Q.74 Lysozyme catalyzes the breakdown of
(A) NAG-NAM (B) lipopolysaccharide (C) teichoic acid (D) lipoprotein A
Lysozyme specifically catalyzes the hydrolysis of β-1,4 glycosidic bonds between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in bacterial peptidoglycan. The correct answer is (A) NAG-NAM, making it a key defense against Gram-positive bacteria.
Lysozyme Function Overview
Lysozyme (muramidase) is an antimicrobial enzyme abundant in tears, saliva, and phagocytes. It targets peptidoglycan in Gram-positive bacterial cell walls, hydrolyzing β-(1→4) linkages between NAG and NAM, creating holes that cause osmotic lysis.
Discovered by Fleming (1922), its mechanism involves Glu35 (proton donor) and Asp52 (stabilizes oxocarbenium ion) at the active site.
Correct Answer: (A) NAG-NAM
Lysozyme cleaves the β-1,4 glycosidic bond between N-acetylglucosamine (NAG/GlcNAc) and N-acetylmuramic acid (NAM/MurNAc) in peptidoglycan: (NAG-β-1,4-NAM)<sub>n</sub> → NAG + NAM fragments.
This weakens the rigid cell wall sacculus, especially lethal for Gram-positive bacteria lacking outer membrane protection.
Why Not the Other Options?
Option (B) Lipopolysaccharide (LPS)
LPS forms the outer leaflet of Gram-negative bacterial outer membranes (lipid A + core polysaccharide + O-antigen). Lysozyme cannot access LPS due to the outer membrane barrier and lacks hydrolase activity against its glycosidic or phosphoester bonds.
Option (C) Teichoic Acid
Teichoic acids are glycerol-phosphate polymers in Gram-positive cell walls, linked to peptidoglycan or membrane (LTA/WTA). They provide cations/charge but lysozyme targets the NAG-NAM backbone, not teichoic acid chains.
Option (D) Lipoprotein A
Braun’s lipoprotein (Lpp) anchors outer membrane to peptidoglycan in Gram-negative bacteria via lipidated cysteine. It’s a structural protein, not a carbohydrate substrate for lysozyme’s glycosidase activity.
Bacterial Cell Wall Components Table
| Component | Location | Lysozyme Target? | Bond Type |
|---|---|---|---|
| NAG-NAM | Peptidoglycan (both Gram±) | ✅ Yes | β-1,4 glycosidic |
| Lipopolysaccharide | Gram⁻ outer membrane | ❌ No | Kdo-lipid A |
| Teichoic acid | Gram⁺ wall | ❌ No | Glycerol-P |
| Lipoprotein A | Gram⁻ OM-peptidoglycan | ❌ No | Thioether |
Mechanism Snapshot
Peptidoglycan: ...-NAG-β(1→4)-NAM-β(1→4)-NAG-β(1→4)-NAM-...
Lysozyme cleaves ↑ here
Exam Strategy
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Mnemonic: “Lysozyme Loves NAG-NAM” (both start with N)
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Gram+ specificity: Peptidoglycan fully exposed vs. Gram⁻ protection
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Common distractors: LPS/teichoic (wall components but wrong bonds)
