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Mexhanism of penicillin

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I now have excellent textbook content and diagrams. Here is a thorough answer:

Mechanism of Action of Penicillin

Penicillin belongs to the beta-lactam class of antibiotics. Its action targets the bacterial cell wall - a structure that has no equivalent in human cells, making it a highly selective antibiotic.

1. The Target: Peptidoglycan and the Cell Wall

The bacterial cell wall is a rigid, cross-linked polymer called peptidoglycan. It is composed of repeating units of two amino sugars:
  • N-acetylglucosamine (NAG/G)
  • N-acetylmuramic acid (NAM/M)
These form long polysaccharide chains. A pentapeptide side chain hangs off each NAM unit, and these side chains must be cross-linked to each other to give the wall its mechanical strength. The cross-linking is what penicillin attacks.
Bacterial cell wall structure showing outer membrane, peptidoglycan layer, PBPs, and periplasmic space with beta-lactamases
Gram-negative cell wall: note the outer membrane (absent in gram-positives), the peptidoglycan layer, PBPs on the cytoplasmic membrane, and beta-lactamases in the periplasmic space. - Katzung's Basic & Clinical Pharmacology, 16th Ed.

2. The Transpeptidation Reaction (Normal)

The final step in cell wall synthesis is transpeptidation - an enzyme (transpeptidase / PBP) removes the terminal D-alanine from the pentapeptide and forms a covalent cross-link between adjacent peptide chains. These cross-links give the peptidoglycan its rigidity and integrity.

3. How Penicillin Blocks This

Penicillin (and all beta-lactams) is a structural analog of the natural substrate D-Ala-D-Ala (the end of the pentapeptide). Because of this mimicry:
  1. Penicillin binds to the active site of PBPs (penicillin-binding proteins) - these are the transpeptidase enzymes located on the inner surface of the cytoplasmic membrane.
  2. The beta-lactam ring opens and covalently and irreversibly acylates the active serine residue of the PBP.
  3. This permanently blocks the transpeptidation reaction, preventing new cross-links from forming.
  4. Peptidoglycan synthesis halts; the cell wall cannot be completed or repaired.
Transpeptidation inhibition by beta-lactam: normal cross-linking vs. blocked reaction
Top: Normal transpeptidation cross-links peptidoglycan chains. Bottom: Beta-lactam binds transpeptidase at the PBP site, no cross-linking occurs. - Katzung's Basic & Clinical Pharmacology, 16th Ed.

4. Bactericidal Action: Autolysins

Inhibiting transpeptidation alone produces bacteriostasis in some organisms. The bactericidal effect comes from an additional event:
"A final bactericidal event is the removal or inactivation of an inhibitor of autolytic enzymes in the cell wall. This activates the autolytic enzymes and results in cell lysis."
  • Jawetz Medical Microbiology, 28th Ed.
  • Autolysins are enzymes bacteria use normally to remodel their cell wall during growth.
  • Penicillin triggers or disinhibits autolysins, which then degrade the existing peptidoglycan.
  • The weakened cell wall cannot withstand the high internal osmotic pressure, leading to cell swelling and lysis.
  • Organisms with defective autolysins are inhibited but not killed - they are called "tolerant".

5. Key Points

FeatureDetail
Drug classBeta-lactam antibiotic
TargetPenicillin-binding proteins (PBPs = transpeptidases)
Reaction inhibitedTranspeptidation (cross-linking of peptidoglycan)
EffectBactericidal
Mechanism of killingAutolysin activation + loss of cell wall integrity -> lysis
SelectivityActive only against growing bacteria (requires active cell wall synthesis)
Gram-positive vs negativeMore potent vs gram-positives (thicker peptidoglycan, no outer membrane barrier)

6. Resistance Mechanisms

From Katzung's Basic & Clinical Pharmacology, 16th Ed.:
  1. Beta-lactamase production - enzymes hydrolyze the beta-lactam ring (most common; e.g., S. aureus, H. influenzae, E. coli)
  2. Altered PBPs - reduced affinity for penicillin (e.g., MRSA has PBP2a; penicillin-resistant S. pneumoniae has altered PBPs)
  3. Impaired penetration - gram-negative outer membrane acts as a barrier; porin loss limits drug entry
  4. Efflux pumps - active transport removes drug from the periplasmic space before it can bind PBPs
Sources: Katzung's Basic & Clinical Pharmacology, 16th Ed. | Jawetz Melnick & Adelberg's Medical Microbiology, 28th Ed. | Medical Microbiology 9th Ed.
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