Chemistry Reference
In-Depth Information
them difficult and time-consuming to synthesize by organic chemistry. Che-
mical methods for the preparation of peptides and oligonucleotides have been
developed which are robust and automated. This has not been possible for the
chemical preparation of complex oligosaccharides and glyco-conjugates.
Thus, researchers have turned to enzymatic methods.
Enzymatic synthesis of oligosaccharides using glycosyltransferases of
the Leloir pathway can overcome the problems encountered in the chemical
synthesis of specific oligosaccharides. However, these enzymatic syntheses
require sugar nucleotides and glycosyltransferases, both of which have not
been readily available or, at least, not cost-effectively. Such synthetic methods
will not be covered in detail in this chapter, but are well reviewed elsewhere
(Prenosil et al., 1987a; Nilsson, 1988; Rastall and Bucke, 1992; Koizumi et al.,
1998). Koizumi et al. (1998) discuss the large-scale production of uridine
5
0
-diphosphate (UDP)-galactose and globotriose. Globotriose is the trisac-
charide portion of a verotoxin receptor and has the structure
-
D
-Gal (1
!
4)-
-
D
-Gal (1
!
4)-
D
-Glc.
The instability of glycosyltransferases as synthetic reagents has led
some workers to develop immobilized glycosyltransferases for oligosacchar-
ide synthesis (Rastall and Bucke, 1992). For specific synthesis of oligosac-
charides of medical interest, a large number of different glycosyltransferases
are required, and their availability is a limiting factor. As more glycosyltrans-
ferases are isolated from nature or formed by recombinant techniques, it has
become increasingly possible to form the specific physico-chemical structures
required.
The usual substrates for galactosyltransferases are UDP-galactose and
either a free N-acetylglucosamine or an N-acetylglucosamine bound to a
protein molecule. The complexity and cost of methods involving glycosyl-
transferases have precluded their use in food applications to date. However,
several companies are interested in the production of longer chain, specifi-
cally tailored oligosaccharides for use in the functional food sector. The main
application of synthesis systems using glycosyltransferases remains in the
biomedical and pharmaceutical areas.
In 2007, the Sigma-Aldrich Company (USA) listed four glycosyltrans-
ferases (Table 5.3) in their catalogue, which were also available in different
formulations and as kits with accompanying substrates and cofactors (Sigma-
Aldrich, 2007). Other major biochemical supply houses carry some glycosyl-
transferase products. There are also a number of specialist glycoscience
commercial groups worldwide, some of which are developing specific oligo-
saccharide pharmaceuticals, others providing specialist analytical services in
the area and others offering custom manufacture and the supply of specific
oligosaccharides (see Playne, 2002b for details).
