mannanases are also active and stable in a wide range of pH, pH 6–11 [119, 120].
The two most alkaline active mannanases which are optimally active at pH 12.5 are
reported from Streptomyces sp. and B. subtilis subsp. inaquosorum CSB31.
The optimum temperature for the activity of alkaline active mannanases falls in
the range of 50–75
C, which is within the range (40–75
C) known for most
mannanases [121]. The size of these enzymes is in the range of 30–60 kDa
(Table 4). However, mannanases with very low or high molecular weights have
also been reported. The mannanase from B. halodurans PPKS-2 has a molecular
weight of 22 kDa [102], while that of Bacillus JAMB-750 is 130 kDa [100].
5.2 Multiplicity of Xylanases and Mannanases
Several xylan-degrading organisms produce multiple xylanases. For example, Streptomyces sp. B-12-2 produces five endo-xylanases [122]. Aspergillus niger and
Trichoderma viride exhibited, respectively, up to 15 and 13 xylanases in their culture
supernatant [123]. Phanerochaete chrysosporium produced about 30 xylanases
[124]. Alkaliphiles are also known in producing multiple xylanases. For instance,
B. halodurans C-125 produces two extracellular xylanases [98] and an intracellular
xylanase [62]. Two extracellular endo-xylanases are reported from an alkaliphilic
Bacillus strain [87]. Three xylanases, two extracellular and one cell associated,
are produced by the alkaliphilic Bacillus sp. K-1 [125]. Multiplicity is also a feature
of mannanases. For instance, the fungus Sclerotium rolfsii produces two
β-mannosidases and five β-mannanases [126], and the alkaliphilic Bacillus
sp. AM-001 produces three mannanases [107].
There are many factors which lead to the appearance of multiple xylanases or
mannanases in the cultures of microorganisms. Multiplicity may be the result of
having distinct xylanase or mannanase encoding genes such as in B. halodurans and
Bacillus sp. AM-001 or due to posttranslational processing such as glycosylation,
differential mRNA processing, and post-secretional modification by proteolytic
digestion [12, 36, 123].
As aforementioned, xylans and mannans are complex polysaccharides, and their
glycosidic linkages may not be equally accessible to xylan-degrading enzymes.
Thus, the complete hydrolysis of these hemicelluloses may be difficult to be
achieved by a single xylanase or mannanase. This implies that an efficient hydrolysis
of these polysaccharides requires the action of multiple xylanases or mannanases
which work synergistically with overlapping but different specificities. The better
substrate hydrolysis achieved by the enzymes’ synergy can be correlated to substrate
utilization efficiency of the organism producing the enzymes. This may offer a
competitive advantage to the producer organism. Moreover, organisms may be
adapted to grow under different environmental conditions. In such scenarios, an
organism may produce a single enzyme (e.g., a mannanase) that works efficiently
under broad conditions at which the organism is adapted to grow or it may produce
different enzymes (e.g., mannanases) that work optimally at the different growth
Alkaline Active Hemicellulases
263
The two most alkaline active mannanases which are optimally active at pH 12.5 are
reported from Streptomyces sp. and B. subtilis subsp. inaquosorum CSB31.
The optimum temperature for the activity of alkaline active mannanases falls in
the range of 50–75
C, which is within the range (40–75
C) known for most
mannanases [121]. The size of these enzymes is in the range of 30–60 kDa
(Table 4). However, mannanases with very low or high molecular weights have
also been reported. The mannanase from B. halodurans PPKS-2 has a molecular
weight of 22 kDa [102], while that of Bacillus JAMB-750 is 130 kDa [100].
5.2 Multiplicity of Xylanases and Mannanases
Several xylan-degrading organisms produce multiple xylanases. For example, Streptomyces sp. B-12-2 produces five endo-xylanases [122]. Aspergillus niger and
Trichoderma viride exhibited, respectively, up to 15 and 13 xylanases in their culture
supernatant [123]. Phanerochaete chrysosporium produced about 30 xylanases
[124]. Alkaliphiles are also known in producing multiple xylanases. For instance,
B. halodurans C-125 produces two extracellular xylanases [98] and an intracellular
xylanase [62]. Two extracellular endo-xylanases are reported from an alkaliphilic
Bacillus strain [87]. Three xylanases, two extracellular and one cell associated,
are produced by the alkaliphilic Bacillus sp. K-1 [125]. Multiplicity is also a feature
of mannanases. For instance, the fungus Sclerotium rolfsii produces two
β-mannosidases and five β-mannanases [126], and the alkaliphilic Bacillus
sp. AM-001 produces three mannanases [107].
There are many factors which lead to the appearance of multiple xylanases or
mannanases in the cultures of microorganisms. Multiplicity may be the result of
having distinct xylanase or mannanase encoding genes such as in B. halodurans and
Bacillus sp. AM-001 or due to posttranslational processing such as glycosylation,
differential mRNA processing, and post-secretional modification by proteolytic
digestion [12, 36, 123].
As aforementioned, xylans and mannans are complex polysaccharides, and their
glycosidic linkages may not be equally accessible to xylan-degrading enzymes.
Thus, the complete hydrolysis of these hemicelluloses may be difficult to be
achieved by a single xylanase or mannanase. This implies that an efficient hydrolysis
of these polysaccharides requires the action of multiple xylanases or mannanases
which work synergistically with overlapping but different specificities. The better
substrate hydrolysis achieved by the enzymes’ synergy can be correlated to substrate
utilization efficiency of the organism producing the enzymes. This may offer a
competitive advantage to the producer organism. Moreover, organisms may be
adapted to grow under different environmental conditions. In such scenarios, an
organism may produce a single enzyme (e.g., a mannanase) that works efficiently
under broad conditions at which the organism is adapted to grow or it may produce
different enzymes (e.g., mannanases) that work optimally at the different growth
Alkaline Active Hemicellulases
263