Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
GTs is the number of transferred sugar units. Nonprocessive GTs catalyze the transfer
of a single sugar residue to the acceptor, whereas processive GTs, such as synthases
of cellulose, chitin, and hyaluronan, attach more than one saccharide unit to respective
acceptor molecules. 4 GTs display regio- and stereospecificity with respect to
donor and acceptor structures as well as to the type of glycosidic linkage. However,
some of them recognize more than one donor of a sugar residue and form more than
one type of linkage. Non-Leloir GTs are said to be less efficient catalysts of synthesis
than Leloir GTs. However, glycogen phosphorylase successfully provides synthesis
of various not natural, tailor-made sugars with polymerization degrees up to 20,
including 2-deoxyglucose derivatives and 2-chloro-4-nitrophenyl-maltooligosaccharides.
5
In contrast to glycosidases and non-Leloir GTs, Leloir GTs provide high yield
and regio- and stereoselectivity of synthesis. Therefore, they are captivating tools
for preparative oligosaccharide synthesis, in particular, for synthesis of glycosidic
components of glycopeptides, glycoproteins, proteoglycans, and glycolipids. Table
10.1 lists examples of Leloir GTs successfully tapped for synthetic purposes. Lack
of inexpensive, pure commercial preparations of both these enzymes and sugar
nucleotides, being the substrates of Leloir GTs, limit their practical significance.
Because both chemical and enzymatic methods of synthesis of sugar nucleotides are
laborious and expensive, some new approaches have been developed, including an
in situ generation achieved by coupling of sugar synthesis catalyzed by a Leloir GT,
with UTP synthesis from UDP and PEP catalyzed by pyruvate kinase. 6 Large-scale
production of oligosaccharides can also exploit whole cells of GMOs bearing bacterial
or mammalian GTs. Some enzymes from genetically engineered bacteria have
been found to be specific toward nonnatural substrates.
GTs have also been applied for a solid-phase synthesis of oligosaccharides. 7
This approach facilitates product purification and synthesis of glycopeptides. The
modified glycopeptide contains a chain substituted with a single sugar residue and
attached to a soluble or insoluble support. Specific GTs elongate the carbohydrate
chain, followed by the cleavage of the final glycopeptide from the support, either
catalyzed by a specific protease or achieved by other methods.
10.2.3 GLYCANSUCRASES
Microbial polysaccharides such as dextran, inulin, and levan-type fructans, alternan,
and 1,4-α-D-glucans are produced by transglycosylases (glycansucrases), 8 which
are different from sugar-nucleotide-dependent GTs (Table 10.2). Glycansucrases
derive the portion of energy necessary to catalyze the transfer of a given glycosyl
group from the cleavage of the osidic bond of sucrose, which is their principal donor
of glycosyl moieties. The best producers of extracellular glycansucrases are lactic
acid bacteria of the genera Leuconostoc, Streptococcus, and Lactobacillus. Some of
these bacterial enzymes have been harnessed for large-scale production of both
polymers and oligosaccharides because, in the presence of suitable acceptor molecules
(e.g., maltose) added to sucrose, glycansucrases preferably synthesize oligomers
instead of high-molecular-mass sugars.