Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
been applied to alter the structure of some natural compounds, including sugars.
Mutations within gene clusters involved in carbohydrate synthesis and attachment
to aglycon moieties have resulted in construction of recombinant microbial strains
producing novel antibiotics. The wealth of possible enzymatic activities and specificities
has dramatically increased due to cultivation-independent approaches also,
based on direct DNA isolation from samples of various origin, giving rise to construction
of a huge number of genomic libraries.
10.2 ENZYMES INVOLVED IN CONVERSIONS OF
CARBOHYDRATES
Glycoside hydrolases (GHs) and glycosyltransferases (GTs) have been undoubtedly
the most important biocatalysts exploited for conversions of sugars, independently
on their scale, but recently oxidoreductases, isomerases, and lyases have also gained
much interest. The basic information on GHs, GTs, as well as carbohydrate lyases
and esterases, including their classification, structure and origin, is presented on the
regularly updated CAZY web server. 2 A grouping of these enzymes into families
and clans, based on primary-structure similarities and hydrophobic cluster analysis
data, appears to be superior over commonly applied classification according to their
substrate specificity.
10.2.1 GLYCOSIDE HYDROLASES
Glycosidases catalyze glycoside bond hydrolysis. However, under certain conditions
they can also synthesize such linkages. Although GHs are stereospecific and precisely
recognize the type of a bond (e.g., α-1,4, β-1,6, etc.), they are not always
regiospecific, and produce mixtures of products. GHs are advantageous as catalysts
of synthesis because of availability of their relatively inexpensive pure preparations
and low prices of substrates. However, a poor synthesis yield is their main drawback.
A satisfactory degree of glycosidic bond formation requires a large excess of substrates,
preferably activated glycosyl donors (e.g., glycosyl fluorides) and acceptors,
or organic solvents. The controlled reaction conditions can also reduce the number
of product regioisomers. Transglycosylation activity is displayed by a number of
hydrolases acting either in an exo or in an endo mode (Table 10.1). The application
of glycosidases has been usually restricted to synthesis of short oligosaccharides.
Ultrasounds carried out at high substrate concentrations improve the yield of hydrolysis
catalyzed by α-amylase, glucoamylase, and invertase. 3
10.2.2 GLYCOSYLTRANSFERASES
Glycosyltransferases catalyze the transfer of a monosaccharide unit from a donor to
the saccharide acceptor. Based on the structure of the activating group at the anomeric
center of the donor, they are categorized as Leloir (sugar-nucleotide-dependent) and
non-Leloir (sugar-1P-dependent) enzymes. 2 Another criterion for differentiation of