Lysozyme specificity: Which linkage does lysozyme hydrolyze in bacterial cell-wall peptidoglycan?
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Aβ-1,4 linkages between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in peptidoglycan
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Bα-1,4 linkages between NAM and NAG in peptidoglycan
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Cα-1,4 linkages between N-diacetylmuramic acid and N-diacetylglucosamine in peptidoglycan
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DAll of the above
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Eβ-1,6 linkages between galactose units in plant hemicellulose
Answer
Correct Answer: β-1,4 linkages between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in peptidoglycan
Explanation
Introduction:Lysozyme is a classic antibacterial enzyme found in tears, saliva, and egg white. Its catalytic action targets a specific glycosidic bond in bacterial peptidoglycan, compromising cell-wall integrity.
Given Data / Assumptions:
- Substrate: bacterial peptidoglycan.
- Monomeric sugars: N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG).
- Lysozyme mechanism involves acid–base catalysis in a defined binding cleft.
Concept / Approach:
Peptidoglycan consists of alternating NAM and NAG residues linked by β-1,4 glycosidic bonds, with peptide cross-links attached to NAM. Lysozyme hydrolyzes the β-1,4 linkage between NAM and NAG, weakening the cell wall and leading to osmotic lysis of susceptible bacteria.
Step-by-Step Solution:
1) Identify correct bond type in peptidoglycan: β-1,4 between NAM and NAG.2) Map to lysozyme active site subsites (e.g., A–F) that position the glycosidic bond for catalysis.3) Catalysis proceeds via proton donation/withdrawal by key residues to cleave the β-1,4 bond.4) Consequence: cell-wall weakening and bacteriolysis.Verification / Alternative check:
Structural and kinetic studies of hen egg-white lysozyme confirm specificity for β-1,4 NAM–NAG linkages.
Why Other Options Are Wrong:
B: Linkage is not α-1,4 in peptidoglycan. C: Terminology and linkage are incorrect. D: Only one statement is correct; others contradict known chemistry. E: Plant hemicellulose galactan bonds are not lysozyme targets.
Common Pitfalls:
Confusing β-1,4 (bacteria) with cellulose β-1,4 in plants; substrates and enzyme specificities are different.
Final Answer:
β-1,4 linkages between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in peptidoglycan