Analysis of a octapeptide revealed the presence of the following products 2 Ala,

Question

Analysis of a octapeptide revealed the presence of the following products 2 Ala, 1 Arg, 1 Asp, 1 Met, 2 Tyr, 1 Val, 1 NH The following facts were observed: a. Partial acid hydrolysis of the octapeptide yielded a dipeptide of the structure: ?? b. Chymotrypsin treatment yielded two tetrapeptides, each containing an alanine residue c. Trypsin treatment of one of the tetrapeptides yielded two dipeptides. d. Cyanogen bromide treatment 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? (Use the three letter codes for amino acids in your answer, and if there is more than one possible sequence just enter one. Separate amino acids using spaces.) Previous

Explanation / Answer

a. figure depicts a dipeptide alanine-valine (Ala-Val)

b.Chymotrypsin cleaves carboxyl side of large hydrophobic or aromatic amino acids . here it would have cleaved at carboxyl end of Tyr, but yielded only two tetrapeptides, possible one Tyrosine(Tyr residue) is at the end of the tetrapeptides. (X1-X2-X3-Tyr)

c.Trypsin cleaves carboxyl terminal side of Arginine or Lysine, here it would have cleaved after Arginine

X1-Arg , X2-X3

d. Cyanogen bromide cleaves at carboxyl terminal of methionine residue, and here it yields a free Tyr, so one of the tetrapeptides is X-Arg-Met-Tyr. This tetrapeptide is Ala-Arg-Met-Tyr (since alanine present in each tetrapetide in step b.)

e. N terminal analysis of the other tetrapeptide lists Asparagine, Asn so other tetrapeptide is Asn-Ala-Val-Tyr (Tyr at end since in step b. two tetrapeptides based on chymotrypsin cleavage)

Hence the octapeptide is N terminal -Asn-Ala-Val-Tyr- Ala-Arg-Met-Tyr-C-terminal

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