Lysozyme is a relatively small (129 AA) enzyme that catalyzes
the hydrolysis of specific kinds of polysaccharides comprising the cell walls of bacteria.
In birds, lysozyme is an exceptionally abundant protein in
fowl egg whites where it likely functions both as an antibiotic (see
below) as well as a nutrient for early embryogenesis. In vertebrates, this "glycosidase" is found
mainly in biological secretions (such as tears) where it probably serves as an
anti-bacterial agent by digesting and weakening the rigid bacterial cell wall, thereby
rendering the bacterial susceptible to osmotic lysis. The effect of lysozyme is similar to
the effect of penicillin which also weakens the cell walls of bacteria only by
irreversibly inhibiting a transpeptidase enzyme required for crosslinking peptidoglycan
macromolecules formed in the biosynthesis of the cell wall. Under normal
conditions, bacteria grow very rapidly in some cases doubling more than once in
an hour. However, when cell wall crosslinking is
disrupted, bacteria tend to lyse in hypotonic media as a result of the
mechanical weakening of their cell walls.
Lysozyme hydrolyzes a number of structurally similar substrates but the
best known substrates for this enzyme are alternating polysaccharide copolymers of
N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) which represent the
"unit" polysaccharide structure of many bacterial cell walls. Lysozyme cleaves
at b(1-4) glycosidic linkage,
connecting the C1 carbon of NAM to the C4 carbon of NAG. The optimal substrate is a
(NAG-NAM)3 hexasaccharide, with lysozyme cleaving at the NAM4-b-O-NAG5 glycosidic bond. The
active site of lysozyme includes binding sites for each sugar ring of the hexasaccharide,
these being designated sites "A" through "F." Thus, lysozyme
preferentially cleaves at the glycosidic bond connecting ring "D" to ring
"E" of the hexasaccharides.
Extended polysaccharide binding site of hen egg white lysozyme. Six sugar monomer binding
pockets are indicated, A-F, and the sidechains of two catalytic residues,
and Asp52 (yellow), are indicated as well as the sidechains of
Trp62 (magenta) and
(orange) which line binding pockets A-C.
Smaller saccharides are effective competitive inhibitors of lysozyme.
One such inhibitor, as examined in this web page exercise, is the trisaccharide,
Tri-NAG binds most efficiently to the saccharide binding sites "A-C" and thus it
is positioned away from the key catalytic residues of lysozyme -- namely
-- which are logically located near the junction between the "D" and
"E" binding sites where lysozyme cleaves.
Click on the image below
to open the
lysozyme web page for further
examination of this enzyme's properties.