Question

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. \(Y=\) Threonine;    Sequence: Thr–Ser–Asp–Gly–Ala
• B. \(Y=\) Serine;   Sequence: Ser–Asp–Thr–Ala–Gly
• C. \(Y=\) Serine;    Sequence: Thr–Ser–Asp–Ala–Gly
• D. \(Y=\) Threonine;  Sequence: Ser–Thr–Asp–Gly–Ala

Hint:

Always read peptide sequences from the \(\mathrm{NH_2}\) (N-terminal) end and identify amino acids by their side chains.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
To solve this, we must identify each amino acid based on its side chain (\( R \)) and know which ones are essential.
Step 2: Detailed Explanation:
1. Identifying the side chains:
- \( R_1 = -\text{CH(OH)CH}_3 \): This is the side chain of Threonine (Thr).
- \( R_2 = -\text{CH}_2\text{OH} \): This is the side chain of Serine (Ser).
- \( R_3 = -\text{CH}_2\text{COOH} \): This is the side chain of Aspartic acid (Asp).
- \( R_4 = -\text{H} \): This is the side chain of Glycine (Gly).
- \( R_5 = -\text{CH}_3 \): This is the side chain of Alanine (Ala).
The sequence from N-terminal to C-terminal is: Thr-Ser-Asp-Gly-Ala.
2. Identifying the Essential Amino Acid:
Essential amino acids are those that cannot be synthesized by the body and must be taken in the diet. The list includes Valine, Leucine, Isoleucine, Phenylalanine, Tryptophan, Lysine, Histidine, Methionine, and Threonine.
Serine, Alanine, Glycine, and Aspartic acid are non-essential.
Therefore, Threonine is the essential amino acid present.
Step 3: Final Answer:
The essential amino acid is Threonine, and the sequence is Thr-Ser-Asp-Gly-Ala.