Peptidoglycan chemistry: the N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) sugars in bacterial walls are covalently linked by which glycosidic bond?
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Aα- 1,4-glycosidic bond
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Bβ-1,6-glycosidic bond
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Cα- 1,6-glycosidic bond
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Dβ- 1,4-glycosidic bond
Answer
Correct Answer: β- 1,4-glycosidic bond
Explanation
Introduction / Context:Peptidoglycan’s strength derives from both its glycosidic linkages and peptide cross-bridges. Knowing the specific bond between NAG and NAM explains the action of lysozyme and certain antibiotics on bacterial cell walls.
Given Data / Assumptions:
- Sugars involved: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
- We must choose the correct glycosidic linkage connecting them in the backbone.
Concept / Approach:In peptidoglycan, NAG and NAM residues are joined by β-1,4-glycosidic bonds. This linkage is specifically cleaved by lysozyme, which hydrolyzes the β-1,4 bond and compromises wall integrity, leading to osmotic lysis in susceptible bacteria.
Step-by-Step Solution:
Recall the backbone chemistry: alternating NAG–NAM with β-1,4 linkage.Connect this to lysozyme’s substrate specificity (β-1,4 between NAG and NAM).Select β-1,4-glycosidic bond.Verification / Alternative check:Biochemistry references consistently depict the NAG–NAM linkage as β-1,4, analogous to cellulose’s β-1,4 (though different monomers) and unlike starch’s α linkages.
Why Other Options Are Wrong:
- α-1,4 / α-1,6: typical for starch/glycogen branches, not peptidoglycan.
- β-1,6: not the canonical linkage in the NAG–NAM backbone.
Common Pitfalls:Mixing carbohydrate linkages from eukaryotic storage polysaccharides with bacterial wall polysaccharides.
Final Answer:β- 1,4-glycosidic bond