JEE Main 2026 · 28 January · morning · Single correct · +4 / −1
In the given pentapeptide, find out an essential amino acid (Y) and the sequence present in the pentapeptide :
Choose the correct answer from the options given below:
- (a) (Y), (Sequence); Serine, Thr-Ser-Asp-Ala-Gly
- (b) (Y), (Sequence); Serine, Ser-Asp-Thr-Ala-Gly
- (c) (Y), (Sequence); Threonine, Ser-Thr-Asp-Gly-Ala
- (d) (Y), (Sequence); Threonine, Thr-Ser-Asp-Gly-Ala
Answer and solution
Answer (d) (Y), (Sequence); Threonine, Thr-Ser-Asp-Gly-Ala
Solution
Chemistry ma’am · 4 steps
Start at the free NH₂ end, read each side chain, then ask which one your body cannot make.
- Find the N-terminus. A peptide is written from the end with the free -NH₂ group (N-terminus) to the end with the free -COOH group (C-terminus). Here the free -NH₂ is at the top left, so reading starts there.
- Read the side chains. Each α-carbon sits between an -NH⁻ and a C=O. The group hanging off that carbon tells you which amino acid it is.
Position Side chain on the α-carbon Amino acid Essential? 1 (free NH₂) -CH(OH)CH₃ Threonine (Thr) Yes 2 -CH₂OH Serine (Ser) No 3 -CH₂COOH Aspartic acid (Asp) No 4 -H (just a CH₂ in the chain) Glycine (Gly) No 5 (free COOH) -CH₃ Alanine (Ala) No - Essential one. Essential amino acids cannot be made in the body and must come from food. Of these five, only threonine is essential. Serine, aspartic acid, glycine and alanine are all non-essential.
- So. Y is threonine and the sequence is Thr-Ser-Asp-Gly-Ala, which is option (d).
The idea to keep. Read a peptide from the free NH₂ end to the free COOH end. Glycine has no side chain, alanine has a methyl, and of the two OH amino acids, threonine is essential, serine is not.
Answer from the NTA final answer key. Solution by PrepWiser.