9.4.2.4 Isoamylases (EC 3.2.1.68)
Isoamylases are also known as glycogen-6-glucanohydrolase. They hydrolyze especially the α-1,6 glycosidic linkages in the branched polysaccharides such as amylopectin, β-limit dextrin, or glycogen (Van der Maarel et al. 2002; Hii et al. 2012;
Elleuche and Antranikian 2013).
However, they are unable to hydrolyze the pullulan or branched oligosaccharides
that have α-1,6 linkages. For this reason, they exert little effect on α-limit dextrin
(Bertoldo and Antranikian 2001).
9.4.2.5 α-Glucosidases (EC 3.2.1.20)
α-Glucosidases are also known as α-D-glucoside glucohydrolase. Usually, they take
part in the last step of the degradation of starch (Lévêque et al. 2000).
In contrast to glucoamylases, they prefer disaccharides and oligosaccharides as
the substrate, and they act on the α-1,4 glycosidic linkages in these structures
(Bertoldo and Antranikian 2002). They belong to the GH families of 4, 13, 31,
63, 97, and 122 (Henrissat 1991).
With some exceptions, their optimum pHs are quite acidic and they are thermostable enzymes (Lévêque et al. 2000; Elleuche and Antranikian 2013).
9.4.2.6 Cyclodextrin Glycosyltransferases (EC 2.4.1.19)
They are also known as CGTases or α-1,4-D-glucan α-4-D-(α-1,4-D-glucano)
transferase. They form non-reductive cyclodextrins (in α, ß, γ forms) via α-1,4
glycosidic linkages by degrading starch, amylose, or oligosaccharides (Gawande
et al. 1999). Besides bacteria (e.g., Anaerobranca gottschalkii), they are also found
in archaea (e.g., Thermococcus sp.) (Bertoldo and Antranikian 2002).
9.4.3 Pullulan and Pullulanolytic Enzymes
Pullulan is an exopolysaccharide synthesized by fungus Aureobasidium pullulans
(Wu et al. 2010). It is a linear homopolymer consisting of maltotriose subunits linked
with α-1,6 glycosidic linkages and isopanose/panose subunits linked with α-1,4
glycosidic linkages (Singh et al. 2008b). Moreover, it is renewable, biologically
degradable, and nontoxic (Prajapati et al. 2013) and plays a significant role as a
model substrate in the determination of the activities of the enzymes that hydrolyze
the branched structures of starch. The α-1,6 glycosidic linkages in its structure are
similar to those in the branch points of amylopectin.
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Isoamylases are also known as glycogen-6-glucanohydrolase. They hydrolyze especially the α-1,6 glycosidic linkages in the branched polysaccharides such as amylopectin, β-limit dextrin, or glycogen (Van der Maarel et al. 2002; Hii et al. 2012;
Elleuche and Antranikian 2013).
However, they are unable to hydrolyze the pullulan or branched oligosaccharides
that have α-1,6 linkages. For this reason, they exert little effect on α-limit dextrin
(Bertoldo and Antranikian 2001).
9.4.2.5 α-Glucosidases (EC 3.2.1.20)
α-Glucosidases are also known as α-D-glucoside glucohydrolase. Usually, they take
part in the last step of the degradation of starch (Lévêque et al. 2000).
In contrast to glucoamylases, they prefer disaccharides and oligosaccharides as
the substrate, and they act on the α-1,4 glycosidic linkages in these structures
(Bertoldo and Antranikian 2002). They belong to the GH families of 4, 13, 31,
63, 97, and 122 (Henrissat 1991).
With some exceptions, their optimum pHs are quite acidic and they are thermostable enzymes (Lévêque et al. 2000; Elleuche and Antranikian 2013).
9.4.2.6 Cyclodextrin Glycosyltransferases (EC 2.4.1.19)
They are also known as CGTases or α-1,4-D-glucan α-4-D-(α-1,4-D-glucano)
transferase. They form non-reductive cyclodextrins (in α, ß, γ forms) via α-1,4
glycosidic linkages by degrading starch, amylose, or oligosaccharides (Gawande
et al. 1999). Besides bacteria (e.g., Anaerobranca gottschalkii), they are also found
in archaea (e.g., Thermococcus sp.) (Bertoldo and Antranikian 2002).
9.4.3 Pullulan and Pullulanolytic Enzymes
Pullulan is an exopolysaccharide synthesized by fungus Aureobasidium pullulans
(Wu et al. 2010). It is a linear homopolymer consisting of maltotriose subunits linked
with α-1,6 glycosidic linkages and isopanose/panose subunits linked with α-1,4
glycosidic linkages (Singh et al. 2008b). Moreover, it is renewable, biologically
degradable, and nontoxic (Prajapati et al. 2013) and plays a significant role as a
model substrate in the determination of the activities of the enzymes that hydrolyze
the branched structures of starch. The α-1,6 glycosidic linkages in its structure are
similar to those in the branch points of amylopectin.
278
T. Karaytuğ et al.