Molecular and biotechnological aspects of xylanases

Abstract

Hemicellulolytic microorganisms play a significant role in nature by recycling hemicellulose, one of the main components of plant polysaccharides. Xylanases (EC 3.2.1.8) 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.

Assessment of the Effect of Easily-metabolised Carbon Supplements on Xylanase Production by Newly Isolated Trichoderma asperellum USM SD4 Cultivated on Oil Palm Empty Fruit Bunches

“…Others are lactose, which is often used in the industries for the production of hydrolytic enzymes, and sophorose, which was reported as the most powerful xylanase inducer (Chaudhuri and Sahai 1994;Xu et al 2000;Xiong et al 2004). Possible explanations for these contrasting observations could be differences in the physiological and metabolic properties of the various organisms used, the type of fermentation (SSF or SMF), or more importantly, the composition of the fermentation media including the carbon sources (soluble carbon or lignocellulosic sources) (Kulkarni et al 1999). Reports which suggested inductive ability of xylose and xylan for xylanase production used only the purified forms of these substrates as sole carbon sources via submerged fermentation.…”

“…Industrially, simple and easily-metabolised carbon compounds such as xylose, xylan, lactose, sophorose, etc., are often used as raw materials for xylanase production (Subramaniyan and Prema 2002) usually by submerged fermentation (SMF); and xylose and xylan were particularly identified in previous studies as unique xylanase inducers (Kulkarni et al 1999). However, these simple sugars do not occur freely in nature but are products of the deconstruction of complex substrates such as cellulose and hemicellulose.…”

Mutagenic Improvement of Xylanase Production from Xylanolytic Bacteria and its Phylogenetic Analysis

“…Under these optimum conditions the enzyme showed maximum activity after 5 d incubation. Kulkarni et al (1999) reported that the optimum temperature for endoxylanase from bacterial and fungal sources varies between 40 and 60 °C. It was also reported by Kulkarni et al (1999) that D-xylanases from different organisms are usually stable over a wide pH range (3-10) and show optimum pH in the range of 4-7, however, certain alkaliphilic Bacilli are known which have pH optima for growth and enzyme production at 9-10.…”

“…The substrate derivatives and the enzymatic end products may often play a key positive role in the induction of xylanases. They can also act as the end-product inhibitors, possibly at much higher concentrations (Kulkarni et al 1999).…”

“…Berdasarkan analisis pohon filogenetik 16S rRNA diketahui bahwa mutan kode 19 termasuk dalam kerabat bakteri anaerob penghasil xilanase. (Kulkarni et al 1999). Two principally important enzymes involved in the recycling of hemicellulosic material are endo-1,4-β-xylanases (EC 3.2.1.8), which cleave the xylan backbone, and β-D-xylosidases (EC 3.2.1.37), which cleave xylose monomers from the nonreducing end of xylo-oligosaccharides ).…”