10.2 Enzymes Required for the Degradation of Lignocellulosic Waste

147

Table 10.2

Types of enzymes used to cleave side chains in hemicellulose [3].

Substituent

Enzymes

L-arabinose

α-Arabinofuranosidases arabinoxylanarabinofuranohydrolases

D-xylose

α-Xylosidases

D-glucuronic acid

α-Glucuronidases

Ferulic acid

Feruloyl esterases

Acetyl group

Acetyl xylanesterases

L-fucose

α-Fucosidases

p-Coumaric acid

p-Coumaroylesterases

D-galactose

α-Galactosidases

families viz., glycoside hydrolase (GH) and carbohydrate esterase (CE) families are

required for complete degradation [26]. There are many substituents found in the

hemicellulose structure, such as arabinose, D-xylose, D-glucuronic acid, ferulic acid,

acetyl group, L-fucose, and p-coumaric acid. The enzymes required for debranching

these side chains from the hemicellulose backbone are presented in Table 10.2.

10.2.2.2

Microbial Production of Hemicellulases

The main hemicellulase producing microorganisms are fungi, though many bacteria

and actinomycetes are also reported to produce hemicellulases. The Cladosporium

sp., Fusarium, Penicillium thomii, Penicillium canescens, Penicillium pinophilum,

Alternaria alternata, Ceratocystis paradoxa, Geotrichum, A. niger, Paecilomyces, Tri-

choderma, and Cephalosporium are some of the fungi producing hemicellulases [1].

The hemicellulase producing bacteria are Cellulomonas, Bacillus sp., Micro-

coccus,

Thermotoga,

Staphylococcus,

Pseudoxanthomonas,

Arthrobacter,

and

Rhodococcus [27].

Thermobifida fusca, Streptomyces flavogriseus, C. fimi, Cellulomonas flavigena

ATCC 482, Actinomadura, Thermomonospora curvata, Thermomonospora alba,

Microbispora bispora, Micromonospora, Nocardia, Thermoactinomyces, Saccha-

romonospora viridis, Streptomyces violaceoruber, Streptomyces lividans, Streptomyces

aureofaciens, Microtetrasporaflexuosa, Streptomyces thermocyanaeviolaceus, and

Thermoactinomyces thalophilus are the actinomycetes producing hemicellulase

enzyme [28].

10.2.2.3

Physical Pre-treatments to Break down Hemicellulose

Some of the physical pre-treatments commonly used for hemicellulose are alkali

pre-treatment, wet oxidation, acid pre-treatment, steam-explosion pre-treatment,

use of green solvents, etc. Hemicellulose hydrolysis is achieved by the physical

pre-treatments. The commonly used alkalis are sodium hydroxide, potassium

hydroxide, calcium hydroxide, and ammonium hydroxide [29]. Acid pre-treatment

helps to convert the hemicellulose into sugars. The commonly used acids are