Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

Enzymatic Hydrolysis and Group Transfer Reactions 111
Glu 35
Glu 35
O
RO
OH
O O
H
O
O
HO
NHAc
OH
O
O
NHAc
O
RO
O O−
OH
+
O
HO
HO
NHAc
OH
O
O
NHAc
−O
O
−O
O
Asp 52
Asp 52
Glu 35
Glu 35
O
RO
OH
O O
H
O
OH
NHAc
−O O
O
RO
OH
O O−
H
+
O O H
NHAc
−O O
O
RO
OH
O
AcNH
O
O
Asp 52
Asp 52
Asp 52
Figure 5.38 Mechanism for lysozyme.
of the substrate simply did not Wt! However, if the pyranose ring of this residue
adopts a somewhat Xattened structure, it is able to bind to the active site,
suggesting that the enzyme binds this residue in a strained but more reactive
conformation. This is thought to be another example of the use of strain in
enzyme catalysis mentioned in Section 3.7. Thus the enzyme uses the binding
energy of other residues in the polysaccharide chain to oVset the binding of the
strained ring, which is then closer in structure and free energy to the Xattened
oxonium ion intermediate.
There are also glycosidase enzymes which proceed via inversion of stereochemistry
at the glycosidic centre. It is thought in these cases that there is no
covalent intermediate, and that there is attack directly on the oxonium intermediate
on the opposite face of the molecule. This is not strictly an S N 2 displacement,
since acetals are not susceptible to nucleophilic attack. However, it may be
viewed as a displacement occurring via a dissociative transition state, as shown
in Figure 5.39. These enzymes also contain two active site carboxyl groups, but
they are situated 10–11 Å apart, far enough to accommodate the nuclophilic
water molecule (whereas the two carboxyl groups in lysozyme are 5.5 Å apart).
Glycosyl transferases are involved in the assembly of polysaccharides and
oligosaccharides. Glycosidic bonds are assembled through use of an activated
glycosyl unit, namely a glycosyl phosphate or glycosyl diphospho-uridine