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 main reason for the difference in the isoelectric points of glutamine and glutamic acid is the presence of an extra ionizable carboxyl group in the side chain of glutamic acid, which causes its isoelectric point to be lower than that of glutamine.

Step-by-step solution

Understanding the structure of glutamine and glutamic acid

First, let's review the structure of the two amino acids - glutamine and glutamic acid. Both amino acids have a similar basic structure, which is composed of an amino group (-NH2), a carboxyl group (-COOH), and a side chain (R-group) connected to the central carbon atom (alpha carbon). Glutamine has a side chain with an amide (-CONH2) group, while glutamic acid has a side chain with an additional carboxyl (-COOH) group.

Determine the ionizable groups for each amino acid

In order to determine the pI, we need to focus on the ionizable groups in each amino acid. These are the groups that will lose or gain a proton (H+) depending on the pH value. For glutamine, the ionizable groups are: 1. Amino terminus (-NH2) 2. Carboxyl terminus (-COOH) For glutamic acid, the ionizable groups are: 1. Amino terminus (-NH2) 2. Carboxyl terminus (-COOH) 3. Side chain carboxyl group (-COOH)

Identify the charge of amino acids at different pH values

At different pH values, the ionizable groups will have different charges. At pH values below their pKa (the pH at which half of the molecules have lost a proton), the groups are fully protonated, while above their pKa, the groups are deprotonated. The charge of amino acids at a certain pH depends on the charge of their ionizable groups: For glutamine: 1. Amino terminus: below pH 8, positive; above pH 8, neutral 2. Carboxyl terminus: below pH 2.1, neutral; above pH 2.1, negative For glutamic acid: 1. Amino terminus: below pH 8, positive; above pH 8, neutral 2. Carboxyl terminus: below pH 2.1, neutral; above pH 2.1, negative 3. Side chain carboxyl group: below pH 4.3, neutral; above pH 4.3, negative

Calculate the isoelectric point

The isoelectric point (pI) is the pH value at which the net charge of the amino acid is zero. To find the pI, we need to calculate the average of the pKa values where the charge changes from positive to negative: For glutamine: pI = (pKa1 + pKa2) / 2 pI = (2.1 + 8) / 2 pI = 5.65 For glutamic acid: pI = (pKa1 + pKa3) / 2 pI = (2.1 + 4.3) / 2 pI = 3.08

Account for the difference in isoelectric points

The difference in the isoelectric points of glutamine and glutamic acid can be attributed to the presence of an extra ionizable carboxyl group in the side chain of glutamic acid. This functional group causes the isoelectric point of glutamic acid to be lower than that of glutamine, because it deprotonates more easily, due to its lower pKa value in the side chain, leading to a more negative net charge at a lower pH. Glutamine, on the other hand, does not have an ionizable group in its side chain and thus has a higher isoelectric point when its amino and carboxyl termini groups are considered.