Q.24 Identifr the statement about Penicillin.
I. It is an m•eversible inhibitor
2. It targets glycopeptide transpeptidase
3. It consists of a beta-lactam ring plus proline
4. It forms covalent bond with active site serme
Penicillin forms a covalent bond with the active site serine of penicillin-binding proteins (transpeptidases), making option 4 correct.
Option Analysis
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Option 1: It is an irreversible inhibitor. Correct but incomplete alone; penicillin acts as an irreversible inhibitor by covalently acylating the enzyme’s serine, but this is a consequence of option 4.
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Option 2: It targets glycopeptide transpeptidase. Partially correct. Penicillin targets penicillin-binding proteins (PBPs), which include DD-transpeptidases (also called glycopeptide transpeptidases) that cross-link peptidoglycan in bacterial cell walls.
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Option 3: It consists of a beta-lactam ring plus proline. Incorrect. Penicillin features a β-lactam ring fused to a thiazolidine (five-membered sulfur-containing ring), not proline; a variable acyl side chain (R-group) attaches to the β-lactam.
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Option 4: It forms covalent bond with active site serine. Correct and most precise. The β-lactam ring mimics D-Ala-D-Ala, reacting with Ser-70 in the PBP active site to form a stable acyl-enzyme complex, irreversibly inhibiting cell wall synthesis.
Introduction to Penicillin Action
Penicillin forms covalent bond with active site serine in bacterial transpeptidases, disrupting peptidoglycan cross-linking via its reactive β-lactam ring. Discovered by Fleming, this irreversible inhibition causes cell lysis, key for microbiology exams like GATE Life Sciences.
Mechanism Details
Penicillin’s β-lactam mimics substrate, opening to acylate Ser-70 in PBPs (e.g., DD-transpeptidase), halting glycopeptide linkages between NAG-NAM chains. Not reversible like competitive inhibitors; thiazolidine stabilizes reactivity.
| Feature | Description | Target |
|---|---|---|
| Structure | β-lactam + thiazolidine ring | Not proline |
| Enzyme | Penicillin-binding proteins | Transpeptidase Ser active site |
| Inhibition | Covalent acylation | Irreversible cell wall block |
Exam Relevance
Questions test specificity: covalent serine bond (option 4) over vague targeting or structure errors. Distinguishes from reversible inhibitors in enzyme kinetics.
