biorefinery concept and some of the valorization shining showcases [10, 11] are
expected to give a great momentum for the sustainable development effort and
consequentially raise the demand for plant biomass degrading extremozymes.
Hemicelluloses, which are structurally and compositionally diverse polysaccharides, are among the major constituents of plant biomass. Due to their complex
natures, several enzymes are involved in the hydrolysis of hemicelluloses. These
enzymes are often referred to as hemicellulases. Hemicellulases have been in the
toolbox of biotechnology for many years and are applied such as in food, feed, pulp,
and beverage productions [12–14]. In recent years, such enzymes have also been
considered in valorization of hemicelluloses [15–17], which is believed to have
enormous economic and environmental potential. This remarkable application
potential of hemicellulases has led to intensive screening studies that result in the
discovery of a range of hemicellulases from various extremophiles. Alkaliphiles,
which are the most important sources of alkaline active enzymes, are among these
extremophile groups that have been intensively screened.
Alkaline active hemicellulases are highly desired for high pH applications such as
in pulp and paper, detergent, and textile industries. In addition, these extremozymes
seem to have greater potential in valorization of hemicelluloses to various valueadded products than the non-alkaline active counterparts. It is believed that the use of
alkaline active hemicellulases in valorization processes is advantageous due to better
solubility of hemicelluloses at alkaline condition and possibility of direct hydrolysis
of alkali-extracted hemicelluloses. Alkaline active xylanases and mannanases which,
respectively, hydrolyze the main chains of xylan and mannan are the most studied
alkaline active hemicellulases. The studies have shown the immense biotechnological importance of these enzymes, and this chapter primarily focuses on these two
groups of extremozymes, which play crucial roles in the hydrolysis of the two
abundant hemicelluloses.
2 Hemicelluloses
A broad and inexplicit definition of hemicellulose refers to plant cell wall polysaccharides that occur in close association with cellulose [18] or are noncellulosic
cell wall polysaccharides extracted from plant specimen [19]. However, since the
term is introduced long before the structure was determined, there has been a
controversy regarding these definitions. One of the arguments is that the definitions
do not exclude plant cell wall polysaccharides such as pectin which is not hemicellulose. Currently, hemicelluloses are defined as a group of cell wall polysaccharides
that are neither cellulose nor pectin and have equatorially configured β-(1!4)-linked
glucose, mannose, or xylose backbones [20, 21]. This definition clearly excludes
pectin and encompasses xyloglucans, xylans, mannans, heteromannans, and even
mixed-linkage β-(1-3,1-4)-glucans. However, it seems excluding carbohydrates such
as laminarin, arabinogalactan, and curdlan which have β-(1!3)-linked backbone but
are considered as hemicelluloses. This shows that even the widely accepted
Alkaline Active Hemicellulases
249
expected to give a great momentum for the sustainable development effort and
consequentially raise the demand for plant biomass degrading extremozymes.
Hemicelluloses, which are structurally and compositionally diverse polysaccharides, are among the major constituents of plant biomass. Due to their complex
natures, several enzymes are involved in the hydrolysis of hemicelluloses. These
enzymes are often referred to as hemicellulases. Hemicellulases have been in the
toolbox of biotechnology for many years and are applied such as in food, feed, pulp,
and beverage productions [12–14]. In recent years, such enzymes have also been
considered in valorization of hemicelluloses [15–17], which is believed to have
enormous economic and environmental potential. This remarkable application
potential of hemicellulases has led to intensive screening studies that result in the
discovery of a range of hemicellulases from various extremophiles. Alkaliphiles,
which are the most important sources of alkaline active enzymes, are among these
extremophile groups that have been intensively screened.
Alkaline active hemicellulases are highly desired for high pH applications such as
in pulp and paper, detergent, and textile industries. In addition, these extremozymes
seem to have greater potential in valorization of hemicelluloses to various valueadded products than the non-alkaline active counterparts. It is believed that the use of
alkaline active hemicellulases in valorization processes is advantageous due to better
solubility of hemicelluloses at alkaline condition and possibility of direct hydrolysis
of alkali-extracted hemicelluloses. Alkaline active xylanases and mannanases which,
respectively, hydrolyze the main chains of xylan and mannan are the most studied
alkaline active hemicellulases. The studies have shown the immense biotechnological importance of these enzymes, and this chapter primarily focuses on these two
groups of extremozymes, which play crucial roles in the hydrolysis of the two
abundant hemicelluloses.
2 Hemicelluloses
A broad and inexplicit definition of hemicellulose refers to plant cell wall polysaccharides that occur in close association with cellulose [18] or are noncellulosic
cell wall polysaccharides extracted from plant specimen [19]. However, since the
term is introduced long before the structure was determined, there has been a
controversy regarding these definitions. One of the arguments is that the definitions
do not exclude plant cell wall polysaccharides such as pectin which is not hemicellulose. Currently, hemicelluloses are defined as a group of cell wall polysaccharides
that are neither cellulose nor pectin and have equatorially configured β-(1!4)-linked
glucose, mannose, or xylose backbones [20, 21]. This definition clearly excludes
pectin and encompasses xyloglucans, xylans, mannans, heteromannans, and even
mixed-linkage β-(1-3,1-4)-glucans. However, it seems excluding carbohydrates such
as laminarin, arabinogalactan, and curdlan which have β-(1!3)-linked backbone but
are considered as hemicelluloses. This shows that even the widely accepted
Alkaline Active Hemicellulases
249