Xylanases: Characteristics, Sources, Production, and Applications

Cellulose is the most abundant renewable biological resource and a low-cost energy source based on energy content ($3–4/GJ) ( Lynd et al., 2008 ; Zhang, 2009 ). The production of bio-based products and bioenergy from less costly renewable lignocellulosic materials would bring benefi ts to the local economy, environment, and national energy security ( Zhang, 2008 ). High costs of cellulases are one of the largest obstacles for commercialization of biomass biorefi neries because a large amount of cellulase is consumed for biomass saccharifi cation, for example, ∼ 100 g enzymes per gallon of cellulosic ethanol produced ( Zhang et al., 2006b ; Zhu et al., 2009 ). In order to decrease cellulase use, increase volumetric productivity, and reduce capital investment, consolidated bioprocessing ( CBP ) has been proposed by integrating cellulase production, cellulose hydrolysis, and ethanol fermentation in a single step ( Lynd et al., 2002, 2008 ). Cellulases are the enzymes that hydrolyze β -1,4 linkages in cellulose chains. They are produced by fungi, bacteria, protozoans, plants, and animals. The catalytic modules of cellulases have been classifi ed into numerous families based on their amino acid sequences and crystal structures ( Henrissat, 1991 ). Cellulases contain noncatalytic carbohydrate-binding module s ( CBM s) and/or other functionally known or unknown modules, which may be located at the Nor C-terminus of a catalytic module. In nature, complete cellulose hydrolysis is mediated by a combination of three main types of cellulases: (1) endoglucanases (EC 3.2.1.4), (2) exoglucanases, including cellobiohydrolases (CBHs) (EC 3.2.1.91), and (3) β -glucosidase (BG) (EC 3.2.1.21). To hydrolyze and metabolize insoluble cellulose, the microorganisms must secrete the cellulases (possibly except BG) that are either free or cell-surface-bound. Cellulases are increasingly being used for a large variety of industrial purposes—in the textile industry, pulp and paper industry, and food industry, as well as an additive in detergents and improving digestibility of animal feeds. Now cellulases account for a signifi cant share of the world ’ s industrial enzyme market. The growing concerns about depletion of crude oil and the emissions of greenhouse gases have motivated the production of bioethanol from lignocellulose, especially through enzymatic hydrolysis of lignocelluloses materials—sugar platform ( Bayer et al., 2007 ; Himmel et al., 1999 ; Zaldivar et al., 2001 ). However, costs of cellulase for hydrolysis of pretreated lignocellulosic materials need to be reduced, and their catalytic effi ciency should be further increased in order to make the process economically feasible ( Sheehan and Himmel, 1999 ). Engineering cellulolytic enzymes with improved catalytic effi ciency and enhanced thermostability is important to commercialize lignocelluloses biorefi nery. Individual cellulase can be enhanced by using either rational design or directed evolution. However, improvements in cellulase performance have been incremental, and no drastic activity enhancement has been reported to date. The further improvement on cellulase performance needs the better understanding of cellulose