Enzymology — When the hexasaccharide substrate (NAG)6 is hydrolyzed by human lysozyme, what are the principal oligosaccharide products formed?
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A6 glucosamines + 6 acetic acids (complete monomers)
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BNAG4 + NAG2 (a tetrasaccharide and a disaccharide)
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CNAG3 + NAG3 (two trisaccharides)
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DNAG3 (a single trisaccharide only)
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ENo cleavage occurs; lysozyme is inactive on (NAG)6
Answer
Correct Answer: NAG4 + NAG2 (a tetrasaccharide and a disaccharide)
Explanation
Introduction:Human lysozyme is a muramidase that cleaves beta-1,4 linkages between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in peptidoglycan; it can also act on defined NAG oligomers. This question focuses on the product profile when the model substrate (NAG)6 is hydrolyzed.
Given Data / Assumptions:
- Lysozyme has subsites that bind several sugar residues simultaneously, positioning one glycosidic bond for hydrolysis.
- (NAG)6 denotes a linear hexamer of N-acetylglucosamine residues.
- Cleavage typically yields a tetrasaccharide plus a disaccharide under standard conditions.
Concept / Approach:Consider lysozyme’s active-site subsites (A–F). When (NAG)6 occupies these, the scissile bond between the D and E subsites is hydrolyzed, producing a 4-mer and a 2-mer as the predominant products.
Step-by-Step Solution:
Bind (NAG)6 across subsites A–F.Catalyze hydrolysis at the designated bond, releasing NAG4 and NAG2.Minor side products may occur, but the main products are the tetra- and disaccharides.Verification / Alternative check:Chromatography of reaction mixtures shows peaks corresponding mainly to NAG4 and NAG2 after lysozyme digestion of (NAG)6.
Why Other Options Are Wrong:
a) Complete monomerization is not the typical single-cut outcome.c) Two trisaccharides are not the principal products with standard binding mode.d) A single trisaccharide cannot be the only product from a single cleavage of a hexamer.e) Lysozyme is active on NAG oligomers; inactivity is incorrect.Common Pitfalls:Assuming lysozyme fully degrades oligomers to monomers in one step; ignoring the active-site subsite model that predicts specific cleavage sites.
Final Answer:NAG4 + NAG2.