fat substitutes, while in pharmaceutical industry, together with other ingredients,
they are used for direct tableting [47]. Monomeric sugars obtained from hemicellulose degradation have been used as fermentative substrate for production of different
chemicals and biofuels [6, 42, 53–55]. Hydrolysis of hemicelluloses can be achieved
either through enzymatic or chemical means. Chemical hydrolysis, which commonly
involves acids, is very efficient and inexpensive. However, it is unspecific, results in
undesirable products, has corrosive effects, and generates waste and fermentation
inhibitors. Moreover, this hydrolysis route often produces monomers as major
products and suffers thereby from poor product diversity. On the other hand,
enzymatic saccharification is attractive as it is specific, biocompatible, and environmentally benign and offers a large variety of products. This makes hemicellulosedegrading enzymes, which are collectively known as hemicellulases, the focus of
intense research.
4 Hemicellulases
A vast array of microorganisms producing hemicellulose-modifying enzymes have
been isolated from various habitats and studied. As hemicelluloses constitute a
heterogeneous group of polysaccharides with diverse compositions, structures,
linkages, etc., nature has evolved a suite of enzymes that degrades it. Enzymes
that belong to this group are either glycoside hydrolases that hydrolyze glycosidic
bonds or carbohydrate esterases which hydrolyze ester linkages. Thus, as shown in
Fig. 3, the complete hydrolysis of hemicelluloses requires a concerted action of
enzymes that cleave the main chain as well as the substituent linkages. Endo- and
exo-hemicellulases attack the same types of linkages in the backbone of hemicelluloses. But, the endo-glycoside hydrolases cleave the internal glycosidic linkages,
while the exo-glycoside hydrolases release mono- or disaccharides either from the
reducing or the nonreducing end of the polysaccharide. Hemicellulases such
as endo-β-1,4-xylanases hydrolyze the main chain of xylan, and the exo-acting
β-xylosidases release xylose from the reducing end of the backbone. Although the
endo-xylanases are often perceived acting randomly, it seems that these enzymes
attack xylan not at random but in an orderly fashion. Factors such as the degree of
substrate branching, the length of the branch, the nature of substituents, etc. determine which bond has to be cleaved [36]. The backbone-hydrolyzing hemicellulases,
specifically the endo-glycoside hydrolases, are crucial for hemicellulose hydrolysis.
However, the accessibility of the glycosidic bonds in the backbone structures could
be limited due to the presence of substituents. Thus, debranching enzymes which are
often referred to as accessory enzymes remove the substituents and facilitate the
hemicellulose complete hydrolysis by the endo- and exo-hemicellulases.
The complete hydrolysis of the common native xylan, at least theoretically,
requires the action of several enzymes including arabinofuranosidases,
α-glucuronidases, α-galactosidases, acetyl xylan esterases, ferulic acid esterase, pcoumaric acid esterase, endo-xylanase, and β-xylosidase. Arabinose and methyl
Alkaline Active Hemicellulases
255
they are used for direct tableting [47]. Monomeric sugars obtained from hemicellulose degradation have been used as fermentative substrate for production of different
chemicals and biofuels [6, 42, 53–55]. Hydrolysis of hemicelluloses can be achieved
either through enzymatic or chemical means. Chemical hydrolysis, which commonly
involves acids, is very efficient and inexpensive. However, it is unspecific, results in
undesirable products, has corrosive effects, and generates waste and fermentation
inhibitors. Moreover, this hydrolysis route often produces monomers as major
products and suffers thereby from poor product diversity. On the other hand,
enzymatic saccharification is attractive as it is specific, biocompatible, and environmentally benign and offers a large variety of products. This makes hemicellulosedegrading enzymes, which are collectively known as hemicellulases, the focus of
intense research.
4 Hemicellulases
A vast array of microorganisms producing hemicellulose-modifying enzymes have
been isolated from various habitats and studied. As hemicelluloses constitute a
heterogeneous group of polysaccharides with diverse compositions, structures,
linkages, etc., nature has evolved a suite of enzymes that degrades it. Enzymes
that belong to this group are either glycoside hydrolases that hydrolyze glycosidic
bonds or carbohydrate esterases which hydrolyze ester linkages. Thus, as shown in
Fig. 3, the complete hydrolysis of hemicelluloses requires a concerted action of
enzymes that cleave the main chain as well as the substituent linkages. Endo- and
exo-hemicellulases attack the same types of linkages in the backbone of hemicelluloses. But, the endo-glycoside hydrolases cleave the internal glycosidic linkages,
while the exo-glycoside hydrolases release mono- or disaccharides either from the
reducing or the nonreducing end of the polysaccharide. Hemicellulases such
as endo-β-1,4-xylanases hydrolyze the main chain of xylan, and the exo-acting
β-xylosidases release xylose from the reducing end of the backbone. Although the
endo-xylanases are often perceived acting randomly, it seems that these enzymes
attack xylan not at random but in an orderly fashion. Factors such as the degree of
substrate branching, the length of the branch, the nature of substituents, etc. determine which bond has to be cleaved [36]. The backbone-hydrolyzing hemicellulases,
specifically the endo-glycoside hydrolases, are crucial for hemicellulose hydrolysis.
However, the accessibility of the glycosidic bonds in the backbone structures could
be limited due to the presence of substituents. Thus, debranching enzymes which are
often referred to as accessory enzymes remove the substituents and facilitate the
hemicellulose complete hydrolysis by the endo- and exo-hemicellulases.
The complete hydrolysis of the common native xylan, at least theoretically,
requires the action of several enzymes including arabinofuranosidases,
α-glucuronidases, α-galactosidases, acetyl xylan esterases, ferulic acid esterase, pcoumaric acid esterase, endo-xylanase, and β-xylosidase. Arabinose and methyl
Alkaline Active Hemicellulases
255