GATE 2025 CY – Question 32

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. · 1 mark · Numerical answer

Partial hydrolysis of a pentapeptide yields all possible tripeptides and dipeptides. The dipeptides that are obtained upon hydrolysis are given below. Val-Ala, Gln-His, Phe-Val and Ala-Gln The total number of tripeptides obtained that contain ‘Ala’ as one of the amino acids is____ (in integer).

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: 3

Explanation

Partial hydrolysis of the pentapeptide gives the dipeptides Val-Ala, Gln-His, Phe-Val and Ala-Gln. Each dipeptide is a pair of **adjacent** residues, so overlapping ends fix the sequence.

**Assemble the chain** by overlapping the shared amino acids:
- Phe-Val and Val-Ala overlap at Val: Phe-Val-Ala.
- Val-Ala and Ala-Gln overlap at Ala: Phe-Val-Ala-Gln.
- Ala-Gln and Gln-His overlap at Gln: Phe-Val-Ala-Gln-His.

The pentapeptide is **Phe-Val-Ala-Gln-His**.

**Tripeptides** (three adjacent residues): Phe-Val-Ala, Val-Ala-Gln and Ala-Gln-His, which is 3 tripeptides. Each of them contains Ala, so the number is **3**.