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

Concept: A peptide sequence is always read from the N-terminus (\(\mathrm{NH_2}\) end) to the C-terminus (\(\mathrm{COOH}\) end). Each amino acid residue is identified by its characteristic side chain (\(R\)-group). Among amino acids, some are {essential}, meaning they must be obtained from diet.
Step 1: Identify the direction of the peptide From the given structure:
Left end shows \(\mathrm{H_2N}\) \(\Rightarrow\) N-terminus
Right end shows \(\mathrm{COOH}\) \(\Rightarrow\) C-terminus Thus, the sequence must be read from left to right.
Step 2: Identify each amino acid residue Examine the side chains attached to each \(\alpha\)-carbon:
First residue: Side chain \( -\mathrm{CH(OH)CH_3} \) \(\Rightarrow\) Threonine (Thr)
Second residue: Side chain \( -\mathrm{CH_2OH} \) \(\Rightarrow\) Serine (Ser)
Third residue: Side chain \( -\mathrm{CH_2COOH} \) \(\Rightarrow\) Aspartic acid (Asp)
Fourth residue: Side chain \( -\mathrm{H} \) \(\Rightarrow\) Glycine (Gly)
Fifth residue: Side chain \( -\mathrm{CH_3} \) \(\Rightarrow\) Alanine (Ala)
Step 3: Write the correct sequence Reading from N-terminus to C-terminus: \[ \boxed{\text{Thr–Ser–Asp–Gly–Ala}} \]
Step 4: Identify the essential amino acid Among the amino acids present:
Threonine \(\rightarrow\) Essential
Serine, Aspartic acid, Glycine, Alanine \(\rightarrow\) Non-essential Thus, \[ Y = \boxed{\text{Threonine}} \] Final Answer: \[ \boxed{\text{Option (A): Threonine; Thr–Ser–Asp–Gly–Ala}} \]