Glucuronidases
α-D-glucuronidases are enzymes that exist in family 67 and participate in the
hydrolysis of the α-1,2 glycosidic bonds between 4-0-methyl-α-glucuronic acid
and the terminal, non-reducing xylopyranosyl end unit of small oligosaccharides
(Mierzwa et al. 2005; Decker et al. 2017).
α-1,2 bonds are the areas that slow down the hydrolysis in the enzymatic
hydrolysis process of xylane. Xylan α-1,2 glucuronidases use paranitrophenyl
α-D-glucuronide as the substrate, but this enzyme is specified for a glucuronamide
with an α (1!2) link (Saraswat and Bisaria 1997).
As indicated in Fig. 9.5, this kind of enzymes can be obtained from various
microorganism groups (Khandke et al. 1989; Suresh et al. 2003).
9.3.3.2 Endoarabinases and Arabinofuranosidases
Endoarabinases and arabinofuranosidases are the enzymes that play catalytic role in
the degradation of the hemicellulosic biomass.
Endoarabinases in GH43 family function by making synergic effect with α-Larabinofuranosidases in the hydrolysis of the arabinan which is between the
hemicellulosic and pectic polysaccharides (Verhertbruggen et al. 2009; Prasad
et al. 2019).
Contrary to α-L-arabinases, α-L-arabinofuranosidases are exolytic enzymes of
the GH55 family (Wongratpanya et al. 2015) that hydrolyze α-1,2, α-1,3, and α-1,5
glycosidic bonds of arabinofuranosides in arabinan, as well as the α-1,2 and α-1,3
bonds in arabinoxylan and arabinogalactan. (Matsuo et al. 2000; Yeoman et al.
2010).
There are very few examples of characterized arabinases, which are obtained only
from some bacteria (Hong et al. 2009; Seo et al. 2010), whereas the
arabinofuranosidases are obtained from both bacteria and fungi (Kaji and Tagawa
1970; Gilead and Shoham 1995) (Fig. 9.5).
9.3.3.3 Esterases (EC 3.1)
In addition to pectin, plant cell wall polysaccharides that consist of hemicellulose
compounds such as xylans, mannans, and glucomannans in particular are usually
acetylated and sometimes feruloylated, with O-linked acetyl groups (Yeoman et al.
2010). In a study on the xylans in the hardwoods, it has been stated that the 60–70%
of xylose residues have been esterified with acetic acid (Shao and Wiegel 1995).
For this reason, to obtain efficient results in the hydrolyzation of the xylans and to
catalyze the hydrolysis of ester bonds, some enzymes from the carbohydrate esterase
(CE) family are needed (acetyl esterase [EC 3.1.1.6], acetyl xylan esterase
[EC 3.1.1.72], and ferulic acid esterase [EC 3.1.1.73]) (Linden et al. 1994; Karnaouri
et al. 2019).
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α-D-glucuronidases are enzymes that exist in family 67 and participate in the
hydrolysis of the α-1,2 glycosidic bonds between 4-0-methyl-α-glucuronic acid
and the terminal, non-reducing xylopyranosyl end unit of small oligosaccharides
(Mierzwa et al. 2005; Decker et al. 2017).
α-1,2 bonds are the areas that slow down the hydrolysis in the enzymatic
hydrolysis process of xylane. Xylan α-1,2 glucuronidases use paranitrophenyl
α-D-glucuronide as the substrate, but this enzyme is specified for a glucuronamide
with an α (1!2) link (Saraswat and Bisaria 1997).
As indicated in Fig. 9.5, this kind of enzymes can be obtained from various
microorganism groups (Khandke et al. 1989; Suresh et al. 2003).
9.3.3.2 Endoarabinases and Arabinofuranosidases
Endoarabinases and arabinofuranosidases are the enzymes that play catalytic role in
the degradation of the hemicellulosic biomass.
Endoarabinases in GH43 family function by making synergic effect with α-Larabinofuranosidases in the hydrolysis of the arabinan which is between the
hemicellulosic and pectic polysaccharides (Verhertbruggen et al. 2009; Prasad
et al. 2019).
Contrary to α-L-arabinases, α-L-arabinofuranosidases are exolytic enzymes of
the GH55 family (Wongratpanya et al. 2015) that hydrolyze α-1,2, α-1,3, and α-1,5
glycosidic bonds of arabinofuranosides in arabinan, as well as the α-1,2 and α-1,3
bonds in arabinoxylan and arabinogalactan. (Matsuo et al. 2000; Yeoman et al.
2010).
There are very few examples of characterized arabinases, which are obtained only
from some bacteria (Hong et al. 2009; Seo et al. 2010), whereas the
arabinofuranosidases are obtained from both bacteria and fungi (Kaji and Tagawa
1970; Gilead and Shoham 1995) (Fig. 9.5).
9.3.3.3 Esterases (EC 3.1)
In addition to pectin, plant cell wall polysaccharides that consist of hemicellulose
compounds such as xylans, mannans, and glucomannans in particular are usually
acetylated and sometimes feruloylated, with O-linked acetyl groups (Yeoman et al.
2010). In a study on the xylans in the hardwoods, it has been stated that the 60–70%
of xylose residues have been esterified with acetic acid (Shao and Wiegel 1995).
For this reason, to obtain efficient results in the hydrolyzation of the xylans and to
catalyze the hydrolysis of ester bonds, some enzymes from the carbohydrate esterase
(CE) family are needed (acetyl esterase [EC 3.1.1.6], acetyl xylan esterase
[EC 3.1.1.72], and ferulic acid esterase [EC 3.1.1.73]) (Linden et al. 1994; Karnaouri
et al. 2019).
272
T. Karaytuğ et al.