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, 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

Answer: Met-val-lys-tyr-thr-trp--Ala-arg-pro-phe

i. CNBr cleaves carboxyl side of met [Met-(nine AA)]

ii. Trypsin cleaves on the C side of Lys and Arg, but not on the N side of Pro. This means that Lys is the last amino acid in T2 (the only trypsin fragment containing Met).

Since Met is rst, this gives: Met-val-lys-X-X-X-X-X-X-X

iii. The middle portion of the sequence can be determined from the chymotrypsin digestion. Chymotrypsin cleaves on the C side of Phe, Trp, and Tyr, but not on the N side of Pro.

iv. CT3 has Lys, Met, Tyr, and Val. Since Met, Val, and Lys are the rst three amino acids Tyr must be the fourth. This now gives: Met-val-lys-tyr-X-X-X-X-X-X

v. CT1 has Ala, Arg, Phe and Pro. Since chymotrypsin cleaves on the C side of Phe, the last amino acid should be Phe. Also, CT1 has the first amino acid as Ala. Moreover, trypsin does not cleave on the N side of Pro.

So, the sequence of CT1 stands: Ala-arg-pro-phe

vi. In CT2, chymotrypsin cleaves on the C side of Trp. So, the sequence of CT2 stands as thr-trp and is present in the middle.

Therefore the whole sequence of the decapeptide stands as:

Met-val-lys-tyr-thr-trp-Ala-arg-pro-phe

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