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An oligopeptide is subjected to amino acid analysis and found to have the following composition:
Tyr, Phe, Asp, Val, Arg, Met

The following statements represent the results obtained after analysis:

A. The above oligopeptide is subjected to N-terminal Edman degradation, revealing Tyrosine residue.

B. Trypsin treatment did not affect the peptide.

C. Cyanogen bromide (CNBr) treatment yielded a dipeptide, a tetrapeptide, and free Arginine.

D. Chymotrypsin treatment yielded a dipeptide and a tetrapeptide. The composition of the tetrapeptide was Valine, Arginine, and Methionine.

Based on the above observations, what is the correct sequence of the heptapeptide?

Correct option is A

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1. N-Terminal Analysis (Edman Degradation) → Identifies First Amino Acid

• Edman degradation identified Tyrosine (Tyr) as the N-terminal residue.
• This means that the peptide must start with Tyr.
• All given sequences start with Tyr, so this confirms the first position but does not differentiate among options.

2. Trypsin Digestion → No Effect on Peptide

• Trypsin cleaves at the C-terminal side of Arginine (Arg) and Lysine (Lys).
• Since Trypsin had no effect, this means:
• There is only one Arg at the C-terminal end.
• There are no internal Lys or Arg residues available for cleavage.
• This supports the idea that Arg is at the C-terminal end.

3. Cyanogen Bromide (CNBr) Cleavage → Cuts at Methionine (Met)

• Cyanogen bromide cleaves peptides at the C-terminal side of Methionine (Met).
• The cleavage produced:
1. A dipeptide
2. A tetrapeptide
3. Free Arginine (Arg)
• Since free Arg was released, we confirm that Arg was at the C-terminal end, reinforcing that the sequence must end in Arg.

4. Chymotrypsin Digestion → Cuts at Aromatic Residues (Phe, Tyr, Trp)

• Chymotrypsin cleaves at the C-terminal side of aromatic residues (Phe, Tyr, Trp).
• The cleavage produced:
1. A dipeptide
2. A tetrapeptide (Val, Arg, Met)
• Interpretation of Chymotrypsin cleavage:

• Since Phe is an aromatic residue, it is likely where cleavage occurred.
• The presence of Val, Arg, and Met in the tetrapeptide suggests that Met-Val-Met-Arg forms a larger segment.
• The dipeptide must then be Tyr-Asp, which suggests the sequence: