A protein biochemist attempted to determine the amino acid sequence of a decapep

Question

A protein biochemist attempted to determine the amino acid sequence of a decapeptide Use the results from the trypsin, Chymolrypsin 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

let us do this stepwise, although lot of other information would be needed to decide the actual sequence.

Trypsin cleaves peptide chains mainly at the carboxyl side of the amino acids lysine or arginine,

Chymotrypsin preferentially cleaves peptide amide bonds where the carboxyl side of the amide bond tyrosine, tryptophan, and phenylalanine

step 1 determining ends

N-terminal amino acid - Ala

action of cyanogen bromide produces - methionine + nonapeptide

therefore, we have

Ala- - - - - - - - -Meth

Step 2

Analysing T2 - Lys, Met, Val

Met - Lys-val- - - - - - - - Ala

Step 3 Analysing CT3

CT3 - Lys, Met, Tyr, Val

so,

Met - Lys-val -Tyr--------Ala

now , we are left with5 more to adjust

step 4

From CT2 - Thr and Trp should be together

Met - Lys-val -Tyr-Thr-Trp -----Ala

Step 5

analysing CT1 and T1

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

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