Lysozyme specificity: Which linkage does lysozyme hydrolyze in bacterial cell-wall peptidoglycan?

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:

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