3.4.2 Fungal Hemicellulases
Hemicelluloses are heterogenous and hence need variety of enzymes for their
complete hydrolysis. Over 70% structure of hemicelluloses is made up of xylan. β1,4 bonds in xylan are the target sites of xylanases, hydrolyzing which it generates
oligomers that are later broken down to xylose with the aid of β-xylosidase.
Hemicellulose may also be comprised of other different components like mannan
or arabinofuranosyl or arabinose in different composition, hence supplementary
enzymes such as β-mannanases, arabinofuranosidases, or α-L-arabinases may also
be required for hydrolysis of hemicelluloses. Hemicellulase belongs to glycoside
hydrolases and to carbohydrate esterases, which breaks down ester bonds of acetate
or ferulic acid side groups. Some fungi are the known producer of cocktail of huge
quantities of extracellular cellulases as well as hemicellulases, and prominent among
them are species of Penicillium, Aspergillus, and T. emersonii. Presence of cellulases
along with hemicellulases or pectinases can efficiently enhance the magnitude of
cellulose transformation (Bhatia et al. 2012).
3.4.3 Fungal Ligninases
Lignin is known to be the tough part of lignocellulosic material, but certain fungi
have the inherited potential to degrade this tough part, by the virtue of their secreting
enzymes termed ligninases. Among the most well-studied ligninase-producing
organisms, C. versicolor, P. chrysosporium, and T. versicolor are the white-rot
basidiomycetes, which can proficiently break down lignin. Fungal ligninases belong
to two ligninolytic families: (1) phenol oxidase (laccase) and (2) peroxidases (lignin
peroxidase [LiP] as well as manganese peroxidase [MnP]). Many fungi that are able
to degrade lignin are found deficient in lignin peroxidases (LiPs). LiP is responsible
of oxidation of nonphenolic part of lignin. Oxidative ligninolytic enzymes have to
face the limitation that they cannot cross the cell wall. Size of these enzymes is
responsible for it. Bacterial cellulosomes are well characterized compared to fungal
ones, and hence it becomes mandatory that before enzymatic effect, low-molecular
weight diffusible reactive oxidative substances start the changes to the lignin structure and hemicelluloses (Bhatia et al. 2012). Being an originator of contemporary
heat-stable enzymes, thermophiles are of special interest. Traditional chemical
processes are replaced or supplemented by a recent spawning of sturdy enzymes
that are fitted to tolerate extreme circumstances in processes prevailing in industries.
It is because of these properties of thermophilic enzymes, many thermophilic
microorganisms are making their way toward biotechnological and commercial
use (Mehta et al. 2016). Moreover, the risk of bacterial contamination is significantly
reduced in the thermophilic fermentation due to inability of normal environmental
contaminants to reproduce themselves. It can be said that the thermophilic fermentation can be best carried out even without pasteurization, which makes it efficient
and a cost-effective operation. Moreover, thermophilic fermentation can well coordinate with simultaneous saccharification process (operating at 55
C), thereby again
72
L. Bhatia et al.
Hemicelluloses are heterogenous and hence need variety of enzymes for their
complete hydrolysis. Over 70% structure of hemicelluloses is made up of xylan. β1,4 bonds in xylan are the target sites of xylanases, hydrolyzing which it generates
oligomers that are later broken down to xylose with the aid of β-xylosidase.
Hemicellulose may also be comprised of other different components like mannan
or arabinofuranosyl or arabinose in different composition, hence supplementary
enzymes such as β-mannanases, arabinofuranosidases, or α-L-arabinases may also
be required for hydrolysis of hemicelluloses. Hemicellulase belongs to glycoside
hydrolases and to carbohydrate esterases, which breaks down ester bonds of acetate
or ferulic acid side groups. Some fungi are the known producer of cocktail of huge
quantities of extracellular cellulases as well as hemicellulases, and prominent among
them are species of Penicillium, Aspergillus, and T. emersonii. Presence of cellulases
along with hemicellulases or pectinases can efficiently enhance the magnitude of
cellulose transformation (Bhatia et al. 2012).
3.4.3 Fungal Ligninases
Lignin is known to be the tough part of lignocellulosic material, but certain fungi
have the inherited potential to degrade this tough part, by the virtue of their secreting
enzymes termed ligninases. Among the most well-studied ligninase-producing
organisms, C. versicolor, P. chrysosporium, and T. versicolor are the white-rot
basidiomycetes, which can proficiently break down lignin. Fungal ligninases belong
to two ligninolytic families: (1) phenol oxidase (laccase) and (2) peroxidases (lignin
peroxidase [LiP] as well as manganese peroxidase [MnP]). Many fungi that are able
to degrade lignin are found deficient in lignin peroxidases (LiPs). LiP is responsible
of oxidation of nonphenolic part of lignin. Oxidative ligninolytic enzymes have to
face the limitation that they cannot cross the cell wall. Size of these enzymes is
responsible for it. Bacterial cellulosomes are well characterized compared to fungal
ones, and hence it becomes mandatory that before enzymatic effect, low-molecular
weight diffusible reactive oxidative substances start the changes to the lignin structure and hemicelluloses (Bhatia et al. 2012). Being an originator of contemporary
heat-stable enzymes, thermophiles are of special interest. Traditional chemical
processes are replaced or supplemented by a recent spawning of sturdy enzymes
that are fitted to tolerate extreme circumstances in processes prevailing in industries.
It is because of these properties of thermophilic enzymes, many thermophilic
microorganisms are making their way toward biotechnological and commercial
use (Mehta et al. 2016). Moreover, the risk of bacterial contamination is significantly
reduced in the thermophilic fermentation due to inability of normal environmental
contaminants to reproduce themselves. It can be said that the thermophilic fermentation can be best carried out even without pasteurization, which makes it efficient
and a cost-effective operation. Moreover, thermophilic fermentation can well coordinate with simultaneous saccharification process (operating at 55
C), thereby again
72
L. Bhatia et al.