Q.30 Which one of the following amino acid residues is specifically recognised by chymotrypsin
during peptide bond cleavage?
(A) Phe
(B) Leu
(C) Val
(D) Asp
Chymotrypsin Specificity: Amino Acid
Chymotrypsin specifically recognizes phenylalanine (Phe), tyrosine, and tryptophan at the P1 position during peptide bond cleavage, making (A) Phe the correct answer among the options.
Enzyme Cleavage Mechanism
Chymotrypsin, a serine protease, hydrolyzes peptide bonds where the carbonyl carbon comes from large hydrophobic/aromatic residues at P1 position.
Its S1 hydrophobic pocket perfectly accommodates Phe’s benzyl side chain, Tyr’s phenol, and Trp’s indole, enabling precise substrate binding.
The catalytic triad (Ser195-His57-Asp102) then performs nucleophilic attack on the scissile bond.
Correct Answer: Phe Recognition
Option (A) Phe fits snugly into chymotrypsin’s S1 pocket due to its aromatic phenyl ring, confirmed by X-ray crystallography and substrate studies.
Phe represents the classic specificity determinant, with cleavage rates orders of magnitude higher than other residues.
This selectivity ensures efficient protein digestion in the small intestine.
Options Comparison Table
| Option | Amino Acid | Side Chain Type | Cleavage Specificity |
|---|---|---|---|
| (A) Phe | Phenylalanine | Aromatic (benzyl) | Primary target; fits S1 pocket perfectly |
| (B) Leu | Leucine | Aliphatic (isobutyl) | Secondary/weak substrate; poorer S1 fit |
| (C) Val | Valine | Aliphatic (isopropyl) | Minimal activity; branched chain too small/bulky |
| (D) Asp | Aspartic acid | Charged (carboxyl) | No cleavage; repelled by hydrophobic pocket |
Leu shows slower hydrolysis as a secondary preference, while Val and Asp fail due to incompatible side chain properties.
Exam Context
This question tests protease specificity knowledge crucial for protein degradation pathways in NEET, CSIR NET biochemistry sections.
Understanding S1 pocket geometry differentiates chymotrypsin from trypsin (Lys/Arg) and elastase (small Ala).
