Enzyme Technology
Glycosidases used in synthetic reactions
This is a comparatively neglected topic
probably because polysaccharides can be obtained readily from plant or microbial
sources, because polysaccharide function is not so specifically related to its
structure, and because the theory describing the functional significance of
oligosaccharides is still being developed. As there is an abundance of knowledge
about glycosidases and their specificities, there is no fundamental reason why
they should not be used to synthesise oligosaccharides. There are few examples
in the literature of oligosaccharides being constructed using enzymes such as
dextransucrase (EC 2.4.1.5), levansucrase (EC 2.4.1.10), cyclomaltodextrin
glucanotransferase (EC 2.4.1.19) and b-galactosidase (EC 3.2.1.23) to
build up glucose, fructose or galactose residues on suitable acceptors.
Fructosyltransferases (e.g., inulosucrase, EC 2.4.1.9) have been used to
synthesise compounds such as sucrose-6-acetate, xylosucrose
(b-D-fructofuranosyl-(2,1)-a-D-xylopyranoside) and the low
cariogenic sweetener, 'neosugar' which consists of a mixture of glucose, sucrose
and b-2,1 linked fructans with terminal non-reducing glucose residues
(typical composition before chromatographic refinement: 2% fructose, 26%
glucose, 11% sucrose, 30% 1-kestose
(b-D-fructofuranosyl-(2,1)-b-D-fructofuranosyl-
(2,1)-a-D-glucopyranose), 25% nystose (b-D-fructofuranosyl-
(2,1)-b-D-fructofuranosyl-(2,1)-b-D-fructofuranosyl-(2,1)-a-D-
glucopyranose) and 6% higher oligosaccharides).
sucrose + glucose-6-acetate
sucrose-6-acetate + glucose [7.12]
sucrose + xylose
xylosucrose + glucose [7.13]
sucrose + sucrose
1-kestose + glucose [7.14]
1-kestose + sucrose
nystose + glucose [7.15]
As shown in reaction scheme
[7.16], dextransucrase can be used to produce dextran (a polymer
of a-1,6 linked glucose residues) from sucrose. Dextran is used in gel
chromatographic media, such as Sephadex, in the aqueous 2-phase systems already
described, and as a blood plasma extender. For any of these uses it is
undesirable to have cells remaining in the dextran so the dextransucrase,
produced extracellularly by Leuconostoc mesenteroides is purified before use. The size of dextran
molecules produced may be controlled by including small concentrations of sugars
such as maltose which compete with sucrose as acceptors for glucose residues
from the donor sucrose molecules.
sucrose + dextran (Gn)
fructose + dextran (Gn+1) [7.16]
Thermodynamically controlled
synthesis of oligosaccharides, catalysed by glycosidases, is possible by use of
high substrate concentrations and a 'molecular trap' to remove the products. For
example, b-galactosidase may be used to synthesise N-acetyllactosamine
(b-D-galactopyranosyl-(1,6)-2-acetamido- 2-deoxy-D-glucose) using a
carbon-celite column to remove it, as formed.
[7.17]
There seems to have been
no systematic attempt to construct hetero-oligosaccharides in a stepwise
movement yet thermodynamically or kinetically controlled reactions could be used
just as in peptide synthesis.
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This page was established in 2004 and last updated by Martin
Chaplin on
6 August, 2014
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