9.3.1 Cellulose and Cellulolytic Enzymes
Cellulose (C 6 H 10 O 5 ) n is a polymer that constitutes the 40% of the plant cell wall
(Yeoman et al. 2010). It occurs by the way of binding the glucose units with β-1,4
glycosidic bonds, through hydrogen bonding and van der Waals interactions (Horn
et al. 2012; Chatterjee et al. 2015).
While the glucose units are synthesized as a chain in nature, in the biosynthesis
area they merge on their own and turn into elementary fibril units consisting of
approximately 30 cellulose chains. When these are packaged in larger units, they
form microfibrils that create cellulose fibrils (Lynd et al. 2002). Hydrogen bonds
bind the chains together with in-chain and interchain bonds and provide a hard
structure (Kannam et al. 2017).
Unlike the other polysaccharides, celluloses can be created in crystal forms.
Crystal cellulose molecules in nature are in Iα and Iβ forms. Iα form is thermodynamically more stable and mostly found in terrestrial plants. Crystal form can change
over time, and it can turn into amorphous forms (Horn et al. 2012; Sorieul et al.
2016).
Due to microfibril schemes, this crystal form creates a packing method which
does not allow the small molecules such as enzyme or water to get in, and thus, it
limits the activity of the enzymatic hydrolysis (Cosgrove 2005; Yeoman et al. 2010).
Cellulases, in the glycoside hydrolase family, are enzymes that can hydrolyze the
crystal structure of cellulose into small oligosaccharides and then into glucose
(Carrillo-Reyes et al. 2016) and have a broad substrate specificity (Yeoman et al.
2010). These enzymes are produced by microorganisms (bacteria, fungi, archaea),
plants, and animals (except mammals) (Zhang and Zhang 2013). They catalyze the
hydrolysis of the β-1,4 bonds in cellulose via two catalytic mechanisms: retaining
and the inverting mechanisms (Davies and Henrissat 1995; Mosier et al. 1999;
Sindhu et al. 2016). Moreover, they have a carbohydrate-binding module (CBM)
which binds to the catalytic area with a flexible binder. This module takes part in
binding the enzyme to the crystal cellulose and increasing the cellulase activity
(Hervé et al. 2010; Reyes-Ortiz et al. 2013).
There are three main enzyme groups for effective hydrolysis of the cellulose
(Elleuche et al. 2015; Carrillo-Reyes et al. 2016).
9.3.1.1 Endoglucanases (Endo-1,4-β-Glucanes or
1,4-β-D-Glucan-4-Glucanohydrolases, EC 3.2.1.4)
These types of enzymes randomly separate the β-1,4 glycosidic bonds in the
amorphous regions of the cellulose, which causes a rapid decrease in the polymer
length and thus the occurrence of oligosaccharides in different lengths. Moreover,
they cause a gradual increase in the number of released reducing ends (Sun and
Cheng 2002). Some microorganisms synthesizing endoglucanase enzymes are
shown in Fig. 9.5 (Bauer et al. 1999; Li et al. 2003; Yang et al. 2010).
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267
Cellulose (C 6 H 10 O 5 ) n is a polymer that constitutes the 40% of the plant cell wall
(Yeoman et al. 2010). It occurs by the way of binding the glucose units with β-1,4
glycosidic bonds, through hydrogen bonding and van der Waals interactions (Horn
et al. 2012; Chatterjee et al. 2015).
While the glucose units are synthesized as a chain in nature, in the biosynthesis
area they merge on their own and turn into elementary fibril units consisting of
approximately 30 cellulose chains. When these are packaged in larger units, they
form microfibrils that create cellulose fibrils (Lynd et al. 2002). Hydrogen bonds
bind the chains together with in-chain and interchain bonds and provide a hard
structure (Kannam et al. 2017).
Unlike the other polysaccharides, celluloses can be created in crystal forms.
Crystal cellulose molecules in nature are in Iα and Iβ forms. Iα form is thermodynamically more stable and mostly found in terrestrial plants. Crystal form can change
over time, and it can turn into amorphous forms (Horn et al. 2012; Sorieul et al.
2016).
Due to microfibril schemes, this crystal form creates a packing method which
does not allow the small molecules such as enzyme or water to get in, and thus, it
limits the activity of the enzymatic hydrolysis (Cosgrove 2005; Yeoman et al. 2010).
Cellulases, in the glycoside hydrolase family, are enzymes that can hydrolyze the
crystal structure of cellulose into small oligosaccharides and then into glucose
(Carrillo-Reyes et al. 2016) and have a broad substrate specificity (Yeoman et al.
2010). These enzymes are produced by microorganisms (bacteria, fungi, archaea),
plants, and animals (except mammals) (Zhang and Zhang 2013). They catalyze the
hydrolysis of the β-1,4 bonds in cellulose via two catalytic mechanisms: retaining
and the inverting mechanisms (Davies and Henrissat 1995; Mosier et al. 1999;
Sindhu et al. 2016). Moreover, they have a carbohydrate-binding module (CBM)
which binds to the catalytic area with a flexible binder. This module takes part in
binding the enzyme to the crystal cellulose and increasing the cellulase activity
(Hervé et al. 2010; Reyes-Ortiz et al. 2013).
There are three main enzyme groups for effective hydrolysis of the cellulose
(Elleuche et al. 2015; Carrillo-Reyes et al. 2016).
9.3.1.1 Endoglucanases (Endo-1,4-β-Glucanes or
1,4-β-D-Glucan-4-Glucanohydrolases, EC 3.2.1.4)
These types of enzymes randomly separate the β-1,4 glycosidic bonds in the
amorphous regions of the cellulose, which causes a rapid decrease in the polymer
length and thus the occurrence of oligosaccharides in different lengths. Moreover,
they cause a gradual increase in the number of released reducing ends (Sun and
Cheng 2002). Some microorganisms synthesizing endoglucanase enzymes are
shown in Fig. 9.5 (Bauer et al. 1999; Li et al. 2003; Yang et al. 2010).
9 Microbial and Bioinformatics Approach in Biofuel Production
267