Amino acid analysis of an oligopeptide containing nine residues revealed the presence of the following amino acids: \(\begin{array}{llllllll}\text { Arg } & \text { Cys } & \text { Gly } & \text { Leu } & \text { Met } & \text { Pro } & \text { Tyr } & \text { Val }\end{array}\) 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 8 -residue fragment left after combined trypsin and chymotrypsin action yielded a 6-residue fragment containing Cys, Gly, Pro, Tyr, and Val; and a dipeptide. e. Treatment of the 6 -residue fragment with \(\beta\) -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 nonapeptide?

Step by step solution

01

Analyze given information

It is known that the nonapeptide consists of the amino acids Arg, Cys, Gly, Leu, Met, Pro, Tyr and Val. No other amino acids are released when the peptide is treated with Carboxypeptidase A, implying that none of these amino acids are at the carboxyl end of the peptide. Trypsin and chymotrypsin treatment only releases free Arg, suggesting that Arg is likely near the N-terminus of the peptide, since these enzymes typically break proteins into smaller fragments.

02

CNBr Treatment Implications

After trypsin and chymotrypsin treatment, which should have cleaved on the carboxyl side of Arg, treatment of the remaining 8-residue fragment with Cyanogen bromide (CNBr) gives a 6-residue fragment and a dipeptide. Cyanogen bromide cleaves on the carboxyl side of the amino acid methionine (Met). Thus, it can be inferred that Met is the seventh amino acid in the sequence from the N-terminus.

03

β-mercaptoethanol Treatment

The 6-residue fragment, when treated with β-mercaptoethanol, yielded two tripeptides. Edman analysis only yielded PTH-Cys, indicating that Cys is the N-terminus of one of the tripeptides and that the sequence of each of the tripeptides is alphabetical according to the single-letter designation. Therefore, the two tripeptides could be CysGlyPro and TyrVal. However, since it is stated that the sequence of the entire nonapeptide from the N-terminus is alphabetical per the single-letter designation for amino acids, these tripeptide sequences could also be rearranged.

04

Assembling the Sequence

Combining all the information, the sequence could be assembled as follows: starting with Arg at the N-terminus (since only free Arg is released upon trypsin and chymotrypsin treatment), followed alphabetically by the tripeptide sequences CysGlyPro and TyrVal, and ending in Met and Leu given that none of the other amino acids were released by carboxypeptidase A treatment.

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