Chapter 29: Q59. (page 1196)

An octapeptide contains the following amino acids: Arg, Glu, His, Ile, Leu, Phe, Tyr, and Val. Carboxypeptidase treatment of the octapeptide forms Phe and a heptapeptide. Treatment of the octapeptide with chymotrypsin forms two tetrapeptides, A and B. Treatment of A with trypsin yields two dipeptides, C and D. Edman degradation cleaves the following amino acids from each peptide: Glu (octapeptide), Glu (A), Ile (B), Glue (C), and Val (D). Partial hydrolysis of tetrapeptide B forms Ile-Leu in addition to other products. Deduce the structure of the octapeptide and fragments A-D.

Short Answer

Expert verified

The structure of octapeptide is Glu-Arg-Val-Tyr-Ile-Leu-His-Phe.

Fragment A is Glu-Arg-Val-Tyr.

Fragment B is Ile-Leu-His-Phe.

Fragment C is Glu-Arg.

Fragment D is Val-Tyr.

Step by step solution

Octapeptide

An octapeptide is a continuous polypeptide chain and comprises eight amino acids.It cleaves into minute fragments when octapeptide is treated with chymotrypsin, trypsin, and carboxypeptidase.

Amino acid sequence of an octapeptide

First of all, it is given that Glu is an octapeptide, so it starts from Glu. When this octapeptide is treated with chymotrypsin, it forms two tetrapeptides (A and B).

A starts with Glu, and B starts with Ile, so at fifth position Ile is present. When A (Glu-Arg-Val-Tyr) is treated with trypsin, it forms two dipeptides (C and D). C cleavage is Glu-Arg and D starts from Val-Tyr.

When octapeptide is treated with carboxypeptidase, it forms heptapeptide (Glu-Arg-Val-Tyr-Ile-Leu-His) and Phe.