328
chapter
16
Carbohydrate Metabolism
111
: Glycoproteins, Glycolipids, GPI Anchors, Proteoglycans, and Peptidoglycans
H£
-NH— C / H *j*
o>
t »-
CH
3
I
'Cv.
I
H
COOH
D-Ala
__
l
__.
Enz-OH
/
3
CH
3
0
NH— C — H
D.
H
I
II
\
R — NH,
R — N— CH— C —
</S)
+
Enz
Enz-OH
R-D-Ala-D-Ala
Active acyl-enzyme
V
I
\ /S x
R— NH— C ------
CT
\
CH
3
i
i
V
O
'
N^c/
CH
3
COOH
Enz-OH
R - N H - C —
C
^
\
,CH
3
,cz
HNV
"O
.
Enz
/
CH,
COOH
Penicillin
Stable penicilloyl-enzyme
complex
(inactive enzym e)
FIGURE 16-22
Mechanism of action of penicillin. The conformation of the D-alanyl-D-alanine portion of the peptidoglycan of bacterial
cell walls is similar to that of the lactam ring of penicillin, so that the enzyme cleaves the lactam ring (at the site
marked t) to form a stable penicilloyl-enzyme complex.
and act neither as transpeptidases nor as D-alanine car-
boxypeptidases
in vitro
using model substrates. This does
not necessarily mean that they lack these activities
in
vivo,
but rather that they are denatured, that some factor
essential for activity is lost during purification, or that
they may require larger, more complex, or more specific
molecules as
substrates.
Large PBPs are frequently
necessary for cell viability. In contrast, small PBPs, i.e.,
those having molecular weights from about 40,000 to
50,000, are generally less sensitive to penicillins and
are insensitive to most cephalosporins. Small PBPs are
relatively abundant, are enzymatically active
in vitro,
and
do not appear to be necessary for cell viability.
Interest has centered on large PBPs as lethal targets for
/1-lactams because of the relative insensitivity of small
PBPs to these antibiotics and because of the importance
of large PBPs for cell viability.
The interactions of /1-lactams with PBPs indicate that
these compounds are structural analogues of R-D-alanyl-
D-alanine, the natural substrate of peptidoglycan transpep-
tidases and D-alanine carboxypeptidases, where R is the
remainder of the pentapeptide. The mechanisms of the
transpeptidase and carboxypeptidase reactions are thought
to involve formation of an acyl-enzyme intermediate that
can react with either a primary amine (e.g., an a-amino
group) to form a peptide bond, or with water to form a car-
boxylic acid. In both reactions D-alanine is released before
the acyl enzyme is formed. When a /1-lactam antibiotic
enters the binding site, the /1-lactam bond is hydrolyzed,
and the resulting acyl group reacts with the active-site
serine hydroxyl group to form a stable acyl enzyme
(Figure 16-22). Thus, penicillin functions as a suicide sub-
strate (Chapter 6). The /1-1 act am in antibiotics is highly
reactive because of the strain inherent in a four-membered
ring. In addition, the other bonds prevent the release of
a group equivalent to the D-alanine that is released dur-
ing reaction with the usual substrates. This reaction by
which penicillin forms a stable enzyme complex blocks
the active site of the enzyme. Some specific interaction
between the thiazolidine or dihydrothiazine ring and the
enzyme may stabilize the enzyme-inhibitor complex. /1-
Lactam antibiotics bind to and acylate the catalytic site of
the enzyme.
Glycopeptide antibiotics, including vancomycin and te-
icoplanin, are large, rigid molecules that inhibit a late
stage in bacterial cell wall peptidoglycan synthesis. The
three-dimensional structure contains a cleft into which
peptides of highly specific configuration can fit (L-aa-
D-aa-D-aa). Such sequences are found only in bacterial
cell walls making these glycopeptides selectively toxic.
As a result of binding to L-aa-D-Ala-D-Ala- groups in
wall intermediates, glycopeptides inhibit, apparently by
steric hinderance, the formation of backbone glycan chains
(catalyzed by peptidoglycan polymerase) from the sim-
ple wall subunits as they are extruded through the cyto-
plasmic membrane. As a result, subsequent transpeptida-
tion reactions that impart rigidity to the cell wall are also
inhibited.
Bacteria susceptible to penicillin can mutate to re-
sistant forms by developing an enzyme, lactamase, that
