7
in the hydrolysis of native cellulose, namely, endoglucanases (EG), exoglucanases/
cellobiohydrolases (CBH), and β-glucosidase (BGL) (Schulein 1998). In cellulaseproducing organisms, there are multiple enzymes under each of these classifications, which act synergistically to break down cellulose. The classical model for
cellulase hydrolysis emerged from the work done on Trichoderma reesei in the late
1990s by several groups as reviewed in Payne et al. (2015). In this model, EGs
(Cel7B in Trichoderma reesei) attack the amorphous regions on the surface of cellulose microfibril revealing new reducing and nonreducing ends in the cellulose
chain, which then serve as sites for attack by exoglucanases. The exoglucanases can
also attack the available free ends of the cellulose chains. In T. reesei, the exoglucanase that attacks the reducing end of the cellulose chain is cellobiohydrolase I
(CBHI/Cel7A) and that which attacks the non-reducing end is cellobiohydrolase II
(CBHII/Cel6A). The cellobiose released by exoglucanases is cleaved to glucose
units by the final enzyme in the cascade – beta glucosidase. The role of each cellulase and its synergism is described below.
1.2.1.2 Endoglucanase
Endoglucanases or endo 1,4-β-D-glucan glucanohydrolases (EC 3.2.1.4) are
enzymes which randomly act on the cellulose polymer producing nicks in the amorphous regions of cellulose (endo-initiating) to expose the reducing and nonreducing
ends by cleaving the endo β-1,4 linkages between adjacent glucose units. They are
generally measured by detecting the reducing groups released from the soluble carboxymethylcellulose substrate (Sheehan and Himmel 1999). Endoglucanases are
classically considered as the initiators of cellulose hydrolysis by the cellulase complex, since their action is essential for exposing the reducing and nonreducing ends
in the cellulose polymer, essential for the action of exoglucanases/cellobiohydrolases. Recent view on endoglucanases also proposes its role to help cellobiohydrolases to overcome blockage at amorphous regions of cellulose (Payne et al. 2015).
Endoglucanases are represented in several glycosyl hydrolase (GH) families, and in
the model organism T. reesei there are six endoglucanases represented in families
GH5, GH7, GH12, GH45, and GH74 (Kubicek 2012).
1.2.1.3 Exoglucanases
Exoglucanases or exocellulases are of two types, namely, cellulose 1,4-β-Dcellobiosidase (reducing end) EC 3.2.1.176 (cellobiohydrolase I/CBHI) and cellulose 1,4-β-D-cellobiosidase (nonreducing end) EC 3.2.1.91 (cellobiohydrolase II/
CBHII). These enzymes attack the available reducing or nonreducing free ends or
the ends generated by the action of EGs to liberate cellobiose units. While CBHI
attacks the reducing ends of the chain, CBHII attacks the nonreducing ends (Cantarel
et al. 2009). The current view on exoglucanases does not consider them as having
exclusive exoglucanase action, but as exoglucanases with endo-initiating action
(Kurasin and Valjamae 2011). In the model organism T. reesei, cellobiohydrolases
are represented in glycosyl hydrolases families GH6 and GH7.
1 Enzymes for Bioenergy
in the hydrolysis of native cellulose, namely, endoglucanases (EG), exoglucanases/
cellobiohydrolases (CBH), and β-glucosidase (BGL) (Schulein 1998). In cellulaseproducing organisms, there are multiple enzymes under each of these classifications, which act synergistically to break down cellulose. The classical model for
cellulase hydrolysis emerged from the work done on Trichoderma reesei in the late
1990s by several groups as reviewed in Payne et al. (2015). In this model, EGs
(Cel7B in Trichoderma reesei) attack the amorphous regions on the surface of cellulose microfibril revealing new reducing and nonreducing ends in the cellulose
chain, which then serve as sites for attack by exoglucanases. The exoglucanases can
also attack the available free ends of the cellulose chains. In T. reesei, the exoglucanase that attacks the reducing end of the cellulose chain is cellobiohydrolase I
(CBHI/Cel7A) and that which attacks the non-reducing end is cellobiohydrolase II
(CBHII/Cel6A). The cellobiose released by exoglucanases is cleaved to glucose
units by the final enzyme in the cascade – beta glucosidase. The role of each cellulase and its synergism is described below.
1.2.1.2 Endoglucanase
Endoglucanases or endo 1,4-β-D-glucan glucanohydrolases (EC 3.2.1.4) are
enzymes which randomly act on the cellulose polymer producing nicks in the amorphous regions of cellulose (endo-initiating) to expose the reducing and nonreducing
ends by cleaving the endo β-1,4 linkages between adjacent glucose units. They are
generally measured by detecting the reducing groups released from the soluble carboxymethylcellulose substrate (Sheehan and Himmel 1999). Endoglucanases are
classically considered as the initiators of cellulose hydrolysis by the cellulase complex, since their action is essential for exposing the reducing and nonreducing ends
in the cellulose polymer, essential for the action of exoglucanases/cellobiohydrolases. Recent view on endoglucanases also proposes its role to help cellobiohydrolases to overcome blockage at amorphous regions of cellulose (Payne et al. 2015).
Endoglucanases are represented in several glycosyl hydrolase (GH) families, and in
the model organism T. reesei there are six endoglucanases represented in families
GH5, GH7, GH12, GH45, and GH74 (Kubicek 2012).
1.2.1.3 Exoglucanases
Exoglucanases or exocellulases are of two types, namely, cellulose 1,4-β-Dcellobiosidase (reducing end) EC 3.2.1.176 (cellobiohydrolase I/CBHI) and cellulose 1,4-β-D-cellobiosidase (nonreducing end) EC 3.2.1.91 (cellobiohydrolase II/
CBHII). These enzymes attack the available reducing or nonreducing free ends or
the ends generated by the action of EGs to liberate cellobiose units. While CBHI
attacks the reducing ends of the chain, CBHII attacks the nonreducing ends (Cantarel
et al. 2009). The current view on exoglucanases does not consider them as having
exclusive exoglucanase action, but as exoglucanases with endo-initiating action
(Kurasin and Valjamae 2011). In the model organism T. reesei, cellobiohydrolases
are represented in glycosyl hydrolases families GH6 and GH7.
1 Enzymes for Bioenergy