£]-1,4 Xylans are heteropolysaccharides in plant biomass that has a backbone of £]-1,4 linked xylopyranosyl residues.This backbone may carry O-acetyl, £\-D-glucopyranosyluronic acid, and 4-O-methyl-£\-D-glucipyranosyluronic acid substituents. The abundance of this type of polysaccharide in biomass is illustrated by birchwood, which can be composed of 35% O-acetyl-4-O-methylglucurono-xylan. Other xylose-containigng polysaccharides in biomass include £]-1,3 xylans, £]-(1,4;1,3)-xylans (such as rhodymenan), and xyloglucans.

The degradation of £]-1,4xylans involves xylanolytic systems that include £]-1,4 xylanases (1,4-£]-D-xylan xylanohydrolase; EC 3.2.1.8) and £]-xylosidases (1,4-£]-D-xylan xylohydrolase; EC 3.2.1.37);the former are generally considered to be those enzymes that hydrolyze the xylan backbone, whereas the latter to be those that hydrolyze xylo-oligomers. Trichoderma spp. are known to produce enzymes with high xylanolytic activity.Different xylanases and various components of their xylanolytic system have been identified and purified. Some of the xylanases have been characterized extensively with respect to their physicochemical, hydrolytic, and molecular properties. The implication of xylanases in biological processes is not surprising when one considers the abundance of their substrates in nature. These enzymes have generally been examined from the perspective of biomass turnover and the microorganisms involved, whether they be saprophytes or pathogens. However, recent work has suggested another biological role for microbial xylanases. Xylanases from T. viride were found to be able to induced, in tobacco, the biosynthesis of ethylene and two of there classes of pathogenesis-related proteins. It therefore appears that certain xylanases can elicit defense mechanisms in plants. This action may be mediated by specific signal oligosaccharides, collectively known as oligosaccharins, which were apparently not produced by all xylanolytic enzymes from T. viride. However, the possibility that the enzyme themselves, or their fragments, serve as the elicitors has not been ruled out.

Most studies of xylanases have considered their biochemistry and biotechnology. Presently, there is great interest concerning the application of cellulase-free xylanases for the prebleaching of kraft pulps. Xylanases could soon become a major industrial enzyme, handled in the high volumes required by the pulp and paper industry. This prospect has led researchers to consider the allergenic properties of these enzymes. More importantly, it has stimulated research activities directed at understanding and manipulating the actions of xylanases. The objective of this review is to discuss the properties of xylanases characterized in Trichoderma spp. with respect to their role in substrate hydrolysis and their potential for use in proposed applications.--------------------Ken K.Y. Wong and John N.Saddler , "Trichoderma Xylanases, Their properties and Application", Critical Review in Biotechnology,12(5/6):413-435(1992)

The three-dimensional structure of endo-1,4-£]-xylanase II (XYNII) from Trichoderma ressei has been determined by X-ray diffraction techniques and refined to a conventional R-factor if 18.3% at 1.8 Angstroms resolution. The 190 amino acid length protein was found to exist as a single domain where the main chain folds to form two mostly antiparallel £]-sheets, which are packed against each other in parallel. The £]-sheet structure is twisted, forming a large cleft on one side of the molecule. The structure of XYNII resembles that of Bacillus 1,3-1,4-£]-glucanase. The cleft is an obvious suggestion for an active site, which has putative binding sites for at least four xylose residues. The catalytic residues are apparently the two glutamic acid residues (Glu86 and Glu177) in the middle of the cleft. One structure was determined at pH5.0, corresponding to the pH optimum of XYNII. The second structure was determined at pH6.5, where enzyme activity is reduced considerably. A clear structural change was observed, especially in the position of the side chain of Glu177. The observed conformational change is probably important for the mechanism of catalysis in XYNII. -------Anneli T&oum1; rr&oum1; nen, Anu Harkki and Juha Rouvinen,Three-dimensional structure of endo-1,4-£]-xylanase II from Trichoderma ressei: two conformational states in the active site, The EMBO Journal,2493, 13, 1994.

Fig1. Ribbon representation of the XYNII molecule showing an alpha-helix and beta-strands. The structure is reminiscent of the shape of a 'right hand'. This protein has approximated dimensions of 32*34*42 Angstroms.

Fig 2. The active site cleft in pH 5.0.

Fig 3. The active site cleft in pH 6.5.

The active site cleft has a length and depth of around 25 and 9 Angstroms,respectively. The average width of the cleft is around 4 Angstroms. But there are two regions where the width is smaller: one between Trp 18 and Pro 126 in the middle of the cleft, and the other between Tyr96 and Tyr 170 at the end of the cleft. When the pH changes from 5.0 (optimum) to 6.5 (activity-reduced), the largest change was observed for Glu177, where the torsion angle between C-£] and C-£^ atoms was changed around 100 degree (from -89 to 171 degree). This places the carboxyl part of the side chain in a different position, the displacement being 2.8 angstroms calculated from the position of the C-£_atom, and this change alters the hydrogen bonding pattern. There are other conformational changes in other residues such as Tyr 73, and Tyr 88, but I do not describe in detail here .If you want to know more detail information, you can check the paper3 in references.


Reference:
1. Ken K. Y. Wang and John N. Saddler,Trichoderma xylanase, their
properties and application, Critical Reviews in Biotechnology, 12(5/6): 413, 435, 1992.
2. Jennifer A. Thomson, Molecular biology of xylan degradation,
FEMS Microbiology Review, 104, 65, 82, 1993.
3. Anneli Torronen, Anu Harkki and Juha Rouvinen, Three-dimensional
structure of endo-1,4-£]-xylanase II from Trichoderma reesi: two
conformational states in the active site, The EMBO Journal, 13(11), 2493,
2501, 1994

PDB Result