Biology Reference
In-Depth Information
reactions [11-14]. The other enzyme in
glycosyltransferases, which are often employed in the synthesis of
polysaccharides, is sucrase-type enzyme [9]. This class of enzymes is
the glycosyltransferase that is highly specialized in transfer either of
a glucose or of a fructose moiety in a substrate of sucrose. Thus, the
sucrase-type enzymes form either glucose-based polysaccharides
(glucans) or fructose-based polysaccharides (fructans) of different
types with respect to glycosidic linkages and side chains. Before
going into the main topic of this topic, which is the phosphorylase-
catalyzed enzymatic chain elongation, herein in this chapter, the
synthesis of polysaccharides catalyzed by hydrolases and sucrase-
type enzymes are briefly overviewed.
in both
in vivo
and
in vitro
2.2
Synthesis of Polysaccharides Catalyzed by
Hydrolases
For the synthesis of polysaccharides by the polymerization
catalyzed by hydrolases, the substrates (monomers) should be
designed as the structure of a transition state analogue [11-14].
On the basis of this concept, two types of monomers, i.e., glycosyl
fluorides and sugar oxazolines, have been designed to be efficiently
recognized by hydrolases [15,16]. The anomeric carbon is activated
by introducing fluoride or an oxazoline group (1,2-oxazoline derived
from 2-acetamido-2-deoxysugar), giving the substrates that have
structures close to a transition state of enzymatic reactions and
efficiently form the enzyme-substrate complexes. In the two types of
hydrolases, i.e., endo- and exotypes, the former has been found to be
an efficient catalyst for the enzymatic synthesis of polysaccharides.
Endotype hydrolases (endoglycosidases) cleave a glycosidic linkage
of the inner unit of polysaccharides. The shapes at the catalytic
domain of endoglycosidases look like cleft. Glycosyl fluorides have
been found to act as a glycosyl donor for enzymatic glycosylation
catalyzed by endoglycosidases. The merit using glycosyl fluorides as
the glycosyl donor originates from the unique properties of a fluorine
atom [17]. A first point is that the size of a fluorine atom is comparable
to that of a hydroxy group and it can accordingly be accepted by an
active site of an enzyme. Second, in glycosyl halides, only glycosyl
fluoride is stable as an unprotected form, which is necessary for the
