Xylanase Case Study

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1.1. The occurrence and structure of xylanase In plant cell walls, the major reservoir of fixed carbon in nature, have three major polymeric constituents: cellulose (insoluble fibers of β-1,4-glucan), hemicelluloses (non-cellulosic polysaccharides including glucans, mannans, and xylans), and lignin (a complex polyphenolic structure). After the deposition of a primary wall during elongation, many plant cells (specially in vascular tissues) undergo secondary thickening during differentiation. The β-1,4-xylans are mainly found in secondary cell walls, the major component of mature cell walls in woody tissue. Also it represent the major hemicelluloses in the primary cell wall of monocots (Wong 1993). Hemicelluloses are composed of complex mixtures of polymeric carbohydrates, including xylan, xyloglucan (heteropolymer of D-xylose and D-glucose),

oat spelt), hardwoods and softwoods (Collins et al., 2005) and ( Shallom et al., 2003). Because of the diversity in chemical structure of xylan derived from cereal, woods or other plant materials, it is widely known that for complete hydrolysis of xylan it requires enzymes with different catalytic properties (Weng et al., 2010). Among these hydrolytic enzymes, xylanases and xylosidases play the most important role in depolymerization of the xylan backbone, by the time other enzymes act on the side chain cleavage (Zhang et al., 2011). Based on the amino acid sequence homology, hydrophobic cluster, and three dimensional structural analysis, xylanases are classified mainly into two glycoside hydrolyse (GH) families, 10 and 11, which enzyme activities are also present in GH 5, 7, 8, 16, 26, 43, 52 and 62 (Khandeparker et al.,. 2011and Ruller, 2008) . GH10 comprises enzymes with a number of known activities which are xylanase (EC 3.2.1.8), endo-1,3-beta-xylanase (EC 3.2.1.32) and cellobiohydrolase