Question

Vasopressin, a nonapeptide hormone secreted by the pituitary gland, functions by stimulating the kidney to retain water. Its sequence was determined by the following evidence: 1. Vasopressin is a cyclic compound containing a disulfide bridge between two cysteine residues. 2. When the disulfide bridge is reduced, vasopressin has the constitution Arg, Asn, Cys2, Gln, Gly, Ile, Phe, Pro. 3. Partial hydrolysis of vasopressin yields seven fragments: Asp-Cys, Ile-Glu, Cys-Phe, Arg-Gly, Phe-Ile-Glu, Glu-Asp-Cys, Cys-Pro-Arg. 4. Gly is the C-terminal group. 5. Both Glu and Asp are present as their side chain amides (Gln and Asn) rather than as free side-chain acids. From the above information, determine the primary sequence of vasopressin and then draw the structure of the Phe-Ile-Glu fragment as it would exist in the intact peptide.

Answer 1

The method of overlapping chains will be used. Information to note from the question is:

· Each amino acid (except Cys) occurs only once in the sequence.

· Gly is C- terminal.

· Glu and Asp obtained during partial hydrolysis = Gln and Asp in the hormone.

Now, the seven chains obtained by partial hydrolysis are :

1. Asp - Cys

2. Ile – Glu

3. Cys – Phe

4. Arg – Gly

5. Phe – Ile - Glu

6. Glu – Asp – Cys

7. Cys – Pro – Arg

Gly is in C terminal. So, the fragment which contains Gly (fragment 4) is the C-terminal fragment.

Thus, we have : - Arg – Gly.

To find who Arg is connected to, look for another fragment with Arg. Fragment 7 has it. So, add 7 to the sequence : -Cys – Pro – Arg – Gly (here, the – before Cys indicates that more fragments will join there since it is the C- terminal sequence)

There are two Cys residues. So, proceeding further would lead to ambiguity. Try to overlap the other chains :

From 5 and 2 we have : Phe – Ile – Glu-   . Adding 6 to this : Phe – Ile – Glu – Asp – Cys- .

(Here, the ‘-‘ is on C terminal side since we know that these are not the C-terminal sequences)

The fragment 1 is redundant to the above sequence. Adding 3 to this sequence :

Cys - Phe – Ile – Glu – Asp – Cys- .

Thus, finally, we need to join these two sequences:

Cys - Phe – Ile – Glu – Asp – Cys – and -Cys – Pro – Arg – Gly

Joining, we have : Cys - Phe – Ile – Glu – Asp – Cys – Pro – Arg – Gly

Since Asn and Gln are present as Asp and Glu due to hydrolysis, the primary sequence in the hormone is:

Cys - Phe – Ile – Gln – Asn – Cys – Pro – Arg – Gly

The structure of -Phe-Ile-Glu- as in the peptide is basically : -Phe-Ile-Gln - :

CNH – CH NH – C H – | $ HN - CH – £- си en City ď the Czech - Glina Ile - Phe -