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10 Enzyme Technology for the Degradation of Lignocellulosic Waste

sulfuric acid, hydrochloric acid, trifluro acetic acid, phosphoric acid, and nitric acid.

Disruption of hemicellulose microfibrils occurs in steam-explosion pre-treatment

[30]. The wastes are subjected to high pressures and temperatures for a short time

followed by depressurization of the system. Sometimes, acid catalysts are used

to aid the steam-explosion system. Wet oxidation and steam explosion combined

together help in the processing large amounts of biomass [31].

10.2.3

Degradation of Lignin

Lignin is a phenolic polymer and is found in every terrestrial plant. Lignins are

derived from 3-hydroxyl-cinnamyl alcohols or monolignols namely, p-coumaryl

alcohol, coniferyl alcohol, and sinapyl alcohol. The monolignols form the building

blocks of lignin. Two monolignol radicals dimerize to form a starting dimeric unit.

It then undergoes cross-coupling with another monolignol/dimeric radical, and

thereby polymerization sets in to biosynthesize lignin. Substituted phenylpropanoid

building units connected to each other by carbon–carbon linkages forming the

backbone of lignin. A precise and simplified definition for lignin is lacking because

of its structure, non-repetitive multi-units, diversity in composition depending on

origins, etc. [32].

10.2.3.1

Microbial Production of Lignin Degrading Enzymes

Due to the diversity in the interunits of lignin, it is not easy to break the internal

bonds. Only a few microorganisms are able to degrade lignins. White-rot basidio-

mycetes, actinomycetes, Phanerochaete chrysosporium, Trichoderma, Streptomyces

cinnamomensis, Lentinus squarrosulus, Schizophyllum commune, Bjerkandera

adusta, Xanthomonas sp., Fomitopsis palustris, and Aspergillus are able to efficiently

degrade lignin.

Phanerochaete chrysosporium is extensively studied in lignin degradation and also

been commercially used for lignocellulosic waste degradation. It produces 10 lignin

peroxidases (LiP), 5 manganese peroxidases (MnP), and several other lignocellu-

lolytic enzymes [33]. Another phytopathogenic fungus, Chondrostereum purpureum

also produces many lignocellulolytic enzymes [34].

10.2.3.2

Enzymes Responsible for the Degradation of Lignin

Lignin degradation is a difficult task to achieve by microorganisms. There are many

complexities in the lignin structure. First, lignin is a large polymer, and hence,

the enzymes required should be extracellular. Second, since the interunit bonds

are carbon–carbon and ether bonds, oxidative degradation rather than hydrolytic

breakdown is essential. And third, owing to its irregular structure, the enzymes

needed should be less specific in nature [35]. The enzymes utilized for lignin

degradation are known as ligninases or lignin-modifying enzymes. Ligninases

are grouped into two classes viz., heme peroxidases and phenol oxidases. LiP,

MnP, versatile peroxidases (VP), and dye decolorizing peroxidases come under the