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lowers the activation energy compared with the nonenzymatic
case.
Similar to the general glycosylation, enzymatic formation of a
glycosidic linkage between C-1 atom of a monosaccharide and one
of the hydroxy groups of the other monosaccharide can be realized
by the reaction of an activated glycosyl donor and a glycosyl acceptor
(Fig. 2.2) [6]. First, the glycosyl donor is recognized by an enzyme
to form a glycosyl-enzyme intermediate (or transition state). Then,
the intermediate is attacked by the hydroxy group of the glycosyl
acceptor, giving a glycoside. On the basis of the above characteristics
of the enzymatic reactions, it has generally been well accepted that
enzymatic glycosylation is a very powerful tool for the stereo- and
regioselective construction of the glycosidic linkages under the mild
conditions, where a glycosyl donor and a glycosyl acceptor can be
employed in their unprotected forms, leading to the direct formation
of unprotected saccharide chains in aqueous media [7]. Thus,
repetition of the enzymatic glycosylations forms polysaccharides
with well-defined structure. As mentioned earlier, enzymes involved
in the synthesis of polysaccharides are categorized into two main
classes: hydrolytic enzymes (hydrolases) and glycosyltransferases
(Fig. 2.3). The latter is precisely subclassified further into synthetic
enzymes (Leloir glycosyltransferases) [3], phosphorolytic enzymes
(phosphorylases) [8], and others [9].
Phosphorylases, which this topic focuses on as a catalyst for
the efficient production of polysaccharide materials, catalyze
phosphorolytic cleavage of a glycosidic linkage in the saccharide
chain in the presence of inorganic phosphate to produce glycose
1-phosphate and the saccharide chain with one smaller DP.
Because the bond energy of the produced phosphate is comparable
with that of the glycosidic linkage, the phosphorylase-catalyzed
reactions exhibit reversible nature. Therefore, phosphorylases can
O
HO
O
O
glycosyl acceptor
O
O
+
X
EnzH
O
Enz
glycoside
glycosyl donor
glycosyl-enzyme intermediate
+
EnzH
+
HX
Figure 2.2
General reaction scheme for enzymatic glycosylation.
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