Question

A protein biochemist attempted to determine the amino acid sequence of a decapeptide. Use the results from the trypsin, chymotrypsin, and cyanogen bromide treatments to suggest the amino acid sequence of this decapeptide.

Trypsin digestion gave two fragments with the following residues (not in order):

• T1: Ala, Arg, Phe, Pro, Thr, Trp, Tyr
• T2: Lys, Met, Val

Chymotrypsin digestion gave three fragments with the following residues (not in order):

• CT1: Ala, Phe
• CT2: Pro, Arg
• CT3: Thr, Trp
• CT4: Lys, Met, Tyr, Val

N‑terminal analysis of the CT1 tetrapeptide indicated that the N‑terminal amino acid was Ala.

The cyanogen bromide gave only one single amino acid, methionine, and a nonapeptide.

What is a possible sequence of the decapeptide? Use three‑letter abbreviations in your answer, and add a dash between each residue.

Answer 1

CNBr cleaves at C-terminal of Met.

As it resulted in Met and nonapeptide. So, Met is at the N-terminal.

Analysis of Trypsin Cleavage:

Trypsin cleaves at C- terminal of Lys and Arg but not if these are followed by Pro.

Now T2 has Met, so it is N-terminal fragment.

Hence, the T2 , in sequence,  is Met-Val-Lys

T1 contains Arg but is not cleaved, so, it is Arg-Pro

Analysis of Chymotrypsin digestion:

Chymotrypsin cleaves at C terminal of Phe, Tyr and Trp but not if followed by Pro

CT4 has Met, so, the sequence from N-terminal is

Met-Val-Lys-Tyr

Now, for CT1, Ala is N-terminal amino acid. Hence, CT1 is as Ala - Phe

Now, chymotrypsin does not cleaves if next to preferred residue is Pro.

So, CT2 in sequence is Arg-Pro

Hence, two possibilities of decapeptide are:

Met-Val-Lys-Tyr-Ala-Phe-Pro-Arg-Thr-Trp

Met-Val-Lys-Tyr-Thr-Trp-Ala-Phe-Pro-Arg