Question

Account for the fact that the isoelectric point of glutamine (pI 5.65) is higher than the isoelectric point of glutamic acid (pI 3.08).

Short answer

Answer: The isoelectric point (pI) of glutamine is higher than that of glutamic acid because glutamine has a side chain with no ionizable groups, while glutamic acid's side chain has a carboxyl group, which is acidic. The additional acidic group in glutamic acid's side chain makes the molecule more negatively charged at higher pH levels compared to glutamine, resulting in a lower pH at which the net charge becomes neutral.

Step-by-step solution

1. Understanding the structure of glutamine and glutamic acid

Glutamine and glutamic acid are both amino acids, but they differ in their side chains. Glutamine has a side chain with an amide functional group, while glutamic acid has a side chain with a carboxylic acid functional group.

2. Understanding the concept of isoelectric point (pI)

The isoelectric point (pI) of an amino acid is the pH at which the molecule carries no net electrical charge. At the pI, the molecule exists as a zwitterion, which means that the positive charges (generally from protonated amino groups) and negative charges (generally from deprotonated carboxyl groups) are equal in the molecule.

3. Properties that influence the pI of amino acids

The pI of an amino acid depends on the pKa values of its ionizable groups. The pKa value indicates the pH at which half of the molecules of a specific group are ionized. An amino acid has at least two ionizable groups: the α-amino group (-NH3^+) and the α-carboxyl group (-COO^-). However, the side chain can also contain ionizable groups.

4. Ionizable groups in glutamine and glutamic acid

In glutamine, the ionizable groups are the α-amino group (pKa ~ 9.0) and α-carboxyl group (pKa ~ 2.0), whereas in glutamic acid, there are three ionizable groups: the α-amino group (pKa ~ 9.0), α-carboxyl group (pKa ~ 2.0), and the side chain carboxyl group (pKa ~ 4.0).

5. Calculating the pI of glutamine and glutamic acid

To determine the pI of an amino acid, we should take the average of the pKa values on either side of the zwitterionic form. In the case of glutamine, there are no ionizable groups in the side chain, so the pI is calculated as follows: pI = (pKa of α-amino group + pKa of α-carboxyl group) / 2 = (9.0 + 2.0) / 2 = 5.65. For glutamic acid, since it has a carboxyl group in its side chain, the pI is calculated using the pKa values of the ionizable groups on either side of the zwitterionic form. In this case, it is the pKa value of the α-carboxyl group and the side chain carboxyl group: pI = (pKa of α-carboxyl group + pKa of side chain carboxyl group) / 2 = (2.0 + 4.0) / 2 = 3.08.

6. Explaining the difference in pI values

The difference in pI values between glutamine and glutamic acid can be attributed to the difference in their side chains. Glutamine has a side chain with no ionizable groups, whereas glutamic acid has a side chain with a carboxyl group, which is acidic and leads to a lower pI. The presence of the additional acidic group in the side chain of glutamic acid makes the molecule more negatively charged at a higher pH compared to glutamine, thus having a lower pH at which the net charge becomes neutral.