**4. Enzymology of cellulose degradation**

Cellulases are classified as hydrolases, i.e., they add water molecules to cleave glycosidic bonds. Cellulases purified from different microorganisms found to poses

**65**

**Figure 9.**

*Mechanism of action of cellulases [54].*

*Overview of the Process of Enzymatic Transformation of Biomass*

successive breaks in a polysaccharide chain [23].

3. β-Glucosidases finally breaks cellobiose to glucose.

i.Endo-exo: among exo-glucanases and endo-glucanases.

iv.Intramolecular synergy between catalytic domains and CBHs.

identified [56]:

and nonreducing ends).

different molecular characteristics including molecular weight, amino acid composition, isoelectric point) absorbability for cellulose, catalytic activity and substrate

1.Endo-β-1,4-glucanases (Cx) attacks soluble cellulose derivative in a random fashion forming nonreducing ends, producing new chain ends to be attacked by exoglucanases. These enzymes may be processive or nonprocessive. In processive enzymes, enzyme-substrate complex formation is followed by several

2.Exo-β-1,4-glucanases (C1) (avicelase) attack the reducing or nonreducing end of the cellulose polymer. Processive exo-β-1,4-glucanases are named as cellobiohydrolases. The end product of exo-glucanase hydrolysis are cellobiose and glucose units,

These enzymes act synergistically (**Figure 9**) [54]. An endo-acting enzyme generates new reducing and nonreducing ends. Exo-acting enzyme releases cellobiose from ends produced by endo-enzymes acting which is finally hydrolyzed by β-glucosidases to glucose [55]. Mainly four types of synergism have been

ii.Exo-exo: among exo-glucanases those processing from different ends (reducing

iii.Synergy between exo-glucanases and β-glucosidases that removes cellobiose.

In general cellulases comprise of two distinct domains, i.e., Small cellulose-binding module (CBM) which is noncatalytic, Large domain having catalytic characteristics

specificity [53]. Three chief classes of cellulases recognized to date are:

*DOI: http://dx.doi.org/10.5772/intechopen.85036*

*Overview of the Process of Enzymatic Transformation of Biomass DOI: http://dx.doi.org/10.5772/intechopen.85036*

*Elements of Bioeconomy*

**Figure 7.**

**Figure 8.**

*Cellulose hydrolysis in acidic media [47].*

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delignification of lignocellulosic biomass. But it also removes some acetyl and uronic acid substitutions on hemicelluloses, which expose the biomass for enzymatic hydrolysis of cellulose and hemicelluloses [49]. A major limitation of alkaline pretreatments is formation of some salts which are either irrecoverable or incorporated as salts into the biomass [50]. Reactor costs for alkali pretreatment are lower than those for acid pretreatments [51]. For a given quantity of biomass, lowest operating cost is for lime pretreatment [39]. However the use of more pricey salts at higher concentrations is the major drawback that

Cellulases are classified as hydrolases, i.e., they add water molecules to cleave glycosidic bonds. Cellulases purified from different microorganisms found to poses

poses environmental threats and may also hinder the recycling process [52].

**4. Enzymology of cellulose degradation**

*Ether bond cleavage in alkaline solution [48].*

different molecular characteristics including molecular weight, amino acid composition, isoelectric point) absorbability for cellulose, catalytic activity and substrate specificity [53]. Three chief classes of cellulases recognized to date are:


These enzymes act synergistically (**Figure 9**) [54]. An endo-acting enzyme generates new reducing and nonreducing ends. Exo-acting enzyme releases cellobiose from ends produced by endo-enzymes acting which is finally hydrolyzed by β-glucosidases to glucose [55]. Mainly four types of synergism have been identified [56]:


In general cellulases comprise of two distinct domains, i.e., Small cellulose-binding module (CBM) which is noncatalytic, Large domain having catalytic characteristics

**Figure 9.** *Mechanism of action of cellulases [54].*

**Figure 10.** *Cellulases [59].*

(CD). Both the domains are found to be connected by a linker region (**Figure 10**) [57]. Till date, about 300 different CBMs have already been identified. CBMs are categorized into 45 families on the basis of their amino acid similarity [58]. This variation in affinity may be due to variation in spatial structure created by the presence of CBMs [60].
