section
n
.2
Peptidoglycans
193
FIGURE 11-16
Primary sequence of the hen egg-white lysozyme. Amino acid residues thought to be part of the active site are identified
by the arrows.
Lysis o f Peptidoglycans by Lysozymes
Lysozyme breaks down the peptidoglycans by hy-
drolysis
of the
/3(1
—>
4)
glycosidic bond between
N-acetylglucosamine and N-acetylmuramic acid. Lyso-
zyme occurs in tears, nasal and bronchial secretions, gas-
tric secretions, milk, and tissues and may have a protective
effect against air- and food-borne bacterial infections.
The structure and catalytic mechanism of lysozyme
have been extensively studied. Its catalytic mechanism
provides an example of steric distortion induced in the
substrate by the enzyme before products are formed. Hen
egg-white lysozyme is abundant and easy to purify. It
is a roughly ellipsoidal, compact molecule (4.5 x 3.0x
3.0 nm) of molecular weight 14,600 consisting of a sin-
gle chain of 129 amino acid residues with four disul-
fide bridges (Figure 11-16). It is also the first enzyme
whose structure was elucidated by x-ray crystallogra-
phy, that contains no prosthetic group or metal ions, and
that consists of regions of antiparallel /3-pleated sheet,
a-helix (small amount), and nondescriptive (“random”)
structure (large amount). As in hemoglobin and myoglobin
(Chapter 4), its interior contains almost entirely nonpo-
lar amino acid residues. Lysozymes and a-lactalbumin
show striking similarity in sequence and structure, which
suggests a common evolutionary ancestry. However,
a-lactalbumin functions as an enzyme modifier in the
synthesis of lactose in the mammary gland after parturi-
tion (Chapter 15), so this may be an example of divergent
evolution. The mechanism of lysozyme action has been
elucidated by the use of synthetic substrates and inhibitors.
GlcNAc-GlcNAc-GlcNAc is an inhibitor that forms a sta-
ble enzyme-inhibitor complex. The active site of the en-
zyme has been established by x-ray crystallography with
and without inhibitor and also by model building. The
enzyme contains a central crevice running horizontally
across the molecule. The hexasaccharide
GlcNAc-MurNAc-GlcNAc-MurNAc-GlcNAc-MurNAc
A
B
C
D
E
F
binds in the cleft noncovalently, with residue C making
the largest contribution. This binding distorts the normal
chair conformation of sugar residue D. Since the bond be-
tween residues D and E is hydrolyzed, this binding region
contains the active site of the enzyme. The rate of catalysis
is not significant unless the substrate is a pentamer. The
main features of a proposed mechanism for hydrolysis of
the bond between D and E are shown in Figure 11-17.
Ring D is distorted resulting in formation of a transition
state intermediate in which the conformation lies between
those of substrate and products, and which has the maxi-
mum energy. With the strained conformation (half-chair)
of ring D, the susceptible bond between D and E is drawn
in close proximity to Glu-35 and Asp-52 residues of the
