phosphorylation prior to the condensation step. The activating group acts as a
leaving group (LG) and is either a nucleoside diphosphate (in the Leloir pathway
[1795]) or a simple phosphate (in non-Leloir enzymes [1796]).
Trans-glycosidases interconvert carbohydrate chains by transferring one sugar
unit from a (di)saccharide (acting as donor) onto an acceptor and they represent an
alternative pathway for gluconeogenesis, which is independent on phosphorylated
donors. Due to their role in biosynthesis, glycosyl transferases and transglycosidases are generally rather specific with respect to their substrate(s) and the
nature of the glycosidic bond to be formed [1797].
Glycosidases belong to the class of hydrolytic enzymes and have a catabolic
function in vivo as they hydrolyze glycosidic linkages to form mono- or oligosaccharides from polysugars. Consequently, they are generally less specific when
compared to glycosyl transferases. Glycosidases can be used for glycoside synthesis in the reverse (condensation) direction.
Glycosyl phosphorylases use phosphate (instead of water) for the breakdown of
a glycosidic bond, which yields a glycosyl phosphate rather than a non-activated
monosugar. The phosphorolytic degradation of oligosaccharides is a more energyefficient alternative to hydrolysis by glycosidases.
2.6.1.1 Glycosyl Transferases
In the Leloir-pathway, a sugar is phosphorylated in a first step by a kinase to give a
sugar-1-phosphate. This activated sugar subsequently reacts with a nucleoside triphosphate under catalysis of a nucleoside transferase and forms a chemically activated
nucleoside diphosphate sugar (NDP, Scheme 2.212) [1798]. These key nucleoside
diphosphate sugars constitute the activated ‘donors’ in the subsequent condensation
with the hydroxyl group ‘acceptors’ a mono- or oligosaccharide, a protein or a lipid,
= phosphate
P
HO
O
HO
O
HO
O
disaccharide or
(oligo+1)-saccharide
D
A
O-R
HO
HO
O
A
O-R
HO
O
HO
O-NDP
D
+
Glycosyl Transferase
(Leloir)
NDP
HO
HO
O
A
O-R
HO
O
HO
O
D
+
Glycosyl
Phosphorylase
P
P
Trans-Glycosidase
Glycosidase
H 2 O
HO
HO
O
A
O-R
HO
O
HO
OH
D
+
D = monosaccharide donor A = mono- or oligosaccharide acceptor NDP = nucleoside diphosphate
+
HO
O
HO
O
HO
O
D
A'
O-R'
HO
HO
O
A
O-R
HO—A'
Scheme 2.212 Four different types of enzymatic glycosylation reactions
236
2 Biocatalytic Applications
leaving group (LG) and is either a nucleoside diphosphate (in the Leloir pathway
[1795]) or a simple phosphate (in non-Leloir enzymes [1796]).
Trans-glycosidases interconvert carbohydrate chains by transferring one sugar
unit from a (di)saccharide (acting as donor) onto an acceptor and they represent an
alternative pathway for gluconeogenesis, which is independent on phosphorylated
donors. Due to their role in biosynthesis, glycosyl transferases and transglycosidases are generally rather specific with respect to their substrate(s) and the
nature of the glycosidic bond to be formed [1797].
Glycosidases belong to the class of hydrolytic enzymes and have a catabolic
function in vivo as they hydrolyze glycosidic linkages to form mono- or oligosaccharides from polysugars. Consequently, they are generally less specific when
compared to glycosyl transferases. Glycosidases can be used for glycoside synthesis in the reverse (condensation) direction.
Glycosyl phosphorylases use phosphate (instead of water) for the breakdown of
a glycosidic bond, which yields a glycosyl phosphate rather than a non-activated
monosugar. The phosphorolytic degradation of oligosaccharides is a more energyefficient alternative to hydrolysis by glycosidases.
2.6.1.1 Glycosyl Transferases
In the Leloir-pathway, a sugar is phosphorylated in a first step by a kinase to give a
sugar-1-phosphate. This activated sugar subsequently reacts with a nucleoside triphosphate under catalysis of a nucleoside transferase and forms a chemically activated
nucleoside diphosphate sugar (NDP, Scheme 2.212) [1798]. These key nucleoside
diphosphate sugars constitute the activated ‘donors’ in the subsequent condensation
with the hydroxyl group ‘acceptors’ a mono- or oligosaccharide, a protein or a lipid,
= phosphate
P
HO
O
HO
O
HO
O
disaccharide or
(oligo+1)-saccharide
D
A
O-R
HO
HO
O
A
O-R
HO
O
HO
O-NDP
D
+
Glycosyl Transferase
(Leloir)
NDP
HO
HO
O
A
O-R
HO
O
HO
O
D
+
Glycosyl
Phosphorylase
P
P
Trans-Glycosidase
Glycosidase
H 2 O
HO
HO
O
A
O-R
HO
O
HO
OH
D
+
D = monosaccharide donor A = mono- or oligosaccharide acceptor NDP = nucleoside diphosphate
+
HO
O
HO
O
HO
O
D
A'
O-R'
HO
HO
O
A
O-R
HO—A'
Scheme 2.212 Four different types of enzymatic glycosylation reactions
236
2 Biocatalytic Applications