Protein biochemist attempted to determine the amino acid sequence of a decapepti

Question

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

The possible sequence of the decapeptide can be given by.

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

The explanation is as follows:-

Treatment of decapeptide by cyanogen bromide gives back methionine and a nonapeptide.Since Cyanogen bromide attacks the methionine sulfur atom and with subsequent reactions breaks the C terminal bond of the peptide linkage ,we can assume the C terminal of the decapeptide series is Methionine.

Met-(9 amino acid)

From chymotripsin and tripin treatment gives back two residues containing methionine (Lys,Met,Trp,Val) and (Lys,Met,Val).Since Trypsin Predominatly cleaaves the C-terminal side of lysin except when they are bound to C terminal Prolin.Thus we can safely assume the amino acid series in T2 residue as follows

Met-val-Lys

Now combining with the residue of CT3 we can write

Met-Val-Lys-Tyr

Also from CT1 we are having N terminal Amino acid as Alanine and since Trypsin also predominant cleaves the C terminal of the Arginine except when they bound to C terminal Prolin the sequence in the CT1 can be written as Ala-Phe-Pro-Arg

The mechanism of peptide bond cleavge by Chymotripsin is by hydrolysis reaction.The main substrates are Tyrosin and Tryptophan which are cleaved at carboxyl terminal.From the CT2 residue we can safely conclude Thr was attached to Tyr and Trp to Ala

The reason was because Tyrosin was C terninal amino acid and Alanin was N terminal Amino acid.

Thus the final sequence is

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

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