A cyclic heptapeptide when treated with 6 M HCl at 110 o C for 24 hours yielded

Question

A cyclic heptapeptide when treated with 6 M HCl at 110 oC for 24 hours yielded the following six amino acids: Ala, Pro, Met, Ser, Phe, and Tyr. Deduce the primary sequence of the peptide based upon the following observations. Prove a brief rational for your answer.

2. A cyclic heptapeptide when treated with 6 M HCl at 110 C for 24 hours yielded the following six amino acids: Ala, Pro, Met, Ser, Phe, and Tyr. Deduce the primary sequence of the peptide based upon the following observations. Prove a brief rational for your answer When the heptapeptide was treated with chymotryupsin, a tripeptide and a tretrapeptide were formed. The tripeptide was determined to consist of three different amino acids one of which is Ala. When the tripeptide was reacted with fluorodinitrobenzene (FDNB) followed by hydrolysis, DNP-Ser was detected. The tetrapeptide was found to consist of three different amino acids. Pro, Met, and Phe. When the tetrapeptide was treated with FDNB followed by hydrolysis, DNP-Met was detected. Treatment of the heptapeptide with cyanogen bromide (CNBr) yielded a dipeptide and a pentapeptide.

Explanation / Answer

The heptapeptide contains the following six amino acids,

Ala, Pro, Met, Ser, Phe and Tyr

Chymotrypsin cleaves the peptide amide bonds which contains large hydrophobic side chains like tyrosine, tryptophan and phenylalanine. FDNB (Flurodinitro benzene) is known as Sanger's reagent which acts on N terminal amino acid. So, the tripeptide sequence must be Ser-Ala-Tyr

The tetrapeptide contains three different amino acids: Pro, Met and Phe. Sanger's reagent gave the amino acid Met. Cyanogen bromide cleaves carboxyl side of methionine. So, the sequence of tetrapeptide is Met-Met-Pro-Phe

Therefore, the whole sequence of the peptide is as follows,

N terminal - Ser-Ala-Tyr-Met-Met-Pro-Phe- C terminal

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