DƯỢC LÍ Goodman & Gilman's The Pharmacological Basis of Therapeutics 12th, 2010

Glycopeptide
polymer
NAM
penicillins
cephalosporins
D-Alanine
Glycopeptide
polymer
NAM
Glycopeptide
polymer
NAM
Glycopeptide
polymer
NAM
Reaction site
Transpeptidase (PBP)
L-Alanine
D-Glutamate
L-Lysine
Glycine
D-Alanine
NAM =
N-Acetylmuramic
acid
Figure 53–2. Action of β-lactam antibiotics in Staphylococcus
aureus. The bacterial cell wall consists of glycopeptide polymers
(a NAM-NAG amino-hexose backbone) linked via bridges
between amino acid side chains. In S. aureus, the bridge is (Gly) 5
-
D-Ala between lysines. The cross-linking is catalyzed by a transpeptidase,
the enzyme that penicillins and cephalosporins inhibit. NAM,
N-acetyl-muramic acid; NAG, N-acetyl-glucosamine.
diphosphate (UDP)–acetylmuramyl-pentapeptide, accumulates in cells
when subsequent synthetic stages are inhibited. The last reaction in the
synthesis of this compound is the addition of a dipeptide, D-alanyl-Dalanine.
Synthesis of the dipeptide involves prior racemization of
L-alanine and condensation catalyzed by D-alanyl-D-alanine synthase.
D-Cycloserine is a structural analog of D-alanine and acts as a competitive
inhibitor of both the racemase and the synthase (Chapter 56).
During reactions of the second stage, UDP-acetylmuramylpentapeptide
and UDP-acetylglucosamine are linked (with the
release of the uridine nucleotides) to form a long polymer. The third
and final stage involves completion of the cross-link. This is accomplished
by peptidoglycan glycosyltransferases outside the cell membrane
of gram-positive and within the periplasmic space of
gram-negative bacteria (Figure 53–3B). The terminal glycine residue
of the pentaglycine bridge is linked to the fourth residue of the pentapeptide
(D-alanine), releasing the fifth residue (also D-alanine)
(Figure 53–2). It is this last step in peptidoglycan synthesis that is
inhibited by the β-lactam antibiotics and glycopeptide antibiotics
such as vancomycin (by a different mechanism than the β-lactams;
Chapter 54). Stereo models reveal that the conformation of penicillin
is very similar to that of D-alanyl-D-alanine. The transpeptidase probably
is acylated by penicillin; i.e., penicilloyl enzyme apparently is
formed, with cleavage of the —CO—N— bond of the β-lactam
ring. Although the transpeptidase activity has been targeted with the
β-lactams and vancomycin, the glycosyltransferase is a relatively
unexplored target for antibiotics (Lovering et al., 2007).
Although inhibition of the transpeptidase just described is
demonstrably important, there are additional, related targets for the
actions of penicillins and cephalosporins; these are collectively
termed penicillin-binding proteins (PBPs). All bacteria have several
such entities; e.g., S. aureus has four PBPs, whereas Escherichia
coli has at least seven. The PBPs vary in their affinities for different
β-lactam antibiotics, although the interactions eventually become
covalent. The higher molecular weight PBPs of E. coli (PBPs 1a and
1b) include the transpeptidases responsible for synthesis of the peptidoglycan.
Other PBPs in E. coli include those that are necessary for
maintenance of the rod-like shape of the bacterium and for septum
formation at division. Inhibition of the transpeptidases causes spheroplast
formation and rapid lysis. However, inhibition of the activities
of other PBPs may cause delayed lysis (PBP 2) or the production of
long filamentous forms of the bacterium (PBP 3). The lethality of
penicillin for bacteria appears to involve both lytic and nonlytic mechanisms.
Penicillin’s disruption of the balance between PBP-mediated
peptidoglycan assembly and murein hydrolase activity results in
autolysis. Non-lytic killing by penicillin may involve holin-like proteins
in the bacterial membrane that collapse the membrane potential
(Bayles, 2000).
Mechanisms of Bacterial Resistance to Penicillins and Cephalosporins.
Although all bacteria with cell walls contain PBPs, β-lactam antibiotics
cannot kill or even inhibit all bacteria because bacteria can be
resistant to these agents by myriad mechanisms. The microorganism
may be intrinsically resistant because of structural differences in the
PBPs that are the targets of these drugs. Furthermore, a sensitive
strain may acquire resistance of this type by the development of highmolecular-weight
PBPs that have decreased affinity for the antibiotic.
Because the β-lactam antibiotics inhibit many different PBPs in
a single bacterium, the affinity for β-lactam antibiotics of several
PBPs must decrease for the organism to be resistant. Altered PBPs
with decreased affinity for β-lactam antibiotics are acquired by
homologous recombination between PBP genes of different bacterial
species. Four of the five high-molecular-weight PBPs of the most
highly penicillin-resistant Streptococcus pneumoniae isolates have
decreased affinity for β-lactam antibiotics as a result of interspecies
homologous recombination events (Figure 53–4). In contrast, isolates
with high-level resistance to third-generation cephalosporins contain
alterations of only two of the five high-molecular-weight PBPs
because the other PBPs have inherently low affinity for the thirdgeneration
cephalosporins. Penicillin resistance in Streptococcus sanguis
and other viridans streptococci apparently emerged as a result of
replacement of its PBPs with resistant PBPs from S. pneumoniae
(Carratalá et al., 1995; Spratt, 1994). Methicillin-resistant S. aureus
are resistant via acquisition of an additional high-molecular-weight
PBP (via a transposon from an unknown organism) with a very low
affinity for all β-lactam antibiotics. The gene (MecA) encoding this
new PBP also is present in and responsible for methicillin resistance
in the coagulase-negative staphylococci (Moran et al., 2006).
Other instances of bacterial resistance to the β-lactam antibiotics
are caused by the inability of the agent to penetrate to its site of
action (Jacoby and Munoz-Price, 2005) (Figure 53–5). In grampositive
bacteria, the peptidoglycan polymer is very near the cell
surface (Figure 53–3). Some gram-positive bacteria have polysaccharide
capsules that are external to the cell wall, but these structures
are not a barrier to the diffusion of the β-lactams; the small
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CHAPTER 53
PENICILLINS, CEPHALOSPORINS, AND OTHER β-LACTAM ANTIBIOTICS