Thermostable Bacterial Xylanases

Hemicellulose is the second most abundant component in lignocellulosics available in nature. It is a storage polymer occurring in seeds and a prominent structural component of cell walls in plants. Hemicelluloses in agricultural residues constitute up to 40%. Monomers of various hemicelluloses are useful in various biotechnological processes like the production of different antibiotics, alcohols, animal feeds, and biofuels. Xylan is the most abundant of all hemicelluloses. It has a linear backbone of β-1,4-linked D-xylopyranose residues. Immense interest in the enzymatic hydrolysis of xylan has been due to the applications of hydrolysates in feedstocks, production of biochemicals, and paper pulp bleaching. Biodegradation of xylan requires action of several enzymes, among which xylanases play a key role. A wide variety of microorganisms are known to produce xylanases. The interest in thermostable xylanases has markedly increased due to their potential applications in pulping and bleaching processes, in food and feed industry, textile processing, enzymatic saccharification of lignocellulosic materials, and waste treatment. Since elevated temperatures have a significant influence on the bioavailability and solubility of organic compounds, most of these processes are carried out at high temperatures. The elevation in temperature is accompanied by a decrease in viscosity and an increase in the diffusion coefficient of organic compounds, and thus, higher rates of reactions are expected. Thermophilic organisms are of special interest as sources of thermostable xylanases. The development of new analytical techniques and the commercial availability of new matrices have led to the purification and characterization of a large number of xylan-degrading bacterial enzymes. The recombinant DNA technology has permitted selection and overproduction of xylanolytic enzymes that are suitable for industrial applications. The developments in cloning and expression, directed evolution, physicochemical and functional characteristics, and biotechnological applications and commercialization of thermostable xylanases of bacterial origin have been reviewed.