Most of the accessory enzymes remove substituents from oligomeric substrates
released by the endo-enzymes, xylanases and mannanases [56]. Some accessory
enzymes that can act on intact hemicelluloses have been known. However, even
these enzymes prefer oligomeric substrates over intact (polymeric) hemicelluloses
[57].
Although complete hydrolysis of hemicelluloses requires both main chain- and
side group-cleaving enzymes, the type of the hemicellulose, the method employed to
extract the hemicellulose, and the enzymatic reaction conditions determine which
enzyme may be required for complete hydrolysis. This is mainly related to the
difference in the presence or absence of side groups and the stability of the chemical
bonds that exist in the hemicelluloses. It is obvious that the set of enzymes required
for complete hydrolysis of homoxylan and heteroxylan are different. Similarly, the
extraction methods used to recover hemicelluloses from lignocellulosic biomass are
known to affect the nature of the hemicellulose, and this can influence which suite of
enzymes to be considered for complete digestion. For instance, the alkaline extraction, which is the prominent method of hemicellulose extraction, is often accompanied by deacetylation and removal of the uronic acid groups [58–60]. Thus, in
principle, complete hydrolysis of alkali-extracted hemicelluloses does not require
acetylmannan esterase and acetylxylan esterase, α-glucuronidase, and to some extent
ferulic acid esterase. Thus, hydrolysis of alkali-extracted xylan is more attractive, not
only for it requires relatively fewer number of enzymes for complete conversion to
its monomers but also for its efficiency. This has been demonstrated recently by
Lyczakowski and coworkers [61] who have shown an efficient hydrolysis of
glucuronic acid-free xylan. Moreover, accomplishing hydrolysis at alkaline condition avoids extensive washing and subsequent pH readjustment of alkali-extracted
hemicellulose, while it benefits from the better solubility of hemicellulose which in
turn enhances the digestion efficiency.
5 Alkaline Active Hemicellulases
Alkaliphiles, organisms thriving in alkaline habitats, are the main sources of alkaline
active enzymes. Several alkaliphiles are known in producing intracellular as well as
extracellular hemicellulases. However, it is the extracellular enzymes that are
often alkaline active as it is evolved to work in the alkaline habitat. On the other
hand, since the cytoplasmic pH is often in the neutral to slightly alkaline pH
range, intracellular hemicellulases are often active around neutral condition [62].
Extremely alkaline active hemicellulases have also been reported from
non-alkaliphilic organisms. The β-mannanases of Bacillus subtilis subsp.
inaquosorum CSB31 [63] and Streptomyces sp. CS147 (Mn147) [64], which are
optimally active at pH 12.5 and pH 11, respectively, can be examples of alkaline
active enzymes of non-alkaliphilic origin. Similarly, Anoxybacillus kamchatkensis
[65] and Actinomadura sp. Cpt20 [66] which are adapted to grow around pH 7
Alkaline Active Hemicellulases
257
released by the endo-enzymes, xylanases and mannanases [56]. Some accessory
enzymes that can act on intact hemicelluloses have been known. However, even
these enzymes prefer oligomeric substrates over intact (polymeric) hemicelluloses
[57].
Although complete hydrolysis of hemicelluloses requires both main chain- and
side group-cleaving enzymes, the type of the hemicellulose, the method employed to
extract the hemicellulose, and the enzymatic reaction conditions determine which
enzyme may be required for complete hydrolysis. This is mainly related to the
difference in the presence or absence of side groups and the stability of the chemical
bonds that exist in the hemicelluloses. It is obvious that the set of enzymes required
for complete hydrolysis of homoxylan and heteroxylan are different. Similarly, the
extraction methods used to recover hemicelluloses from lignocellulosic biomass are
known to affect the nature of the hemicellulose, and this can influence which suite of
enzymes to be considered for complete digestion. For instance, the alkaline extraction, which is the prominent method of hemicellulose extraction, is often accompanied by deacetylation and removal of the uronic acid groups [58–60]. Thus, in
principle, complete hydrolysis of alkali-extracted hemicelluloses does not require
acetylmannan esterase and acetylxylan esterase, α-glucuronidase, and to some extent
ferulic acid esterase. Thus, hydrolysis of alkali-extracted xylan is more attractive, not
only for it requires relatively fewer number of enzymes for complete conversion to
its monomers but also for its efficiency. This has been demonstrated recently by
Lyczakowski and coworkers [61] who have shown an efficient hydrolysis of
glucuronic acid-free xylan. Moreover, accomplishing hydrolysis at alkaline condition avoids extensive washing and subsequent pH readjustment of alkali-extracted
hemicellulose, while it benefits from the better solubility of hemicellulose which in
turn enhances the digestion efficiency.
5 Alkaline Active Hemicellulases
Alkaliphiles, organisms thriving in alkaline habitats, are the main sources of alkaline
active enzymes. Several alkaliphiles are known in producing intracellular as well as
extracellular hemicellulases. However, it is the extracellular enzymes that are
often alkaline active as it is evolved to work in the alkaline habitat. On the other
hand, since the cytoplasmic pH is often in the neutral to slightly alkaline pH
range, intracellular hemicellulases are often active around neutral condition [62].
Extremely alkaline active hemicellulases have also been reported from
non-alkaliphilic organisms. The β-mannanases of Bacillus subtilis subsp.
inaquosorum CSB31 [63] and Streptomyces sp. CS147 (Mn147) [64], which are
optimally active at pH 12.5 and pH 11, respectively, can be examples of alkaline
active enzymes of non-alkaliphilic origin. Similarly, Anoxybacillus kamchatkensis
[65] and Actinomadura sp. Cpt20 [66] which are adapted to grow around pH 7
Alkaline Active Hemicellulases
257