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An introduction to the enzyme - Xylanase
Before talking about xylanase lets first talk about xylan. As we know that every enzyme requires a
substrate. So, xylan is the required substrate for xylanase. Now what exactly xylan is and where it is
present ? Xylan is the second most abundant polysaccharide after cellulose and a major component
in plant cell wall. The plant cell wall is a composite material in which cellulose, hemicelluloses
(mainly xylan) and lignin are closely joined together. Organic wastes from renewable forest and
agricultural residues comprise polysaccharides like cellulose, hemicellulose and lignin in the average
ratio of 4:3:3.The exact percentage of these, however, varies from source to source.
Cellulose is the major structural component of plant cell walls and some algae. About 33% of all the
plant material is cellulose. It is a polysaccharide consisting of linear chains of several hundred to
thousand 𝛽( 1→4) linked D-glucose units.
Hemicellulose includes D-xylose, D-mannose, D-galactose and L-arabinose as principal momomers
and xylan, mannan, galactan and arabin as the main heteropolymers (polymer consisting of two or
more monomers). Xylan is a major component of hemicelluloses. Xylans are heteropolymers
consisting principally of xylose and arabinose.
Lignin is a complex compound most commonly derived from wood and is an integral component in
the secondary cell wall of plants and some algae. Its most common function is strengthening of
wood. It fills the spaces between cellulose, hemicelluloses and pectin components. It is covalently
linked to hemicelluloses and thereby crosslinks different plant polysaccharides.Lignin is bound to
xylans by an ester linkage to 4-O-methyl-D-glucuronic acid residues.
The structure of xylans found in cell walls of plants can differ greatly depending on their origin, but
they always contain β-1,4-linked D-xylose backbone. Different structures attached to the xylan
backbone can result in a large variety of xylan structures found in plants. Although most of the
xylans are branched structures, some linear polysaccharides have been isolated. The structures of
xylans are complex, and several enzymes are involved in their breakdown. Xylan degrading enzymes
are produced by a variety of microorganisms.
Xylanase is a hydrolase that catalyses the hydrolysis of complex sugars (primarily xylan and certain
related compounds) to simpler sugars and the primary product being is xylose. Xylanase catalyses
the reaction by breaking the 1,4-β-D-xylosidic linkages in xylan. Its systematic name is 1,4-β-Dxylanohydrolase, recommended name is endo-1,4-β-xylanase. The complete enzymatic hydrolysis
of xylan into its constituent monosaccharides requires the synergistic action of many xylanolytic
enzymes. This is due to the fact that xylans from different sources exhibit a significant variation in
composition and structure.The most important enzyme is endo-1,4-β-xylanase (EC 3.2.1.8), which
initiates the conversion of xylan into xylooligosaccharides. Xylosidase, debranching enzymes (Larabinofuranosidase and glucuronidase) and esterases are also required for the complete hydrolysis
of xylan.
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Research has mainly focused on only two of the xylanase containing glycoside hydrolase,GH
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, but to a lesser extent.
Structure of xylan, xylopyranosyl residues linked through 𝜷-1,4 linkages; point of xylanase
action is shown by arrow.
Sources of Xylanase: Xylanases are produced by a variety of the microorganisms like bacteria,
fungi and actinomycetes. Among bacteria, Aeromonas, Bacillus, Bacteroids, Microbacter,
Cellulomonas, Ruminococcus, Paenibacillus and Streptomyces have been reported to produce
xylanases. Some researchers have demonstrated Bacillus sp. to be the most promising and potential
sources of xylanase production.
Properties of the enzyme: Xylanases are genetically single chain glycoproteins, ranging from 6–80
kDa, active between pH 4.5–6.5 and at temperature between 40 and 60 °C. Xylanases from different
sources differ in their requirements for temperature, pH, etc. for optimum functioning.There are
reports about xylanases which work even at extreme temperatures like 90°C and can tolerate high pH
as 8-11.
Applications and Uses: Xylanases have wide biotechnological uses and potential applications in
industrial processes which includes kraft pulp bleaching, de-inking of newsprint, in poultry industry,
in wine production, in baking, in forage digestions, agricultural waste degradation, yielding cellulose
from rayon etc. Currently the most important application of enzymes is in the prebleaching of kraft
pulp. Xylanase enzymes have been found to be most effective for that purpose. Xylanase
prebleaching technology is now in use at several mills worldwide. This technology has been
successfully transferred to full industrial scale in just a few years.
Future prospects and areas to be focussed: Most of the available xylanases cannot withstand harsh
process conditions including extreme temperature, pH, presence of oxidizing agents, inhibition etc.
Another bottleneck is high cost of the production of the enzyme. Therefore, there is need for robust
xylanases which can work under hostile industrial processes and production of the enzyme must be
accomplished with cost effective substrates. As xylan is too expensive to be used as a substrate for
industrial production of enzymes, agro-residual wastes from agrobased industries could be a good
alternative.
By Kalpana Manhas Jasrotia