Enzyme mechanism — In classic lysozyme catalysis of bacterial cell wall cleavage, which listed feature does NOT contribute to the reaction?

Introduction:
Lysozyme is a classic model enzyme that hydrolyzes the β(1→4) glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine in bacterial peptidoglycan. This question distinguishes true mechanistic contributors from features belonging to other enzyme classes.

Given Data / Assumptions:

• Active site residues: Glu35 and Asp52.
• Transition state resembles an oxocarbenium ion.
• Substrate binds in multiple subsites (−4 to +2), with distortion at the −1 site.

Concept / Approach:
Glu35 (protonated due to an elevated pKa in the hydrophobic pocket) donates a proton to the glycosidic oxygen (general acid catalysis). Asp52 acts as a nucleophile forming a transient covalent glycosyl-enzyme intermediate; subsequent water attack (assisted by Glu35 acting as a base) completes hydrolysis. A key feature is distortion of the bound sugar into a half-chair, facilitating transition state formation.

Step-by-Step Solution:

Verification / Alternative check:
Mutagenesis and kinetic isotope effect studies support the covalent intermediate at Asp52 and the pKa modulation of Glu35. Structural snapshots capture sugar distortion in the active site, consistent with catalysis by distortion and electrostatic stabilization.

Why Other Options Are Wrong:

• High pKa of Glu35: essential for acid/base catalysis in a hydrophobic microenvironment.
• Strained D sugar: properly places the ring to form the transition state.
• Covalent intermediate at Asp52: core to the mechanism.
• Electrostatic stabilization: contributes significantly to lowering the barrier.

Common Pitfalls:
Transposing features of serine proteases (Ser195, His57, Asp102 triad) onto glycosidases like lysozyme; the chemistries are distinct.

Final Answer:
Formation of a covalent intermediate at Ser195 (a catalytic residue of serine proteases).