Chapter 24, Problem 24.44SP

Complete hydrolysis of an unknown basic decapeptide gives Gly, Ala, Leu, IIe, Phe, Tyr, Glu, Arg, Lys, and Ser. Terminal residue analysis shows that the N terminus is Ala and the C terminus is lle. Incubation of the decapeptide with chymotrypsin gives two tripeptides, A and B, and a tetrapeptide, C. Amino acid analysis shows that peptide A contains Gly, Glu, Tyr, and NH₃; peptide B contains Ala, Phe, and Lys, and peptide C contains Leu, lle, Ser, and Arg. Terminal residue analysis gives the following results.

	N terminus	C terminus
A	Gln	Tyr
B	Ala	Phe
C	Arg	Ile

Incubation of the decapeptide with trypsin gives a dipeptide D, a pentapeptide E, and a tripeptide F. Terminal residue analysis of F shows that the N terminus is Ser and the C terminus is Iie. Propose a structure for the decapeptide and for fragments A through F.