hydrolyze internal 1,4-glucosidic linkages within the cellulosic chain of cellulose,
leading to formation of gluco-oligosaccharides. Large numbers of reducing as well
as nonreducing ends are formed in the oligosaccharides; (2) Exoglucanases (Exo1,4-β-D-glucan-4-glucanohydrolase) cleave only external β-1,4-glucosidic bonds
from nonreducing terminal of cellulose and also oligosaccharides and tear off
glucose units; (3) Cellobiohydrolases (Exo-1,4-β-D-glucan-4-cellobiohydrolase EC
3.2.1.91) generate cellobiose units from nonreducing terminal of cellulose. The
distinction between exoglucanases and cellobiohydrolases is always not clear, and
(4) β-glucosidases (β-D-glucoside glucohydrolase EC 3.2.1.21) aids in breakdown
of cellobiose and dextrins of low molecular weight to glucose. Organisms producing
cellulases differ from each other in possessing different number of components of
cellulases, as there are multiple enzyme components present in cellulases. There are
two types of cellobiohydrolases, which are immunologically different from each
other, these are CBH I and CBH II. Trichoderma spp., particularly T. reesei, is the
known producer of these cellobiohydrolases and is known to secrete them in
extracellular medium (Kubicek and Pentill 1998). Both of these cellobiohydrolases
work in synergistic or cooperative way. Reducing and nonreducing ends are
generated when endoglucanase works upon linear cellulose molecules. These ends
are targeted by exoglucanase, which highlights more inner sites for endoglucanase
binding. Cellobiose causes feedback inhibition of CBH. If somehow this cellobiose
is converted into glucose with the help of β-glucosidase, this feedback inhibition is
greatly reduced, and cellulolytic activity continues (Wood et al. 1995). Even synergistic interactions (exo–exo synergism) between isozymic forms of exoglucanase
(CBH I as well as CBH II) occurred in solubilizing crystalline cellulose (Henrissat et
al.1985).
3.4
Enzymatic Hydrolysis of Pretreated Biomass
The cellulose and hemicelluloses that are released once the pretreatment is over are
required to be hydrolyzed to their monomeric constituents’ sugars, viz., hexoses and
pentoses. This step is mediated with the intervention of various enzymes like
cellulases and hemicellulases. Cellulases lead to breakdown of cellulose, and
hemicellulases do the same with hemicelluloses. The preliminary transformation
of lignocellulose to sugars is a main congestion in the procedure of biofuel generation, and novel biotechnological unfolding is required to elevate their effectiveness,
which would reduce the comprehensive rate of bioethanol manufacture (Bhatia et al.
2012). When we use enzymes, we do not have to face corrosion problems. Maintenance costs are lower and conditions of processing are mild, which together give
higher output. If the hydrolysis is carried out enzymatically, this would make the
bioethanol production economically feasible in a long run. There are many hurdles
in biological translation of lignocellulosic residues to ethanol. Most prominent is the
absence of most of the enzymes or their insufficient production that are required to
completely degrade cellulose and hemicelluloses. Remarkably, thermostability of
these enzymes is another barrier toward effective biotransformation of
70
L. Bhatia et al.
leading to formation of gluco-oligosaccharides. Large numbers of reducing as well
as nonreducing ends are formed in the oligosaccharides; (2) Exoglucanases (Exo1,4-β-D-glucan-4-glucanohydrolase) cleave only external β-1,4-glucosidic bonds
from nonreducing terminal of cellulose and also oligosaccharides and tear off
glucose units; (3) Cellobiohydrolases (Exo-1,4-β-D-glucan-4-cellobiohydrolase EC
3.2.1.91) generate cellobiose units from nonreducing terminal of cellulose. The
distinction between exoglucanases and cellobiohydrolases is always not clear, and
(4) β-glucosidases (β-D-glucoside glucohydrolase EC 3.2.1.21) aids in breakdown
of cellobiose and dextrins of low molecular weight to glucose. Organisms producing
cellulases differ from each other in possessing different number of components of
cellulases, as there are multiple enzyme components present in cellulases. There are
two types of cellobiohydrolases, which are immunologically different from each
other, these are CBH I and CBH II. Trichoderma spp., particularly T. reesei, is the
known producer of these cellobiohydrolases and is known to secrete them in
extracellular medium (Kubicek and Pentill 1998). Both of these cellobiohydrolases
work in synergistic or cooperative way. Reducing and nonreducing ends are
generated when endoglucanase works upon linear cellulose molecules. These ends
are targeted by exoglucanase, which highlights more inner sites for endoglucanase
binding. Cellobiose causes feedback inhibition of CBH. If somehow this cellobiose
is converted into glucose with the help of β-glucosidase, this feedback inhibition is
greatly reduced, and cellulolytic activity continues (Wood et al. 1995). Even synergistic interactions (exo–exo synergism) between isozymic forms of exoglucanase
(CBH I as well as CBH II) occurred in solubilizing crystalline cellulose (Henrissat et
al.1985).
3.4
Enzymatic Hydrolysis of Pretreated Biomass
The cellulose and hemicelluloses that are released once the pretreatment is over are
required to be hydrolyzed to their monomeric constituents’ sugars, viz., hexoses and
pentoses. This step is mediated with the intervention of various enzymes like
cellulases and hemicellulases. Cellulases lead to breakdown of cellulose, and
hemicellulases do the same with hemicelluloses. The preliminary transformation
of lignocellulose to sugars is a main congestion in the procedure of biofuel generation, and novel biotechnological unfolding is required to elevate their effectiveness,
which would reduce the comprehensive rate of bioethanol manufacture (Bhatia et al.
2012). When we use enzymes, we do not have to face corrosion problems. Maintenance costs are lower and conditions of processing are mild, which together give
higher output. If the hydrolysis is carried out enzymatically, this would make the
bioethanol production economically feasible in a long run. There are many hurdles
in biological translation of lignocellulosic residues to ethanol. Most prominent is the
absence of most of the enzymes or their insufficient production that are required to
completely degrade cellulose and hemicelluloses. Remarkably, thermostability of
these enzymes is another barrier toward effective biotransformation of
70
L. Bhatia et al.