9.3.3 Hemicelluloses
Hemicelluloses are heterogeneous polymers which consist of the pentose (xylose
and arabinose) and hexose sugars (mannose, glucose and galactose) and sugar acids
that are present in the primary and secondary cell walls of the plants (Saha 2003;
Scheller and Ulvskov 2010; Sorieul et al. 2016). They are energy rich structures due
to the sugar mixtures in their composition. They are attached to lignin sheaths
through covalent bonds and to the cellulose by hydrogen bonds (Bailey et al.
1992; Sethi and Scharf 2013; Amoozegar et al. 2019).
Hemicelluloses found in different plant species contain a wide variety of polysaccharides such as glucomannan, galactomannan, galactoglucomannan,
glucuronoarabinoxylan, xyloglucan, mixed-linkage glucans, ß-D(1–4) bonds,
arabinogalactan, and ß-(1–6) glycosidic bonds (Buchanan et al. 2015; Sorieul et al.
2016). Besides, softwood hemicelluloses contain mostly glucomannan, whereas the
hardwood hemicelluloses contain rather more xylan (Saha 2003; Huang et al.
2016b).
For this reason, according to the complex structures of the hemicelluloses and the
kind of pretreatment, quite a lot of hemicelluloytic enzymes are needed for the
breakdown of the substrate (Yeoman et al. 2010; Álvarez et al. 2016). These
hemicelluloytic enzymes contain endoxylanases (EC 3.2.1.8), xylosidases
(EC 3.2.1.37), xyloglucanases (EC 3.2.1.151), endomannanases (3.2.1.78),
mannosidases (EC 3.2.1.25), fucosidases (EC 3.2.1.63), arabinofuranosidases
(EC 3.2.1.55), glucuronidases (EC 3.2.1.31), xylan esterases (EC 3.1.1.72), ferulic
acid esterases (EC 3.1.1.73), and p-coumaric acid esterases (EC 3.1.1.B10) (Bosetto
et al. 2016; Razeq et al. 2018; López-Mondéjar et al. 2019).
Therefore, in contrast to cellulases, the hemicellulases are a broader spectrum of
enzymes.
9.3.3.1 Xylan and Xylolytic Enzymes
Xylan is the hemicellulose that forms the ~ 1 = 4 – 1 = 3 of plant biomass and is the most
abundant component in the cell walls of most plants (Prade 1996). Also, it is a kind
of polysaccharide that consists of xylopyranosyl units bound with ß-1,4 glycosidic
bonds (Polizeli et al. 2005).
In a general sense, a xylan polymer usually contains a linear backbone of the
ß-1,4-D-xylopyranoside residues which can change places with acetyl,
arabinofuranosyl, and 4-0-methyl glucuronyl groups (Yeoman et al. 2010;
Amoozegar et al. 2019).
Most of the xylans contain variable groups in their main and side chains. This
variability depends on the source of the plant material (Puls 1997; Wainø and
Ingvorsen 2003).
The complete hydrolysis of xylan requires the synergistic action of various
enzymes, in proportion to its complex structure (Yeoman et al. 2010).
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T. Karaytuğ et al.
Hemicelluloses are heterogeneous polymers which consist of the pentose (xylose
and arabinose) and hexose sugars (mannose, glucose and galactose) and sugar acids
that are present in the primary and secondary cell walls of the plants (Saha 2003;
Scheller and Ulvskov 2010; Sorieul et al. 2016). They are energy rich structures due
to the sugar mixtures in their composition. They are attached to lignin sheaths
through covalent bonds and to the cellulose by hydrogen bonds (Bailey et al.
1992; Sethi and Scharf 2013; Amoozegar et al. 2019).
Hemicelluloses found in different plant species contain a wide variety of polysaccharides such as glucomannan, galactomannan, galactoglucomannan,
glucuronoarabinoxylan, xyloglucan, mixed-linkage glucans, ß-D(1–4) bonds,
arabinogalactan, and ß-(1–6) glycosidic bonds (Buchanan et al. 2015; Sorieul et al.
2016). Besides, softwood hemicelluloses contain mostly glucomannan, whereas the
hardwood hemicelluloses contain rather more xylan (Saha 2003; Huang et al.
2016b).
For this reason, according to the complex structures of the hemicelluloses and the
kind of pretreatment, quite a lot of hemicelluloytic enzymes are needed for the
breakdown of the substrate (Yeoman et al. 2010; Álvarez et al. 2016). These
hemicelluloytic enzymes contain endoxylanases (EC 3.2.1.8), xylosidases
(EC 3.2.1.37), xyloglucanases (EC 3.2.1.151), endomannanases (3.2.1.78),
mannosidases (EC 3.2.1.25), fucosidases (EC 3.2.1.63), arabinofuranosidases
(EC 3.2.1.55), glucuronidases (EC 3.2.1.31), xylan esterases (EC 3.1.1.72), ferulic
acid esterases (EC 3.1.1.73), and p-coumaric acid esterases (EC 3.1.1.B10) (Bosetto
et al. 2016; Razeq et al. 2018; López-Mondéjar et al. 2019).
Therefore, in contrast to cellulases, the hemicellulases are a broader spectrum of
enzymes.
9.3.3.1 Xylan and Xylolytic Enzymes
Xylan is the hemicellulose that forms the ~ 1 = 4 – 1 = 3 of plant biomass and is the most
abundant component in the cell walls of most plants (Prade 1996). Also, it is a kind
of polysaccharide that consists of xylopyranosyl units bound with ß-1,4 glycosidic
bonds (Polizeli et al. 2005).
In a general sense, a xylan polymer usually contains a linear backbone of the
ß-1,4-D-xylopyranoside residues which can change places with acetyl,
arabinofuranosyl, and 4-0-methyl glucuronyl groups (Yeoman et al. 2010;
Amoozegar et al. 2019).
Most of the xylans contain variable groups in their main and side chains. This
variability depends on the source of the plant material (Puls 1997; Wainø and
Ingvorsen 2003).
The complete hydrolysis of xylan requires the synergistic action of various
enzymes, in proportion to its complex structure (Yeoman et al. 2010).
270
T. Karaytuğ et al.