Depolymerization of cellulose to glucose requires the synergistic action of three key cellulases,
endoglucanase (E.C. 3.2.1.4), exoglucanase (E.C. 3.2.1.176) (E.C. 3.2.1.91) and β-glucosidase (E.C. 3.2.1.21).
They belong to the glycoside hydrolase (GH) family and catalyze the hydrolysis of glyosidic linkages
depolymerizing cellulose to fermentable sugars. Cellulases are naturally produced by a wide spectrum of
bacteria and fungi. These enzymes usually exist as cellulosomes attached to the cell wall of bacteria but
are secreted into environments in fungi. They exist either as monomers or multimers with each
monomer having a simple architecture containing a cellulose binding domain (CBD) and a catalytic
domain (CD) interlinked by a linker peptide. Thermophilic, mesophilic and psychrophilic cellulases are
quite different in their structures and amino acid compositions. Post-translational modifications such as
glycosylation contribute to enzyme function, multiplicity and stability. Recent advances in recombinant
DNA technology allow fast identification of novel cellulase genes, large scale production of cellulases and
their genetic modifications to make tailor-made enzymes for various applications. Cellulases have been
traditionally used in food processing and textile industries. The rapid depletion of fossil fuels and
production of cheap fermentable sugars from abundant renewable resources have increased the demand
for cellulases in lignocellulose-based biorefinery. However, the lack of a better understanding of the
mechanisms of individual cellulases, their synergistic actions and their high prices are the major
bottlenecks yet to be overcome for large scale commercial applications of cellulases in lignocellulosebased
biorefinery.
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