and
113 (Yeoman et al. 2010).
On the other hand, exo-acting mannosidases are hydrolases that release the
mannose away from oligosaccharides by attacking its terminal links on the
non-reducing ends in addition to transforming mannobiose into mannose units that
are included in the GH family 1, 2, and 5 (Moreira 2008; Van Zyl et al. 2010;
Yeoman et al. 2010).
With the help of ß-mannases, ß-glucosidases release the 1,4 glucopyranose units
at the non-reducing end of the oligomers that are formed as a result of the degradation of glucomannan and galactomannan, and they are in GH families of 1 and
3 (Mccleary and Matheson 1987; Bhatia et al. 2002; Cairns and Esen 2010).
Moreover, to remove the side groups from galactomannan, glucomannan, and
galactoglucomannan, auxiliary enzymes are required, specifically α-galactosidase
and acetyl mannan esterase (Tenkanen et al. 1995; Ganter et al. 2001; Moreira 2008).
9.3.4 Lignin and Ligninolytic Enzymes
Lignin, which is the most abundant source of raw material in nature after cellulose
and consists of phenylpropanoid units linked through covalent bonds, is the main
structural component of the plant cell wall (Li et al. 2009; Sriharti et al. 2017). In
addition to having an important mechanical role for the plants, it also helps the plants
to gain resistance against microbial attacks (Vance et al. 1980; Li et al. 2009).
The enzymes that participate in the degradation of lignin are laccases and
peroxidases (lignin peroxidase and manganese peroxidase) (Kuhad et al. 1997;
Plácido and Capareda 2015; Sindhu et al. 2016).
1. Laccases (EC 1.10.3.2) are extracellular glycoproteins that contain copper
(Mayer and Staples 2002; Alcalde 2007). Due to their low substrate specificities,
they enable the deterioration of phenolic structured compounds (Zouari-Mechichi
et al. 2006). The potential of the laccases to degrade lignocelluloses depends on
the phenolic compounds (e.g., 3-hydroxyanthranilic acid; 2,2
0 -azino-bis(3 ethylbenzothiazoline-6-sulfonic acid; vanilin)) that function as redox mediators (Camarero et al. 2004). When the mediators are absent, the activity of the
laccases is limited (Saloheimo et al. 2002; Fillat and Roncero 2009; Plácido and
Capareda 2015). Laccase enzyme does not require additional compounds such as
manganese or hydrogen peroxide for its activity; therefore, it is the most interesting among the environmental applications. Moreover, in hypersaline environments, they synchronously carry out actions such as degradation and color
removal of lignin (Molitoris et al. 2000).
Various microorganisms catalyze the enzymatic break down of lignin are
shown in Fig. 9.5 (Althuri et al. 2017; Siroosi et al. 2018).
274
T. Karaytuğ et al.