The pKa's of isoleucine's α-Carboxyl group and α-Amino group are 2.3 and 9.8,respectively.Calculate the isoelectric point.
A)2.3
B)6.0
C)9.8
D)The isoelectric point cannot be calculated without the pKa value for the side chain.

Question

Quizplus · Accepted Answer

The isoelectric point (pI) is the pH at which the molecule has no net charge. It is calculated as the average of the pKa's of the ionizable groups that have lost or gained a proton, so the equation for the pI of an amino acid with both α-COOH and α-NH2 groups is: pI = (pKa1 + pKa2)/2.

In this case, the pKa1 is 2.3 for the α-COOH group and the pKa2 is 9.8 for the α-NH2 group. Therefore, the pI of isoleucine is:

pI = (2.3 + 9.8)/2 = 6.05

Rounding to one decimal place gives an answer of choice B, which is the best choice.

The PKa's of Isoleucine's α-Carboxyl Group and α-Amino Group Are