Last updated: August 4, 2019
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Hemicelluloses in
wood are made up of xylan and glucomannans. Xylan is a major carbohydrate and
its composition varies. Degradation of glucomannans and xylans require several
synergistic enzymes, endoxylanases and endomannanases hydrolyses main backbone of
xylans and glucomannan, respectively. Xylanases are placed in glycosyl
hydrolase families 10 and 11 and differ from each other with respect to their
catalytic properties. The catalytic domains of these two families are different
in their molecular masses, net charges and isoelectric point. These properties
might play some role in specificity and activity. Complete hydrolysis of xylans into free monomers requires numerous
enzymes like endo-1,4-?-xylanase, acetyl esterase, ?-glucuronidase and
?-xylosidase. The major difference between endo-1,4-?-xylanase and
1,4-?-xylosidase are; former generate xylan oligosaccharides while later works
on oligosaccharides generated by endo-1,4-?-xylanase to produce xylose. Tenkanen and
coworkers stated that enzymes from Trichoderma
reesei synergistically hydrolyze beech wood xylan. Later it was perceived
that, endoxylanases produced by single fungi show different specificities
towards xylans, showing complex nature of substrate. It has been demonstrated
that the ?-glucuronidases, ?-arabinosidases, and acetyl esterases are varying
in specificities in respect to neighboring substituents and xylan chain length. In addition, Clostridium stercorarium produced eight
different enzymes to degrade arabinoxylan however, only three of them required
for hydrolysis. Therefore, the efficient hydrolysis of native xylan appears to
comprise not only four different enzymes but also multiple isoenzyme system.

Xylanases are
produced by many species of bacteria, fungi and plants. The optimum temperature
from bacterial and fungal origin are ranges between 40 to 60°C but
themostability of bacterial xylanases are higher than fungal enzymes. Eriksson
et al described that white-rot fungus Phanerochaete
chrysosporium produced multiple endoxylanases with varying molecular weight
and ranges between 28 and 37 kDa. A tadpole shaped endoglucanases from T. reesei of almost 5 nm in diameter and
20 nm long are observed showing acidic pH optima. Two glycoproteins of 38 and 62 kDa with acidic pH optima were
purified from Irpex lacteus which
depolymerizes larch xylan. The pH optima of fungal xylanases ranges between pH 4.5-5.5 while
bacterial enzymes displayed maximum activity at pH 6.0-7.0.  Xylanases from Bacillus sp. and Streptomyces
viridosporus are active at alkaline pH.

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Mannanases are heterogeneous
group of enzymes similar to xylanases. The complete hydrolysis of
O-acetylgalactoglucomanann required many enzymes such as endomannases,
?-galactosidases, acetylglucomannan esterases and ?-mannisidases. Degradation opens
with rupturing of polymer by endomannases; acetylglucomannan esterase removes
acetyl groups, similar to xylan esterase in xylans. After that ?-Galactosidases
remove substituted galactose residues and finally ?-mannosidase and ?-glycosidase
breakdown ?-1,4 bonds and release oligomers. Mannanases are larger proteins
than xylanases with acidic isoelectric points. The molecular weight ranges
between 30-90 kDa. Similar to cellulolytic enzyme, multi domain structure is
reported in mannanase of Trichoderma
reesei; a catalytic core domain and a cellulose binding domain, separated
by a linker. In addition to these groups of enzymes, hemicellulose
biodegradation required some supplementary enzymes like xylan esterases,
ferulic and p-coumaric esterases,
?-l-arabinofuranosidases and ?-4-O-methyl glucuronosidases for the efficient hydrolysis
of xylans and mannans.

Endomannases usually
found in white-rot fungi like Irpex
lacteus, Haematostereum
sanguinolentum and Coriolus
versicolor as well as gram-positive and gram-negative bacteria. They are extensively
studies in several nonwood decaying ascomycetes such as Sporotrichum cellulophilum, Trichoderma reesei, and Sclerotium rolfsii. Additionally,
?-galactosidases, acetylglucomanane esterases and ?-mannonidases are explored
in Aspergillus niger and Polyporus sulfureus.