​A polypeptide was subjected to the following treatments with the indicated results.

I. Acid hydrolysis:

(1) (Ala, Arg, Cys, Glx, Gly, Lys, Leu, Met, Phe, Thr)

II. Aminopeptidase M:

(2) No fragments

III. Carboxypeptidase A + Carboxypeptidase B:

(3) No fragments

IV. Trypsin followed by Edman degradation of the separated products:

(4) Cys-Gly-Leu-Phe-Arg

(5) Thr-Ala-Met-Gln-Lys

Which one of the following represents the primary structure of the peptide?

Correct option is D

To determine the primary structure of the peptide, we analyze the given experimental data.

Step 1: Acid Hydrolysis (Complete Breakdown)

• The given set of amino acids: Ala, Arg, Cys, Glx, Gly, Lys, Leu, Met, Phe, Thr.
• This confirms all amino acids present but does not give sequence information.

Step 2: Aminopeptidase M

• No fragments were obtained, indicating that the N-terminus is blocked, possibly due to cyclization.

Step 3: Carboxypeptidase A + B

• No fragments, meaning that the C-terminus is blocked, further supporting cyclization.

Step 4: Trypsin Digestion + Edman Degradation

• Trypsin cleaves at Lys (K) and Arg (R).
• Two fragments obtained:
  1. Cys-Gly-Leu-Phe-Arg (Fragment 1)
  2. Thr-Ala-Met-Gln-Lys (Fragment 2)

From these results:

• Since Trypsin cleaves after Arg and Lys, we deduce that Arg is at the C-terminal of one fragment, and Lys is at the C-terminal of the other.
• However, since Aminopeptidase and Carboxypeptidase gave no results, the peptide is most likely cyclic.

Final Answer:

The cyclic nature of the peptide is supported by:

• No free N-terminal or C-terminal (Step 2 and Step 3 results).
• Matching sequence fragments with the cyclic representation.

Thus, the correct answer is:(4) 

Explanation:

• A cyclic peptide means that the amino acids form a closed-loop structure rather than a linear sequence.
• Since both trypsin digestion products match the sequence in the given cyclic structure, the primary structure must be cyclic.
• Linear options (1, 2, 3) are ruled out due to missing free ends.