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GATE 2026 CY – Question 51

Organic Chemistry · Biomolecules: Structure, properties and reactions of mono- and di-saccharides, physicochemical properties of amino acids, chemical synthesis of peptides, chemical structure determination of peptides and proteins, structural features of proteins, nucleic acids, lipids, steroids, terpenoids, carotenoids, and alkaloids. · 2 marks · Multiple select

Chymotrypsin selectively cleaves a peptide at the carboxyl side of the amino acids having an aryl sidechain. The tripeptide(s) formed on hydrolysis of the peptide, Val-Phe-Leu-Met-Tyr-Pro-Gly-Trp-Cys, with chymotrypsin is(are)

  1. Leu-Met-Tyr
  2. Phe-Leu-Met
  3. Pro-Gly-Trp
  4. Tyr-Pro-Gly

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Correct answer: (A) Leu-Met-Tyr; (C) Pro-Gly-Trp

Explanation

Apply the cleavage rule: chymotrypsin cuts on the **carboxyl side** of residues with an aryl side chain (Phe, Tyr, Trp).

Peptide: Val-**Phe**-Leu-Met-**Tyr**-Pro-Gly-**Trp**-Cys

**Cut after each aromatic residue:**
- after Phe: Val-Phe | Leu...
- after Tyr: ...Met-Tyr | Pro...
- after Trp: ...Gly-Trp | Cys

**Fragments:**
1. Val-Phe (dipeptide)
2. Leu-Met-Tyr (tripeptide)
3. Pro-Gly-Trp (tripeptide)
4. Cys (single amino acid)

The tripeptides are **Leu-Met-Tyr (A)** and **Pro-Gly-Trp (C)**. (Phe-Leu-Met and Tyr-Pro-Gly cross the cleavage points, so they are not formed.)

Official GATE 2026 answer key: https://gate2026.iitg.ac.in/doc/download/2026/Keys/CY_Keys.pdf