Calculation of pI for a Polypeptide.
The pI is the pH where the molecule exists in an uncharged state. Here are some practice problems for calculating the pI of a polypeptide.
1. What is the pI of LEKAT?
The amino acids are:
The pKa values are as follows:
- pKa1 (carboxyl terminal)= 3.1
- pKa2 (glutamate side chain) = 4.4
- pKa3 (amino terminal) = 8--note that it is different when in a polypeptide.
- pKa4 (lysine amino group) = 10.0
At the pI the net charge of the molecule is zero. To find the uncharged molecule, first figure out what the charge on the polypeptide is after the loss of each proton.
To find the pI, average the two pKa values on either side of the neutral form of the polypeptide.
- Below pH 2, the charge is +2
- As base is added the first proton is removed from the terminal carboxyl group and the charge on the polypeptide is now +1.
- As the pH is increased to 6.4 (two pH units above pKa2) the second proton is removed and the charge on the polypeptide is now 0.
- As the pH is increased to 10 (two pH units above pKa3) the third proton is removed and the charge on the molecule is -1.
- As the pH is increased to 12 (two pH units above pKa4) the forth proton is removed and the charge on the molecule is -2.
- (pKa2 + pKa3)/2 = pI
- (4.4 + 8.0)/2 = 6.2
Draw out the titration curve for LEKAT to convince yourself that the uncharged form is predominant at this pH.
For more practice:
Be sure that you can predict the charge on a polypeptide at any pH. Remember that
when the pH is two units above the pKa, the proton is gone. (Recall that pH is a negative log function, an increase of two units corresponds to the addition of 100 times more base.)
- What is the pI of the polypeptide GATHER?
- What is the pI of the polypeptide HERE?
This returns you to the Biochemistry I page.
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