Xylan structure, microbial xylanases, and their mode of action

Abstract

Xylans, the major portion of the hemicellulose of plant cell walls and grasses, are heteropolymers consisting principally of xylose and arabinose. Microbial xylanases with different multiplicities and properties are reported. Most studies on the mode of action of these xylanases have been carried out with fungi and there is very little information available on bacterial xylanases. Fungal xylanases have three or more substrate binding sites: for exampleAspergillus niger, Ceratocytis paradoxa, Cryptococcus albidus andChainia sp. endoxylanases have four to seven subsites with the catalytic site located at the centre of these sub-sites. The analysis of these sub-sites is either by kinetic or end-product analysis studies. Kinetic studies are used for exo-type enzymes while the end-product analysis studies are more convenient for endo-type enzymes. This review covers microbial xylanases with special emphasis on studies of sub-site mapping. The industrial applications of the microbial xylanases are also discussed.

Efficient Degradation of Lignocellulosic Plant Biomass, without Pretreatment, by the Thermophilic Anaerobe “ Anaerocellum thermophilum ” DSM 6725

“…Like C. thermocellum (34,40), A. thermophilum DSM 6725 generated high concentrations of glucose and cellobiose from cellulose, and similarly, xylan was converted mainly to xylobiose and xylose. These products are typical for cellulose and xylan hydrolysis by many other microorganisms, although the ratios may differ (2,6,40).…”

Xylanase and β-Xylosidase from Penicillium janczewskii: Production, Physico-chemical Properties, and Application of the Crude Extract to Pulp Biobleaching

“…The ion Hg 2+ completely inhibited xylanase but only moderately inhibited β-xylosidase. The inhibition by Hg 2+ seems to be a general property of xylanases, suggesting the presence of thiol groups of cysteine next to or in the active site of the enzyme (Bastawde 1992), but occurring only for some β-xylosidases (Rizzatti et al 2001;Guerfali et al 2008) and not for others (Deleyn and Claeyssens 1977;Knob and Carmona 2009b In addition, β-mercaptoethanol and DTT-stimulated xylanase activity can be explained by the prevention of oxidation of the -SH groups in the presence of these agents, or by a reduction of S-S bridges, restoring the native conformation of the enzyme or some specific region of the catalytic site. The inhibition of both enzymes verified with SDS indicates the importance of hydrophobic interactions for the three-dimensional structures of these enzymes.…”

“…Endo-β-1,4-xylanase (4-β-D-xylan xylanohydrolase, EC 3.2.1.8) is the main enzyme responsible for xylan depolymerization, breaking the principal chain and liberating oligosaccharides, while β-xylosidase (4-β-Dxylan xylohydrolase, EC 3.2.1.37) attacks xylobiose and other xylooligasaccharides, liberating smaller xylooligasaccharides and D-xylose. Other enzymes responsible for removing xylan substituents such as α-L-arabinofuranosidase, α-glucuronidase, acetyl xylan, feruloyl, and p-coumaroyl esterases have been referred to as auxiliary or accessory xylanolytic enzymes (Biely 1985;Bastawde 1992;Kulkarni et al 1999;Polizeli et al 2005). suspended in sterile distilled water.…”

Isolation and identification of xylanase producing fungal isolate

“…Xylanases show great potential for industrial applications in the bioconversion of lignocelluloses to sugar, ethanol, and other useful substances, clarification of juices and wines, improving the nutritional quality of silage and green feed and the de-inking processes of waste papers (Viikari et al, 2001). Interest in xylanolytic enzymes has increased due to their potential use in several industrial processes such as biopulping and bleaching; bioconversion of lignocellulose; food processing, including clarification of beer, wine, and juice; increasing digestibility of animal feedstock; and bread making (Bastawade 1992: Vikari et al 1994,Uffen 1997). …”

“…Xylan is a complex heteropolysaccharide having a highly branched chain of 1, 4-linked xylanopyranosyl residues (Bastawade 1992). Xylanases (EC 3.2.1.8) catalyze the hydrolysis of xylan, the major constituent of hemicelluloses found in plant cell wall.…”