Glutamic Acid Has PKₐ's at 2

Question

Glutamic acid has pKₐ's at 2.2,4.3,and 9.7.What is the isoelectric point for glutamic acid?
A)3.25
B)4.3
C)5.4
D)7.0

Accepted Answer

The isoelectric point (pI) of an amino acid with three pKa values (such as glutamic acid, which has both a carboxylic acid group and an amino group in its side chain) is calculated as the average of the pKa values that flank the zwitterionic form. For glutamic acid, the relevant pKa values are 2.2 (for the carboxyl group) and 4.3 (for the carboxyl side chain), making the pI (2.2 + 4.3) / 2 = 3.25.