Xylanases (EC 3.2.1.8)
Xylanases are the enzymes that catalyze the hydrolysis of xylan which is a heterogeneous polysaccharide (and the fullest extent of these enzymes is called the
xylanolytic enzyme system) (Subramaniyan and Prema 2002).
As shown in Fig. 9.5, the xylanases can be synthesized by certain molds (Ito et al.
1992), bacteria (Nascimento et al. 2002), and yeasts (Mandal 2015).
Like celluloses, xylanases are also a part of the glycoside hydrolase (GH) family
and are mostly derived from GH10 and GH11. However, there are also some
xylanases included in other GH families (CAZy; http://www.cazy.org/).
These enzyme families are similar with respect to the way they depolymerize the
xylan via Koshland type (two-phased catalysis: the products are separated with the
retained stereochemistry of the anomeric configuration). Family 10 enzymes give
products with less molecular weight (tetramer) than the family 11 enzymes
(pentamer) (Christov et al. 2000; Decker et al. 2017).
Crystal structures of many GH10 and GH11 family members have been clarified.
It has been stated that the xylanases of the GH family 10 consist of (α/β) 8 tim-barrel
fold and the xylanases of the GH family 11 generally consist of β-sheets
(Manikandan et al. 2005; Yeoman et al. 2010).
Due to the complex nature of the xylans, their enzymatic hydrolysis is harder than
that of other plant polysaccharides, so the enzymes need to work together.
ß-glycosidases and endoxylanases mediate the depolymerization of the xylan backbone (Decker et al. 2017).
Moreover, according to the xylan type, debranching enzymes such as
α-glucuronidase (EC 3.2.1.139), acetyl xylan esterase (EC 3.1.1.72), and ferulic
and coumaric acid esterases (EC 3.1.1.73) also help moving away the side groups in
xylan structure (Begemann et al. 2011; Amoozegar et al. 2019).
Xylosidases
ß-D-xylosidases are the enzymes that hydrolyze the xylooligomers in various
lengths to the xylose (Sindhu et al. 2016; Decker et al. 2017), and they are in GH
families of 3, 30, 39, 43, 51, 52, and 54 (Yeoman et al. 2010). As shown in Fig. 9.5,
xylosidases are isolated from a series of certain fungi (especially the Aspergillus
genus) (Martin Perez et al. 2017) and bacteria (Panbangred et al. 1984).
In a similar fashion to the ß-glucosidases that are in the cellulose systems, ß-Dxylosidases are crucial in moderating the final product inhibition of xylanases which
is caused by xylobiose (Yeoman et al. 2010). As the concentrations of the final
product (xylose) increases, the performance of the ß-xylosidases typically gets
inhibited (as is the case with the ß-glucosidases). Zanoelo and co-workers have
pointed out that the thermophilic ß-xylosidase, isolated from fungus (Scytalidium
thermophilum), develops a tolerance to final product inhibition (Zanoelo et al. 2004).
It is obvious that this type of feature has great importance for the bio-catalyzers in
the biofuel industry (Yeoman et al. 2010).
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271
Xylanases are the enzymes that catalyze the hydrolysis of xylan which is a heterogeneous polysaccharide (and the fullest extent of these enzymes is called the
xylanolytic enzyme system) (Subramaniyan and Prema 2002).
As shown in Fig. 9.5, the xylanases can be synthesized by certain molds (Ito et al.
1992), bacteria (Nascimento et al. 2002), and yeasts (Mandal 2015).
Like celluloses, xylanases are also a part of the glycoside hydrolase (GH) family
and are mostly derived from GH10 and GH11. However, there are also some
xylanases included in other GH families (CAZy; http://www.cazy.org/).
These enzyme families are similar with respect to the way they depolymerize the
xylan via Koshland type (two-phased catalysis: the products are separated with the
retained stereochemistry of the anomeric configuration). Family 10 enzymes give
products with less molecular weight (tetramer) than the family 11 enzymes
(pentamer) (Christov et al. 2000; Decker et al. 2017).
Crystal structures of many GH10 and GH11 family members have been clarified.
It has been stated that the xylanases of the GH family 10 consist of (α/β) 8 tim-barrel
fold and the xylanases of the GH family 11 generally consist of β-sheets
(Manikandan et al. 2005; Yeoman et al. 2010).
Due to the complex nature of the xylans, their enzymatic hydrolysis is harder than
that of other plant polysaccharides, so the enzymes need to work together.
ß-glycosidases and endoxylanases mediate the depolymerization of the xylan backbone (Decker et al. 2017).
Moreover, according to the xylan type, debranching enzymes such as
α-glucuronidase (EC 3.2.1.139), acetyl xylan esterase (EC 3.1.1.72), and ferulic
and coumaric acid esterases (EC 3.1.1.73) also help moving away the side groups in
xylan structure (Begemann et al. 2011; Amoozegar et al. 2019).
Xylosidases
ß-D-xylosidases are the enzymes that hydrolyze the xylooligomers in various
lengths to the xylose (Sindhu et al. 2016; Decker et al. 2017), and they are in GH
families of 3, 30, 39, 43, 51, 52, and 54 (Yeoman et al. 2010). As shown in Fig. 9.5,
xylosidases are isolated from a series of certain fungi (especially the Aspergillus
genus) (Martin Perez et al. 2017) and bacteria (Panbangred et al. 1984).
In a similar fashion to the ß-glucosidases that are in the cellulose systems, ß-Dxylosidases are crucial in moderating the final product inhibition of xylanases which
is caused by xylobiose (Yeoman et al. 2010). As the concentrations of the final
product (xylose) increases, the performance of the ß-xylosidases typically gets
inhibited (as is the case with the ß-glucosidases). Zanoelo and co-workers have
pointed out that the thermophilic ß-xylosidase, isolated from fungus (Scytalidium
thermophilum), develops a tolerance to final product inhibition (Zanoelo et al. 2004).
It is obvious that this type of feature has great importance for the bio-catalyzers in
the biofuel industry (Yeoman et al. 2010).
9 Microbial and Bioinformatics Approach in Biofuel Production
271