bond of pullulan to produce panose (6-alpha-D-glucosylmaltose). They also catalyse
transglycosylation reactions, with the formation of new α-1-4 or α-1-6 glycosidic
bonds.
2.4 Transferases
Transferases cleave the α-1-4 glycosidic bond of a donor molecule and transfer part
of the donor molecule to a glycosidic acceptor with the formation of a new glycosidic bond. Examples of this group of enzymes include amylomaltase (EC 2.4.1.25)
and cyclodextrin glycosyltransferases or CGTases (EC 2.4.1.19), which form new
α-1-4 glycosidic bonds, and 1,4-α-glucan branching enzymes (EC 2.4.1.18), which
form new α-1-6 glycosidic bonds. CGTases make cyclic oligosaccharides with 6, 7
or 8 glucose residues by intramolecular transglycosylation. They also produce highly
branched high-molecular-weight dextrins, referred to as glycosyltransferase limit
dextrins. Amylomaltase, on the other hand, performs transglycosylation reactions
resulting in the formation of linear products as opposed to cyclic products [66].
3 Starch-Modifying Enzymes from Alkaliphiles
Alkaliphiles are an important source of alkaline-active enzymes. Members of the
genus Bacillus from the alkaliphilic microbial community are major producers of
alkaline-active enzymes of industrial importance such as α-amylases. Most of
these enzymes are secreted outside the cells, thus making their purification easier.
Furthermore, the high growth rate of strains within this genus gives Bacillus species
an added advantage in the production of enzymes of industrial interest. Currently,
industrially important amylases are produced by recombinant DNA technology,
and the source of genes encoding commercial amylases are from Bacillus species.
Considerable diversity of α-amylases from alkaliphilic Bacillus species has been
reported, in terms of activity and stability features of the enzymes, substrate specificity and products formed.
The first report of an alkaline-active α-amylase was from alkaliphilic Bacillus
sp. strain A-40-2 [17]. The enzyme had a molecular weight of 70 kDa and was
most active at pH 10.0–10.5, retaining 50% of activity between pH 9 and 11.5.
It was classified as a saccharifying amylase, as it hydrolysed 70% of starch
to produce mainly glucose, maltose and maltotriose. Several alkaline-active amylases from Bacillus halodurans species have been reported, including Bacillus
sp. A-59 by Horikoshi and co-workers [18]. This species was reported to produce
α-amylase, pullulanase and α-glucosidase [20]. Other alkaline-active enzymes
that modify starch including maltooligosaccharide-forming amylases, cyclodextrin
glycosyltransferases, pullulanases and isoamylases have also been reported extensively and discussed in the context of their industrial applications in subsequent
sections.
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S. O. Hashim
transglycosylation reactions, with the formation of new α-1-4 or α-1-6 glycosidic
bonds.
2.4 Transferases
Transferases cleave the α-1-4 glycosidic bond of a donor molecule and transfer part
of the donor molecule to a glycosidic acceptor with the formation of a new glycosidic bond. Examples of this group of enzymes include amylomaltase (EC 2.4.1.25)
and cyclodextrin glycosyltransferases or CGTases (EC 2.4.1.19), which form new
α-1-4 glycosidic bonds, and 1,4-α-glucan branching enzymes (EC 2.4.1.18), which
form new α-1-6 glycosidic bonds. CGTases make cyclic oligosaccharides with 6, 7
or 8 glucose residues by intramolecular transglycosylation. They also produce highly
branched high-molecular-weight dextrins, referred to as glycosyltransferase limit
dextrins. Amylomaltase, on the other hand, performs transglycosylation reactions
resulting in the formation of linear products as opposed to cyclic products [66].
3 Starch-Modifying Enzymes from Alkaliphiles
Alkaliphiles are an important source of alkaline-active enzymes. Members of the
genus Bacillus from the alkaliphilic microbial community are major producers of
alkaline-active enzymes of industrial importance such as α-amylases. Most of
these enzymes are secreted outside the cells, thus making their purification easier.
Furthermore, the high growth rate of strains within this genus gives Bacillus species
an added advantage in the production of enzymes of industrial interest. Currently,
industrially important amylases are produced by recombinant DNA technology,
and the source of genes encoding commercial amylases are from Bacillus species.
Considerable diversity of α-amylases from alkaliphilic Bacillus species has been
reported, in terms of activity and stability features of the enzymes, substrate specificity and products formed.
The first report of an alkaline-active α-amylase was from alkaliphilic Bacillus
sp. strain A-40-2 [17]. The enzyme had a molecular weight of 70 kDa and was
most active at pH 10.0–10.5, retaining 50% of activity between pH 9 and 11.5.
It was classified as a saccharifying amylase, as it hydrolysed 70% of starch
to produce mainly glucose, maltose and maltotriose. Several alkaline-active amylases from Bacillus halodurans species have been reported, including Bacillus
sp. A-59 by Horikoshi and co-workers [18]. This species was reported to produce
α-amylase, pullulanase and α-glucosidase [20]. Other alkaline-active enzymes
that modify starch including maltooligosaccharide-forming amylases, cyclodextrin
glycosyltransferases, pullulanases and isoamylases have also been reported extensively and discussed in the context of their industrial applications in subsequent
sections.
226
S. O. Hashim