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.