Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

110 Chapter 5
functional group. Glycosyl transfer enzymes also employ acid catalysis for just
the same reason. It will not surprise you to learn that glycosidase enzymes are
highly speciWc: they are speciWc for cleavage at the glycosidic bond of a particular
monosaccharide, and they are also speciWc for cleavage of either an a- ora
b-glycosidic linkage (see Figure 5.36).
The best studied glycosidase enzyme is lysozyme, a mammalian protein
found in such diverse sources as egg whites and human tears. It functions as
a mild anti-bacterial agent by hydrolysing the glycosidic linkage between
MurNAc and GlcNAc residues in the peptidoglycan layer of bacterial cell
walls. The active site contains two carboxylic acid groups which are involved
in catalysis: Glu-35 and Asp-52, shown in Figure 5.37. The mechanism is
illustrated in Figure 5.38.
Upon binding of substrate, Glu-35 donates a proton to the departing
GlcNAc C-4 oxygen, promoting cleavage of the glycosidic bond and formation
of an oxonium ion intermediate. There has been much debate in the literature
over the precise role of Asp-52: whether it stabilises the oxonium ion via an ion
pair electrostatic interaction, or whether it becomes covalently attached to the
oxonium ion. There is evidence to suggest that covalent attachment does take
place reversibly, and upon attack of water on the subsequent oxonium ion
intermediate the product is formed with retention of stereochemistry at the
glycosidic centre.
When the X-ray crystal structure of lysozyme was solved, it was found that
the MurNAc residue whose glycosidic linkage was attacked could not be
modelled into the active site of the enzyme – the lowest energy conformation
Figure 5.37 Structure of chicken egg lysozyme (PDB Wle 193L), showing the catalytic residues
Glu-35 and Asp-52 in red.