How to find out sequence of the peptide?


An unknown nonapeptide is cleaved with concentrated HCl to yield the following amino acids:

GLU, CYS2, GLY, ILE, LEU, PRO, TYR, ASP

Sanger's reagent releases cysteine, and carboxypeptidase releases glycine. Partial hydrolysis of this peptide gives the following fragments:

1. ASP-CYS

2. CYS-TYR

3. TYR-ILE-GLU

4. CYS-PRO-LEU

5. ILE-GLU

6. LEU-GLY

7. GLU- ASP-CYS

What is the original sequence of this peptide?

Solution: Since the question says that it is a nonapeptide, we know there are 9 amino acids. Sanger's reagent gives the first amino acid, and carboxypeptidase gives the last amino acid. Therefore, first amino acid of this sequence is CYS and the last amino acid is GLY.

CYS --- --- --- --- --- --- --- GLY

Now, we can start putting our puzzle together.

We need to start with a fragment that starts with CYS since it is the first amino acid. We also need to overlap our fragments as much as possible to get the correct sequence. Sometimes, multiple possible arise. Not to worry! If you overlap the wrong fragments, you will not get the correct number of amino acids or it will not end with the correct amino acid, and that will tell you to try again.

Let's begin

CYS-TYR (if you choose number 4, you will see that you won't get the correct sequence)

TYR-ILE-GLU

ILE-GLU

GLU- ASP-CYS

ASP-CYS

CYS-PRO-LEU

LEU-GLY

Thus, our sequence comes out to CYS-TYR-ILE-GLU-ASP-CYS-PRO-LEU-GLY

We know this must be the correct sequence, since it is 9 amino acids long. The first amino acid is CYS and the last one is GLY as predicted previously.

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