Solving the Sequence of an Oligopeptide From Sequence Analysis Data Analysis of the blood of a catatonic football fan revealed large concentrations of a psychotoxic octapeptide. Amino acid analysis of this octapeptide gave the following results: 2 Ala 1 Arg 1 Asp 1 Met 2 Tyr 1 Val 1 NH4 1 The following facts were observed: a. Partial acid hydrolysis of the octapeptide yielded a dipeptide of the structure C O C N H H COOH H3C CH3 CH3 C CH H3 N b. Chymotrypsin treatment of the octapeptide yielded two tetrapeptides, each containing an alanine residue. c. Trypsin treatment of one of the tetrapeptides yielded two dipeptides. d. Cyanogen bromide treatment of another sample of the same tetrapeptide yielded a tripeptide and free Tyr. e. N-terminal analysis of the other tetrapeptide gave Asn. What is the amino acid sequence of this octapeptide

The amino sequence of an octapeptide can be determined by :

i.Amino acids present in the peptide

ii.By the behaviour shown by the treatment of these material with different biological and chemical reactions

As shown ammonium was released from the peptide,which shows that the peptide contains either Asn or Gln.

As there is presence of Asp in the amino acid analysis which indicates that the octapeptide contains Asp.

Acid hydrolysis hydrolyzes the peptide bonds which yields a Ala-Val  which is the result of partial acid hydrolysis.

Chymotrypsin is on the C-terminal side of the aromatic amino acids Phe, Tyr, and Trp. The peptide has Tyr which indicates that  the two present tetrapeptides after chymotrypsin treatment are fourth and eighth amino acids i-e N--Tyr-C and Tyr as the fourth amino acid and Ala-Val somewhere in the octapeptide

As Arg is only present on the C terminal side  of the Trypsin cleavage therefore Arg must be the second or the sixth amino acid for the two dipeptides to form as a result of arginine treatment of a tetrapeptide

Cyanogen bromide cleaves on the C-terminal side of Met residues. For free Tyr to be produced from cyanogen bromide treatment of one of the tetrapeptides, Met must be either the third or seventh amino acid. Because Ala was present in both tetrapeptides, Ala must be either the first or fifth amino acid, present in the same tetrapeptide as Arg and Met,i-e N.-Tyr-C and Ala-Val somewhere in the octapeptide and Ala-Arg-Met as the first, second, and third or fifth, sixth, and seventh amino acids and Tyr as the fourth amino acid.

One of the tetrapeptides contains Ala-Val as its second and third amino acids and Asn as its N-terminal amino acid. Therefore two combinations are possible as below

N-Asn-Ala-Val-Tyr-Ala-Arg-Met-Tyr-C or N-Ala-Arg-Met-Tyr-Asn-Ala-Val-Tyr-C