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10 Enzyme Technology for the Degradation of Lignocellulosic Waste

by grinding, milling, steam injection, and pyrolysis [21]. It also involves the use

of acids (hydrochloric acid, sulfuric acid, or phosphoric acid), alkalis (sodium

hydroxide, potassium hydroxide, calcium hydroxide, and ammonium hydroxide),

organic solvents (alcohols, organic acids, ketones, phenols, glycols, and ether), and

ionic liquids.

10.2.2

Degradation of Hemicellulose

The different sugar molecules which form the hemicellulose include D-glucose,

D-xylose, D-arabinose, D-mannose, D-galactose, D-4-O-methyl-glucuronic, D-glucuro-

nic, and D-galacturonic acid, and other ester-linked coumaryl, acetyl, and feruloyl

moieties. The sugars are linked with each other by β-1,4-glycosidic bonds and

β-1,3-glycosidic bonds [22]. Hemicelluloses are not crystalline in nature but are

attached with cellulose microfibrils, and together, they constitute the hard fibers and

secondary wall of plant cells. Among all the heteropolymers found in hemicellulose,

xylan is most abundant [23].

10.2.2.1

Enzymes Responsible for Degradation of Hemicellulose

Due to the complex nature of hemicellulose, it requires a combined effort by

endo-enzymes, exo-enzymes, and accessory enzymes for its degradation. The

endo-enzymes break the main chains internally, the exo-enzyme produces

monomers, and the accessory enzymes breaks the side chains and also the attached

oligosaccharides thereby producing monosaccharides and disaccharides.

Xylan degradation is achieved by two main enzymes viz., ß-1,4-endoxylanase and

ß-1,4-xylosidase. The ß-1,4-endoxylanase splits the xylan backbone and produces

smaller oligosaccharide. The ß-1,4-xylosidase further breaks the oligosaccha-

rides into smaller units, i.e. xylose monosaccharides. Depending upon substrate

specificity, the fungal endoxylanases are two types G10 and G11 [24]. Complete

degradation of xylan is achieved by ß-xylosidase.

ß-1,4-linked

D-glucose forms the backbone of xyloglucan substituted with

D-xylose side chains. The enzymes, xyloglucanases and ß-glucosidases, are needed

for xyloglucan degradation. Xyloglucanase activity is not same for all substrates.

For example, some xyloglucanases break only the glucose backbone of xyloglucan

and not the glucose backbone of any other cellulose. Also, xyloglucanase activities

derived from different fungi are different. For instance, the T. reesei xyloglucanase

has substrate specificity for branched glucose chains, whereas the A. niger xyloglu-

canase belonging to GH12 family breaks xylogluco-oligosaccharides having more

than six glucose residues with at least one non-branched glucose residue [25].

Mannan degradation is also done by two enzymes viz., ß-endomannanases and

ß-mannosidases [4]. Mannans are comprised ß-1,4-linked D-mannose backbone

with D-galactose side chains. The ß-endomannanases break down the galactoman-

nans and produce mannobiose and mannotriose, and these were further broken

down into mannose by ß-mannosidases.

Many accessory enzymes are required to remove all the substituted side chains

from the hemicellulose backbone. A total of nine enzymes belonging to different