Question

The $p{K}_a$ values for the three ionisable groups $X,Y\text{and}Z$ of glutamic acid are 4.3, 9.7 and 2.2 respectively.

The isoelectric point for the amino acid is:

• A. 7
• B. 3.25
• C. 4.95
• D. 5.95

Answer 1

The correct option is B 3.25

Since this is an acidic amino acid, we take the $pKa$ values of the carboxylic groups and then proceed to take an average of those values to get the isoelectric point.

Hence, $pH$ at isoelectric point = $\frac{4.3+2.2}{2}=6.25/2=3.25$