Question

A decapeptide has the following amino acid composition. $$ \mathrm{Ala}_{2} \text {, Arg, Cys, Glu, Gly, Leu, Lys, Phe, Val } $$ Partial hydrolysis yields the following tripeptides. $$ \text { Cys-Glu-Leu + Gly-Arg-Cys + Leu-Ala-Ala + Lys-Val-Phe + Val-Phe-Gly } $$ One round of Edman degradation yields a lysine phenylthiohydantoin. From this information, deduce the primary structure of this decapeptide.

Short answer

Answer: The primary structure of the decapeptide is Lys-Val-Phe-Gly-Arg-Cys-Glu-Leu-Ala-Ala.

Step-by-step solution

List the given information

List down the given information to solve this problem: A decapeptide has the following amino acid composition: $$ \mathrm{Ala}_{2} \text{, Arg, Cys, Glu, Gly, Leu, Lys, Phe, Val} $$ Partial hydrolysis yields the following tripeptides: $$ \text {Cys-Glu-Leu, Gly-Arg-Cys, Leu-Ala-Ala, Lys-Val-Phe, Val-Phe-Gly} $$ Edman degradation yields a lysine phenylthiohydantoin, which implies that Lys is the first amino acid in the sequence.

Identify the terminal amino acids

From the partial hydrolysis, we know that the terminal amino acids are Cys, Gly, Leu, Lys, and Val, as they come first in the given tripeptides. We can also eliminate any repeated amino acids since there is only one of each in the tripeptides. The remaining terminal amino acids are Gly, Leu, and Val. Since we know Lys is the first amino acid from the Edman degradation, the remaining terminals can be assigned to the C-terminus.

Determine the N-terminal amino acid using Edman degradation

Since we know that Lysine phenylthiohydantoin is obtained from Edman degradation, this implies that Lys is the first (N-terminal) amino acid in the sequence. Therefore, the sequence starts with Lys.

Use hydrolysate data to determine the sequence

Knowing that Lys is the N-terminal amino acid, we can examine the tripeptides produced after partial hydrolysis. This will help us to deduce the order of the amino acids in the decapeptide. Lys is found in the tripeptide Lys-Val-Phe. So, the sequence starts as: Lys-Val-Phe

Identify the following amino acids in the sequence

After determining the first three amino acids (Lys-Val-Phe), we can use the terminal amino acids from step 2 to identify the next amino acids in the sequence: - After Phe, the next amino acid should be Gly (from the tripeptide Val-Phe-Gly) - The sequence becomes Lys-Val-Phe-Gly Now, we have the tripeptide Gly-Arg-Cys in the hydrolysate information. So, the sequence becomes Lys-Val-Phe-Gly-Arg-Cys Using the tripeptide Cys-Glu-Leu, we can extend the sequence: Lys-Val-Phe-Gly-Arg-Cys-Glu-Leu Finally, the remaining tripeptide is Leu-Ala-Ala; hence, the complete sequence is: Lys-Val-Phe-Gly-Arg-Cys-Glu-Leu-Ala-Ala

Write the final primary structure of the decapeptide

Using the steps above, we have determined the primary structure of the decapeptide as follows: $$ \text{Lys-Val-Phe-Gly-Arg-Cys-Glu-Leu-Ala-Ala} $$