Question

An octapeptide contains the following amino acids: Met, Thr, Cys, Asp, Phe, Arg, Glu, Gln. Carboxypeptidase treatment of the octapeptide forms Asp and a heptapeptide. Treatment of the octapeptide with Chymotrypsin forms 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: Cys (octapeptide), Cys (A), Gln (B), Cys (C), Glu(D). Partial hydrolysis of tetrapeptide B forms Gln-Thr in addition to other products. Deduce the structure of the octapeptide and fragments A-D.

Answer 1

1. Carboxypeptidase treatment of the octapeptide forms Asp and a heptapeptide

-Carboxypeptidase cleaves the amino acid to the c terminal so Asp is the last amino acid

2. Treatment of the octapeptide with Chymotrypsin forms two tetrapeptides, A and B

3. Treatment of A with Trypsin yields two dipeptides, C and D

c should end with lys or arg but lys is not an option so it should be arg.

4. Edman degradation cleaves the following amino acids from each peptide: Cys (octapeptide), Cys (A), Gln (B), Cys (C), Glu(D)

- n terminal amino acid is cleaved by Edman degradation - so octapeptide n terminus is cys,

A n terminus is cys

B n terminus is Gln

C n terminus is cys

D n terminus is Glu

5. Partial hydrolysis of tetrapeptide B forms Gln-Thr in addition to other products

b looks like gln-thr-...-...

so only met is left in 7th position

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phe de D: prti