Question

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 Ala-Gln 
The total number of tripeptides obtained that contain ‘Ala’ as one of the amino acids is __________

Hint:

To count tripeptides in a linear peptide chain, use a sliding window of 3 residues. Make sure to cross-check against the sequence constraints derived from known dipeptides.

Answer 1

Correct Answer: 3

Let’s first reconstruct the pentapeptide sequence consistent with the given dipeptides:

From the dipeptides:
Val–Ala → Val precedes Ala
Ala–Gln → Ala precedes Gln
Gln–His → Gln precedes His
Phe–Val → Phe precedes Val

Therefore, the likely pentapeptide sequence is:

Phe – Val – Ala – Gln – His

Now, generate all possible tripeptides from this sequence using a sliding window of three amino acids:

Phe–Val–Ala → contains Ala (valid)
Val–Ala–Gln → contains Ala (valid)
Ala–Gln–His → contains Ala (valid)

Other combinations such as Phe–Ala–Gln are not valid since they do not occur in the original linear sequence.

Thus, the tripeptides that contain Ala are:

Phe–Val–Ala
Val–Ala–Gln
Ala–Gln–His

Final Answer: 3