An octapeptide contains the following amino acids: Gln, Glu, Gly, Thr, Arg, Tyr,

Question

An octapeptide contains the following amino acids: Gln, Glu, Gly, Thr, Arg, Tyr, Ile, Ser. Carboxypeptidase treatment of the octapeptide forms Thr and a heptapeptide. Treatment of the octapeptide with Trypsin forms forms two tetrapeptides, A and B. Treatment of A with Chymotrypsin yields two dipeptides, C and D. Edman degradation cleaves the following amino acids from each peptide: Gly (octapeptide), Gly (A), Ser (B), Gly (C), Ile(D). Partial hydrolysis of tetrapeptide B forms Ser-Glu in addition to other products. Deduce the structure of the octapeptide and fragments A-D.

Explanation / Answer

Structure of octapeptide (N' to C') :  Gly-Tyr-Ile-Arg-Ser-Glu-Gln-Thr.

Fragment A : Gly-Tyr-Ile-Arg

Fragment B : Ser-Glu-Gln-Thr

Fragment C : Gly-Tyr

Fragment D : Ile-Arg

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