What is the Isoelectric Point of Lys-Ser-Asp-Ala Tetrapeptide?

[leopard]

How can I find the pI of a tetrapeptide like Lys-Ser-Asp-Ala?

There are four ionizable groups that I have to take into account:

Alpha amino of Lys: 8.95
Amino side chain of Lys: 10.79
Carboxyl side chain of Asp: 3.86
Alpha carboxyl of Ala: 2.34

The two first are positively charged ionizing to uncharged groups. The two last are uncharged groups ionizing to negatively charged groups.

pI is somewhere between 3.76 and 8.95 of course, but that's not a very accurate answer.

 

[epenguin]

Sure ‘somewhere’ is not very accurate. What reason is there for that ‘somewhere’ to be nearer one of those figures than the other?

Firstly you should make sure you could do that question for a molecule with only 2 ionising groups say a carboxyl and an amino group. E.g. with pKs of 3.86 and 8.95?

Then at the isoelectric point (pH) of that molecule in what state are groups with pKs of 2.34 and 10.79 going to be?

 

[Blueshift5]

To find the pI of a tetrapeptide like Lys-Ser-Asp-Ala, you will need to take into account the pKa values of each ionizable group. The pKa value represents the pH at which half of the molecules are ionized and half are in their neutral form.

In this case, you can use the Henderson-Hasselbalch equation to calculate the pI. This equation takes into account the pKa values and the charge of each group.

First, you will need to determine the net charge of the peptide at different pH levels. At a low pH, the amino and carboxyl groups will be protonated, giving the peptide a net positive charge. As the pH increases, the carboxyl group will lose its proton, while the amino group will remain protonated. At a high pH, the carboxyl group will be deprotonated, giving the peptide a net negative charge.

Next, you can plot the net charge of the peptide against the pH and find the point where the net charge is zero. This is the pI of the peptide. You can also use a computer program or online calculator to make this calculation easier.

In the case of Lys-Ser-Asp-Ala, the pI would be closer to 3.76, as the positively charged groups have lower pKa values compared to the negatively charged groups. However, the exact pI may vary depending on the specific pKa values and molecular structure of the peptide.