about 24–37% [7, 25]. The hemicellulose fraction of grass and switchgrass can reach
up to 50% of the total dry weight [7].
Xylan and mannan are the most abundant hemicelluloses which are the dominant
constituents of hardwood and softwood hemicelluloses, respectively. Thus, biotechnological applications involving enzymatic modification of hemicelluloses often
revolve around xylan and mannan hydrolysis.
2.1 Xylan
Xylan is an important hemicellulose. It is the second most abundant polymer in
nature and roughly accounts for one-third of the renewable biomass available on
Earth. It constitutes about 30% of the cell wall material of annual plants, 15–35% of
hardwoods, and 7–10% of softwoods [26, 27]. Xylan often consists of a β-1,4-linked
D-xylose backbone which can be substituted with different side groups such as Larabinose, D-galactose, acetyl, feruloyl, p-coumaroyl, and glucuronic acid residues
which are attached at positions C-2 and C-3 of xylose residues (Fig. 2).
Xylan is known to occur in several structural varieties in terrestrial plants and
algae. It has also been found in other organisms. A glucuronoxylan was reported
from unicellular flagellate Herpetomonas samuelpessoai [28], a high molecular
weight glycan containing mixed linked xylan from a parasitic protozoan Leptomonas
samueli [29], and the β-1,4-D-xylan containing exopolysaccharide from the bacterium Pasteurella multocida [30].
Its structure and composition vary depending on the botanical source and even in
between different tissues of the same plant [31]. The diversity is attributed to the
difference in the nature of substituents, the frequency and position of substitution,
Fig. 2 Simplified illustration of xylan structure
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up to 50% of the total dry weight [7].
Xylan and mannan are the most abundant hemicelluloses which are the dominant
constituents of hardwood and softwood hemicelluloses, respectively. Thus, biotechnological applications involving enzymatic modification of hemicelluloses often
revolve around xylan and mannan hydrolysis.
2.1 Xylan
Xylan is an important hemicellulose. It is the second most abundant polymer in
nature and roughly accounts for one-third of the renewable biomass available on
Earth. It constitutes about 30% of the cell wall material of annual plants, 15–35% of
hardwoods, and 7–10% of softwoods [26, 27]. Xylan often consists of a β-1,4-linked
D-xylose backbone which can be substituted with different side groups such as Larabinose, D-galactose, acetyl, feruloyl, p-coumaroyl, and glucuronic acid residues
which are attached at positions C-2 and C-3 of xylose residues (Fig. 2).
Xylan is known to occur in several structural varieties in terrestrial plants and
algae. It has also been found in other organisms. A glucuronoxylan was reported
from unicellular flagellate Herpetomonas samuelpessoai [28], a high molecular
weight glycan containing mixed linked xylan from a parasitic protozoan Leptomonas
samueli [29], and the β-1,4-D-xylan containing exopolysaccharide from the bacterium Pasteurella multocida [30].
Its structure and composition vary depending on the botanical source and even in
between different tissues of the same plant [31]. The diversity is attributed to the
difference in the nature of substituents, the frequency and position of substitution,
Fig. 2 Simplified illustration of xylan structure
Alkaline Active Hemicellulases
251