Q.24 Antibacterial action of penicillin could be attributed to
- Inhibition of the enzyme transpeptidase
- Inhibition of the cross linking of peptidoglycan side chain
- Hydrolysis of peptidoglycan
- Both 1 and 2
Both 1 and 2 correctly describe the antibacterial action of penicillin, as it inhibits transpeptidase to block peptidoglycan cross-linking.
Option Analysis
Inhibition of the enzyme transpeptidase: Penicillin’s β-lactam ring mimics D-ala-D-ala, binding irreversibly to transpeptidase (a penicillin-binding protein, PBP), preventing it from catalyzing peptide cross-links in peptidoglycan.
Inhibition of the cross linking of peptidoglycan side chain: Without active transpeptidase, the glycan strands of peptidoglycan cannot form covalent cross-links between tetrapeptide side chains (e.g., via glycine bridges), weakening the cell wall and causing osmotic lysis in growing bacteria.
Hydrolysis of peptidoglycan: Penicillin does not directly hydrolyze or break peptidoglycan bonds; it blocks new synthesis/repair, leading to indirect autolysis by bacterial enzymes, not direct hydrolysis.
Both 1 and 2: This is accurate, as the two mechanisms are directly linked—transpeptidase inhibition specifically prevents cross-linking.
The antibacterial action of penicillin could be attributed to inhibition of transpeptidase enzyme, which prevents cross-linking of peptidoglycan side chains in bacterial cell walls. This bactericidal effect targets growing Gram-positive bacteria primarily, causing cell lysis without harming human cells lacking peptidoglycan.
Mechanism Details
Penicillin binds PBPs (transpeptidases) via its β-lactam ring, acylating the active-site serine and halting the final transpeptidation step where L-lysine links to D-ala in peptidoglycan precursors. Weakened walls rupture under turgor pressure during division.
Option Comparison
Hydrolysis is indirect; options 1 and 2 are mechanistically equivalent and correct.
Option Describes Penicillin Action? Explanation 1. Transpeptidase inhibition Yes Direct target enzyme blocked 2. Cross-linking inhibition Yes Consequence of enzyme block Hydrolysis of peptidoglycan No No direct bond breaking Both 1 and 2 Correct answer Linked mechanisms This duo explains why antibacterial action of penicillin could be attributed to both transpeptidase inhibition and peptidoglycan cross-linking blockade in exams.
