What is the possible sequence of the decapeptide? Protein biochemist attempted t

Question

What is the possible sequence of the decapeptide?

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, containing 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, Arg, Phe, Pro CT2: Thr, Trp CT3: 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 dashes between residues.

Explanation / Answer

arboxypeptidase A will cleave at the C-terminus, except for Pro, Lys, and Arg whereas

carboxypeptidase B cleavage occurs at C-terminal Arg and Lys. No action by either indicates

either a circular peptide or proline as C-terminus.

Trypsin cleavage yielded two tetrapeptides and free lysine. Since we are working with a

decapeptide, two moles of lysine must have been released. Further, the lysines must be located

such that trypsin cleavage releases them both. We can further assume that one of the

tetrapeptides ends in R. There are two possibilities:

(1) _ _ _ R K K _ _ _ P

(2) K _ _ _ R K _ _ _ P

From (c) we recognize that R could be located at positions 4 or 6 but only option (1) is consistent

with this observation.

From (d) we learn that M is located at position 8 and that the last two amino acids are N P in this

order. And, from (f) we identify the N terminal amino acid as serine. Thus,

(1) S _ _ R K K _ M N P

F or Y must be located at position 7 to insure that chymotrypsin cleavage produces a tripeptide

from the C-terminus and nothing longer. In addition an aromatic must also be located at

position 3.

The only amino acid unaccounted for is either Glu (E) or Gln (Q) which is located at position 2.

This gives:

(1) S (E/Q) (F/Y) R K K (F/Y) M N P

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