Biology Reference
In-Depth Information
OH
OH
O
hydrolase
O
+
H
2
O
+
HO
R
HO
HO
HO
HO
OH
OH
OR
OH
OH
OH
Leloir glycosyltransferase
O
O
+
+
HO
R
HO
UDP
HO
HO
HO
OH
OH
OUDP
OR
OH
OH
phosphorylase
O
inorganic
phosphate
O
+
+
HO
R
HO
HO
HO
O
HO
OH
OH
OR
OP
O
O
Figure 2.3
Typical enzymes involved in the synthesis of polysaccharides.
be employed in the practical synthesis of saccharide chains via
glycosylation. In such glycosylations catalyzed by phophorylases, the
glycose 1-phosphates are used as a glycosyl donor and the glycose
unit is transferred from the substrate to a nonreducing end of an
appropriate glycosyl acceptor to form a stereo- and regiocontrolled
glycosidic linkage accompanied with the production of inorganic
phosphate.
Leloir glycosyltransferases are biologically important because
they perform the role of synthesizing saccharide chains
in vivo
[10]. The reactions of the enzymes are irreversible in the synthetic
direction because of the requirement for cleavage of the high-
energy linkage of the glycosyl nucleotide substrates. However,
Leloir glycosyltransferases are generally transmembrane-type
proteins, present in nature in very small amounts, and unstable for
isolation and purification. Therefore, the enzymes are expensive
and hardly available. Hydrolases have been frequently employed in
the hydrolysis of polysaccharides, which are industrially important
in the utilization of natural polysaccharides such as starch. The
hydrolase catalysis using natural polysaccharides readily proceed in
the way to hydrolysis under the normal conditions in aqueous media.
However, when an enzyme-substrate complex is formed, hydrolases
catalyzing hydrolysis
in vivo
are able to catalyze a glycosylation
in vitro
to produce the saccharide chains. This view is based on a
hypothesis that the structures of transition states are very close
