21. The formation of a carbocation, also called an oxonium ion, occurs during the reaction
catalyzed by
(A) aldolase
(B) lysozyme
(C) ribonuclease A
(D) ) carboxypeptidase

The correct answer to the query is (B) lysozyme. Lysozyme forms a carbocation intermediate, often termed an oxonium ion, during its hydrolysis of bacterial cell wall polysaccharides.

Correct Answer Explanation

Lysozyme catalyzes the hydrolysis of β-1,4-glycosidic bonds in peptidoglycan, a key bacterial cell wall component. Glu35 acts as a general acid to protonate the glycosidic oxygen, while Asp52 stabilizes the resulting oxocarbenium ion (carbocation-like) on the substrate’s anomeric carbon. This short-lived intermediate resembles an oxonium ion due to its positive charge delocalized over the ring oxygen.​

Option Analysis

Aldolase

Aldolase cleaves fructose-1,6-bisphosphate into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate in glycolysis via a Schiff base with lysine, involving enediol intermediates but no carbocation.​

Lysozyme

As noted, lysozyme’s mechanism explicitly generates a carbocation/oxonium ion intermediate, confirmed by structural and kinetic studies.​

Ribonuclease A

Ribonuclease A hydrolyzes RNA via a 2′,3′-cyclic phosphate intermediate, with His12 and His119 facilitating acid-base catalysis, lacking any carbocation step.​

Carboxypeptidase

Carboxypeptidase A cleaves C-terminal peptide bonds using a zinc-bound water nucleophile, promoting hydrolysis without carbocation formation.​