degradation of polysaccharides. The dilute acid pretreatment is recommended over
lignocellulosic biomass, whose lignin composition is lower than 15%. Generally,
dilute acid processes do not remove lignin (but do modify its chemical structure,
promoting more accessibility) and remain the cellulose, letting elevated enzymatic
hydrolysis rates as result of amplified accessibility of the exposed cellulose
(Karapatsia et al. 2017; Kumar et al. 2018).
2.12 Enzymatic Hydrolysis
The conversion process of cellulose and hemicelluloses into smaller molecules
(commonly called total reducing sugars or fermentable sugars with 6 and 5 carbons,
respectively) is named hydrolysis or saccharification. This process can be developed
in the presence of acids, enzymes, or both simultaneously (Loow et al. 2016; Sharma
et al. 2017). Acid hydrolysis involves organic or inorganic acids (such as sulfuric,
hydrochloride, oxalic, acetic, and uronic). Enzymatic hydrolysis includes different
type of enzymes acting on cellulose and hemicellulose chains (Loow et al. 2016).
Cellulose hydrolysis occurs by action of cellulases, called cellulolytic enzymes too,
and hemicelluloses hydrolysis occurs by action of hemicellulases, called
hemicellulolytic enzymes too (Bhattacharya et al. 2015).
Cellulases are grouping in three enzymes categories. Endoglycanases,
carboxylmethylcellulases, or CMCases (EC 3.2.1.4) are enzymes that hydrolyze
randomly the non-crystalline sections of cellulose generating new non-reducing
and reducing ends. Exoglycanases, cellobiohydrolases, CBH, or Avicelases
(EC 3.2.1.91) are enzymes that act over ends of cellulose chains and the new
non-reducing and reducing ends to produce cellobiose molecules. And
β-glycosidases (EC 3.2.1.21) are responsible to break cellobiose and thus to release
glucose (Dotsenko et al. 2018) (Fig. 2.3).
According to chemical composition of hemicelluloses, its biological breaking is
completed by several groups of enzymes. Endoxylanases, β-xylosidases, α-Larabinofuranosidases, and β-galactosidases are the hemicellulolytic enzymes most
relevant in hydrolysis process of xylan, the principal polymer that makes up
hemicelluloses. Endoxylanases (EC 3.2.1.8) are enzymes that attack the principal
chain of xylan to generate xylobiose. β-Xylosidases (EC 3.2.1.37) are responsible to
break xylobiose and thus to release xylose. α-Arabinofuranosidase (EC 3.2.1.55) and
β-galactosidase (EC 3.2.1.23) enzymes attack the xylan branches to release arabinose and galactose molecules, respectively (Quiroz-Castañeda and Folch-Mallol
2011).
2.13 Conclusion
Fossil fuels are a finite resource and damage the environment, turning their use an
unsustainable practice if new technologies continue to advance demanding more and
more energy. Sugarcane, corn, wood, algae, and many more biomasses are
56
F. L. Shimizu et al.
lignocellulosic biomass, whose lignin composition is lower than 15%. Generally,
dilute acid processes do not remove lignin (but do modify its chemical structure,
promoting more accessibility) and remain the cellulose, letting elevated enzymatic
hydrolysis rates as result of amplified accessibility of the exposed cellulose
(Karapatsia et al. 2017; Kumar et al. 2018).
2.12 Enzymatic Hydrolysis
The conversion process of cellulose and hemicelluloses into smaller molecules
(commonly called total reducing sugars or fermentable sugars with 6 and 5 carbons,
respectively) is named hydrolysis or saccharification. This process can be developed
in the presence of acids, enzymes, or both simultaneously (Loow et al. 2016; Sharma
et al. 2017). Acid hydrolysis involves organic or inorganic acids (such as sulfuric,
hydrochloride, oxalic, acetic, and uronic). Enzymatic hydrolysis includes different
type of enzymes acting on cellulose and hemicellulose chains (Loow et al. 2016).
Cellulose hydrolysis occurs by action of cellulases, called cellulolytic enzymes too,
and hemicelluloses hydrolysis occurs by action of hemicellulases, called
hemicellulolytic enzymes too (Bhattacharya et al. 2015).
Cellulases are grouping in three enzymes categories. Endoglycanases,
carboxylmethylcellulases, or CMCases (EC 3.2.1.4) are enzymes that hydrolyze
randomly the non-crystalline sections of cellulose generating new non-reducing
and reducing ends. Exoglycanases, cellobiohydrolases, CBH, or Avicelases
(EC 3.2.1.91) are enzymes that act over ends of cellulose chains and the new
non-reducing and reducing ends to produce cellobiose molecules. And
β-glycosidases (EC 3.2.1.21) are responsible to break cellobiose and thus to release
glucose (Dotsenko et al. 2018) (Fig. 2.3).
According to chemical composition of hemicelluloses, its biological breaking is
completed by several groups of enzymes. Endoxylanases, β-xylosidases, α-Larabinofuranosidases, and β-galactosidases are the hemicellulolytic enzymes most
relevant in hydrolysis process of xylan, the principal polymer that makes up
hemicelluloses. Endoxylanases (EC 3.2.1.8) are enzymes that attack the principal
chain of xylan to generate xylobiose. β-Xylosidases (EC 3.2.1.37) are responsible to
break xylobiose and thus to release xylose. α-Arabinofuranosidase (EC 3.2.1.55) and
β-galactosidase (EC 3.2.1.23) enzymes attack the xylan branches to release arabinose and galactose molecules, respectively (Quiroz-Castañeda and Folch-Mallol
2011).
2.13 Conclusion
Fossil fuels are a finite resource and damage the environment, turning their use an
unsustainable practice if new technologies continue to advance demanding more and
more energy. Sugarcane, corn, wood, algae, and many more biomasses are
56
F. L. Shimizu et al.