Determine the primary structure of an octopeptide from the following data: acid-

Question

Determine the primary structure of an octopeptide from the following data:

acid-catalyzed hydolysis gives Ala, Arg, Arg, Lys, Met, Tyr, Phe, Val

carboxypeptidase A releases Val

Edman's reagent releases Ala

Treatment with cyanogen bromide produces 2 peptides with the following amino acid compositions: 1. Arg, Tyr, Val 2. Arg, Ala, Lys, Met, Phe

Trypsin-catalyzed hydrolysis forms the following two amino acids and two peptides: 1. Arg 2. Val 3. Arg, Met, Tyr 4.Ala, Lys, Phe

The answer I got was: Ala-Phe-Lys-Arg-Met-Tyr-Arg-Val

Explanation / Answer

The answer obtained is correct. The sequence of the peptide is Ala-Phe-Lys-Arg-Met-Tyr-Arg-Val.

* Carobxypeptidase cleaves the peptide chain at its carboxyl end. Therefore, the carboxyterminus has the amino acid Valine

* Edmans reagent releases the amino acid at the amino terminus. Therefore, Alanine is the amino acid at amino terminus

* Cyanogen bromide cleaves at the carboxy terminus of a methionine. Since one of the fragments has arg, tyr and val, these three must be towards the carboxy terminus of methionine

* Trypsin cleaves peptide chains at the carboxyl side of either lysine or arginine. Since a free arginine is released, it must be preceded by a lysine at its amino terminus. Similarly, a free valine is released, it must be preceded by another arginine at its amino terminus.

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