Question

Glutamic acid is another of the amino acids found in proteins (Chapter 27). Glutamic acid has two carboxyl groups, one with \(\mathrm{p} K_{\mathrm{a}} 2.10\) and the other with \(\mathrm{p} K_{\mathrm{a}} 4.07\). (a) Which carboxyl group has which \(\mathrm{p} K_{\mathrm{a}}\) ? (b) Account for the fact that one carboxyl group is a considerably stronger acid than the other carboxyl group.

Short answer

Answer: In glutamic acid, the alpha-carboxyl group has the lower pKa value (2.10) due to its proximity to the highly polar amino group (-NH2). This proximity allows for stabilizing interactions like hydrogen bonds and electrostatic interactions with the amino group, making the alpha-carboxyl group stronger as an acid compared to the gamma-carboxyl group.

Step-by-step solution

Recall the structure of glutamic acid

Glutamic acid is an amino acid with the following structure: HOOC-CH(NH2)-CH2-CH2-COOH. We can observe that it has two carboxyl groups, one on the alpha carbon (attached to the amino group) and one on the gamma carbon (attached to the side chain).

Determine which carboxyl group has which pKa value

By convention, the lower pKa value corresponds to the stronger acid. In glutamic acid, the alpha-carboxyl group is closer to the highly polar amino group (-NH2). The presence of a polar group nearby strengthens the acidity of the carboxyl group by stabilizing the resulting carboxylate anion through hydrogen bonding or electrostatic interactions. Therefore, the alpha-carboxyl group has the lower pKa value (2.10), and the gamma-carboxyl group has the higher pKa value (4.07).

Explain the difference in acidity between the carboxyl groups

To account for the difference in acidity between the alpha-carboxyl group and gamma-carboxyl group, we need to consider their proximity to the polar amino group (-NH2). The alpha-carboxyl group is closer to the amino group and can interact with it through hydrogen bonds and electrostatic interactions. These interactions stabilize the carboxylate anion formed upon deprotonation of the alpha-carboxyl group, making the alpha-carboxyl group more acidic than the gamma-carboxyl group. On the other hand, the gamma-carboxyl group is farther away from the amino group; therefore, it does not experience the same stabilizing interactions and is weaker as an acid.

Key concepts

pKa Values

Understanding the concept of pKa values is essential for anyone studying biochemistry, especially when it comes to amino acids such as glutamic acid. The term pKa is the negative log of the acid dissociation constant (Ka), and it represents how readily an acid donates its proton to a base.

In very simple terms, a lower pKa value indicates a stronger acid because it's more willing to give up its proton. The pKa value informs us about the reactivity of an acid in different environments, which is crucial in biological systems, where the precise acidity of a molecule can affect everything from enzyme function to metabolic pathways. In the case of glutamic acid, looking at the different pKa values of its carboxyl groups tells us about which hydrogen atoms are more likely to dissociate at given pH levels.

Carboxyl Groups

Carboxyl groups are functional groups present in various organic compounds, including amino acids. They consist of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (—COOH). This unique arrangement allows carboxyl groups to act as acids, as they can release a proton (H+) into solution, forming a carboxylate ion (—COO−).

In glutamic acid, one carboxyl group is attached to the alpha carbon next to the amino group, while the other is on the gamma carbon, three carbons away from the amino group. The position of each carboxyl group relative to other functional groups in the molecule can alter their ability to give up protons. This positional effect is why the two carboxyl groups in glutamic acid have different pKa values.

Amino Acids

Amino acids are the building blocks of proteins and play a critical role in various biological processes. Each amino acid has a central alpha carbon atom to which an amino group (—NH2), a carboxyl group (—COOH), a hydrogen atom, and a distinctive side chain are attached. The properties of an amino acid, including its acidity, are heavily influenced by the nature of its side chain.

Glutamic acid, with its two carboxyl groups, exhibits the influence of different side chain environments on acid strength. Understanding how the amino and carboxyl groups interact within the amino acid can help students predict how the molecule will behave in different pH environments, which is vital for understanding enzyme kinetics and protein folding.

Acid Strength

The strength of an acid refers to its ability to donate a proton. In aqueous solutions, this is commonly expressed as the degree of ionization that occurs when the acid dissociates to form a proton and its conjugate base. Factors affecting the acid strength include the stability of the conjugate base, the presence of electronegative atoms nearby, and the molecule's ability to delocalize the negative charge.

For the amino acid glutamic acid, the proximity of different groups influences how easily its carboxyl groups lose their protons. The alpha-carboxyl group, being close to the amino group, is a stronger acid as it forms a more stable conjugate base through interactions with nearby atoms. This detail is necessary for understanding how proteins behave biochemically and is invaluable in fields such as drug design and molecular biology.