Question

Give the pka values for the protonatable groups of glutamine, and then calculate its pi

Answer 1

Answer :

$pKa_1=2.17\\pKa_2=9.13\\pI=5.65$

Explanation: Glutamine a amino acid which have 2 protonable groups that is $NH_3^+$ and $COOH$ having 2 pKa's.

This amino acid has an extra amide group ($-CONH_2$) group which is not protonable.

The structures with their pKa values are provided in the image below.

pI or isoelectric point is the pH at which the amino acid have no net charge.

In this case, structure B has a net 0 charge, therefore it will have a pI present between $pKa_1$ and $pKa_2$.

pKa values of glutamine are:

$pKa_1=2.17\\pKa_2=9.13$

pI will be

$pI=\frac{pKa_1+pKa_2}{2}$

$pI=\frac{2.17+9.13}{2}$

pI = 5.65