Microbial xylanases and their industrial applications: a review
Despite an increased knowledge of microbial xylanolytic systems in the past few years, further studies are required to achieve a complete understanding of the mechanism of xylan degradation by microorganisms and their enzymes. The enzyme system used by microbes for the metabolism of xylan is the most important tool for investigating the use of the second most abundant polysaccharide (xylan) in nature. Recent studies on microbial xylanolytic systems have generally focussed on induction of enzyme production under different conditions, purification, characterization, molecular cloning and expression, and use of enzyme predominantly for pulp bleaching. Rationale approaches to achieve these goals require a detailed knowledge of the regulatory mechanism governing enzyme production. This review will focus on complex xylan structure and the microbial enzyme complex involved in its complete breakdown, studies on xylanase regulation and production and their potential industrial applications, with special reference to biobleaching.
Molecular and biotechnological aspects of xylanases
Hemicellulolytic microorganisms play a significant role in nature by recycling hemicellulose, one of the main components of plant polysaccharides. Xylanases (EC 126.96.36.199) catalyze the hydrolysis of xylan, the major constituent of hemicellulose. The use of these enzymes could greatly improve the overall economics of processing lignocellulosic materials for the generation of liquid fuels and chemicals. Recently cellulase-free xylanases have received great attention in the development of environmentally friendly technologies in the paper and pulp industry. In microorganisms that produce xylanases low molecular mass fragments of xylan and their positional isomers play a key role in regulating its biosynthesis. Xylanase and cellulase production appear to be regulated separately, although the pleiotropy of mutations, which causes the elimination of both genes, suggests some linkage in the synthesis of the two enzymes. Xylanases are found in a cornucopia of organisms and the genes encoding them have been cloned in homologous and heterologous hosts with the objectives of overproducing the enzyme and altering its properties to suit commercial applications. Sequence analyses of xylanases have revealed distinct catalytic and cellulose binding domains, with a separate non-catalytic domain that has been reported to confer enhanced thermostability in some xylanases. Analyses of three-dimensional structures and the properties of mutants have revealed the involvement of specific tyrosine and tryptophan residues in the substrate binding site and of glutamate and aspartate residues in the catalytic mechanism. Many lines of evidence suggest that xylanases operate via a double displacement mechanism in which the anomeric configuration is retained, although some of the enzymes catalyze single displacement reactions with inversion of configuration. Based on a dendrogram obtained from amino acid sequence similarities the evolutionary relationship between xylanases is assessed. In addition the properties of xylanases from extremophilic organisms have been evaluated in terms of biotechnological applications.
Xylanases, xylanase families and extremophilic xylanases
Xylanases are hydrolytic enzymes which randomly cleave the β 1,4 backbone of the complex plant cell wall polysaccharide xylan. Diverse forms of these enzymes exist, displaying varying folds, mechanisms of action, substrate specificities, hydrolytic activities (yields, rates and products) and physicochemical characteristics. Research has mainly focused on only two of the xylanase containing glycoside hydrolase families, namely families 10 and 11, yet enzymes with xylanase activity belonging to families 5, 7, 8 and 43 have also been identified and studied, albeit to a lesser extent. Driven by industrial demands for enzymes that can operate under process conditions, a number of extremophilic xylanases have been isolated, in particular those from thermophiles, alkaliphiles and acidiphiles, while little attention has been paid to cold-adapted xylanases. Here, the diverse physicochemical and functional characteristics, as well as the folds and mechanisms of action of all six xylanase containing families will be discussed. The adaptation strategies of the extremophilic xylanases isolated to date and the potential industrial applications of these enzymes will also be presented.
Xylanases: An Overview
Endo-1, 4-β-xylanase (Endo-β-1, 4-xylan, xylanohydrolase; EC. 188.8.131.52, commonly called xylanase) is an industrially important enzyme which degrades xylan randomly and produces xylooligosaccharides, xylobiose and xylose. It is mainly present in microbes and plants but not in animals. Xylanases from fungal and bacterial sources have been extensively studied and produced commercially. Its potential use in paper industries has been discussed which is directly related to reduction in environmental pollution. It has role in bio-bleaching paper pulp and increasing pulp brightness. Besides, it can be exploited for ethanol production and as an additive in animal feedstock to improve its nutritional value. Endo-1, 4-β-xylanase can also be exploited in baking and fruit juice industries. Here, we reviewed its distribution, structural aspects and industrial/ biotechnological applications. Besides, we also discussed studies related to cloning of the gene encoding endo-1, 4-β-xylanase with the objectives of overproducing the enzyme and altering its properties to suit commercial applications
Xylanases–from Microbial Origin to Industrial Application
Xylanases are in the focus of research due to their potential to replace many current polluting chemical technologies by biochemical conversion. The field of application for xylanases is vast; it comprises industrial applications like wood pulp bio-bleaching, papermaking and bioethanol production. In addition, these enzymes can be applied as additives in food and beverage industry, and animal nutrition. However, considering the potential applications for these enzymes, the market share of xylanases is still low. Thus, the search for promising xylanases which tolerate relevant processing conditions and therefore can be used in industrial settings is an ongoing task. This review provides an overview of the enzymes reported from 2012 to mid 2014. Further, legal restrictions for the use of (genetically modified) organisms and enzymes are considered. This review provides an integrated perspective on the potential of specific xylanases for industrial applications.
 Beg, Q., Kapoor, M., Mahajan, L. and Hoondal, G.S., 2001. Microbial xylanases and their industrial applications: a review. Applied microbiology and biotechnology, 56(3), pp.326-338.
 Kulkarni, N., Shendye, A. and Rao, M., 1999. Molecular and biotechnological aspects of xylanases. FEMS microbiology reviews, 23(4), pp.411-456.
 Collins, T., Gerday, C. and Feller, G., 2005. Xylanases, xylanase families and extremophilic xylanases. FEMS microbiology reviews, 29(1), pp.3-23.
 Sharma, M. and Kumar, A., 2013. Xylanases: an overview. Biotechnology Journal International, pp.1-28.
 Kalim, B., Böhringer, N., Ali, N. and Schäberle, T.F., 2015. Xylanases–from microbial origin to industrial application. Biotechnology Journal International, pp.1-20.
Microbial xylanases and their industrial applications: a review