Microbial cellulases: Engineering, production and applications

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 lignocellulose-based biorefinery.