As shown in Table 5, the GH10 xylanase is active in a wide range of pH, and
hence when the cells grow under alkaline condition, this enzyme can primarily
degrade xylan, whereas when the growth condition is near neutral, both GH10 and
GH11 xylanases may be involved in the degradation of xylan, which might compensate the effect of lower substrate solubility (at neutral condition), a phenomenon
that can possibly reduce catalytic efficiency. Moreover, the difference in the properties of the two xylanases may have another competitive advantage in hydrolyzing
xylan from different sources. GH11 xylanase prefers to hydrolyze not close to
branching site, while GH10 xylanase hydrolyzes xylan even close to branching
position. GH11 xylanases tend to release bigger fragments than GH10 xylanases.
GH10 xylanases can attack smaller xylooligosaccharides which cannot be digested
by family 11 xylanase. On the other hand, the smaller GH11 xylanase can diffuse
easily and accesses xylan that cannot be reached by the bigger GH10 xylanase. This
suggests that the two enzymes work synergistically to bring better xylan degradation. Xylooligosaccharides released by the two extracellular endo-xylanases will be
taken up by the cells and further hydrolyzed by GH8 xylanase inside the cell. Thus,
the multiplicity is expected to offer some competitive advantage, and this may be
one of the reasons why hemicellulose-degrading organisms tend to produce multiple
xylanases and mannanases having diverse physicochemical properties, structures,
specific activities, and even end-product profiles, thereby increasing the efficiency
and extent of hydrolysis.
5.3 Modular Organization
Many glycoside hydrolases and glycosyltransferases have modular structural architecture containing catalytic and non-catalytic domains connected to each other via
short linker segments. The non-catalytic domains include structures that bind to
cellulose, xylan, chitin, dockerin, etc. and other domains for which the function has
yet to be established. The non-catalytic domains are commonly involved in carbohydrate binding and hence often referred to as carbohydrate-binding modules
(CBMs). Like other glycoside hydrolases, modularity has been reported in alkaline
active hemicellulases [82, 92, 128]. CBM structures are known to bind to insoluble
substrates such as crystalline cellulose and insoluble xylan. This binding enhances
the hydrolysis of such poorly soluble substrates by keeping the catalytic domain of
the enzyme in close proximity to the substrate and by weakening the intermolecular
interaction of the substrate molecules [129, 130]. However, in addition to its
catalytic role, thermal stabilization effect has been widely reported [129]. On the
other hand, there is little information if CBM structures are involved in enhancing
enzyme activity and stability at high pH. But mutational studies on xynAS27 from
Streptomyces sp. S27 revealed the possible roles of the non-catalytic domain on the
activity and stability of alkaline active hemicellulases in alkaline milieu [128]. However, it is not clear if this role is direct or indirect. The great majority of alkaline
active xylanases and mannanases from genus Bacillus are non-modular with some
Alkaline Active Hemicellulases
265
hence when the cells grow under alkaline condition, this enzyme can primarily
degrade xylan, whereas when the growth condition is near neutral, both GH10 and
GH11 xylanases may be involved in the degradation of xylan, which might compensate the effect of lower substrate solubility (at neutral condition), a phenomenon
that can possibly reduce catalytic efficiency. Moreover, the difference in the properties of the two xylanases may have another competitive advantage in hydrolyzing
xylan from different sources. GH11 xylanase prefers to hydrolyze not close to
branching site, while GH10 xylanase hydrolyzes xylan even close to branching
position. GH11 xylanases tend to release bigger fragments than GH10 xylanases.
GH10 xylanases can attack smaller xylooligosaccharides which cannot be digested
by family 11 xylanase. On the other hand, the smaller GH11 xylanase can diffuse
easily and accesses xylan that cannot be reached by the bigger GH10 xylanase. This
suggests that the two enzymes work synergistically to bring better xylan degradation. Xylooligosaccharides released by the two extracellular endo-xylanases will be
taken up by the cells and further hydrolyzed by GH8 xylanase inside the cell. Thus,
the multiplicity is expected to offer some competitive advantage, and this may be
one of the reasons why hemicellulose-degrading organisms tend to produce multiple
xylanases and mannanases having diverse physicochemical properties, structures,
specific activities, and even end-product profiles, thereby increasing the efficiency
and extent of hydrolysis.
5.3 Modular Organization
Many glycoside hydrolases and glycosyltransferases have modular structural architecture containing catalytic and non-catalytic domains connected to each other via
short linker segments. The non-catalytic domains include structures that bind to
cellulose, xylan, chitin, dockerin, etc. and other domains for which the function has
yet to be established. The non-catalytic domains are commonly involved in carbohydrate binding and hence often referred to as carbohydrate-binding modules
(CBMs). Like other glycoside hydrolases, modularity has been reported in alkaline
active hemicellulases [82, 92, 128]. CBM structures are known to bind to insoluble
substrates such as crystalline cellulose and insoluble xylan. This binding enhances
the hydrolysis of such poorly soluble substrates by keeping the catalytic domain of
the enzyme in close proximity to the substrate and by weakening the intermolecular
interaction of the substrate molecules [129, 130]. However, in addition to its
catalytic role, thermal stabilization effect has been widely reported [129]. On the
other hand, there is little information if CBM structures are involved in enhancing
enzyme activity and stability at high pH. But mutational studies on xynAS27 from
Streptomyces sp. S27 revealed the possible roles of the non-catalytic domain on the
activity and stability of alkaline active hemicellulases in alkaline milieu [128]. However, it is not clear if this role is direct or indirect. The great majority of alkaline
active xylanases and mannanases from genus Bacillus are non-modular with some
Alkaline Active Hemicellulases
265