Biochemistry 6th Edition by Reginald Garrett,Charles Grisham

Exercise 8

Solving the sequence of an oligopeptide from sequence analysis data
Amino acid analysis of an oligopeptide containing nine residues revealed the presence of the
following amino acids:
The following was found:
a. Carboxypeptidase A treatment yielded no free amino acid.
b. Edman analysis of the intact oligopeptide released

c. Neither trypsin nor chymotrypsin treatment of the nonapeptide released smaller fragments. However, combined trypsin and chymotrypsin treatment liberated free Arg.
d. CNBr treatment of the eight-residue fragment left after combined trypsin and chymotrypsin action yielded a six-residue fragment containing Cys, Gly, Pro, Tyr and Val; and a dipeptide.
e. Treatment of the six-residue fragment with ? -mercaptoethanol yielded two tripeptides. Brief Edman analysis of the tripeptide mixture yielded only PTH-Cys. (The sequence of each tripeptide, as read from the N-terminal end, is alphabetical if the one-letter designation for amino acids is used.)
What is the amino acid sequence of this octapeptide?