At pH 8.50, what is the ratio of cationic to anionic functional groups in the NqrD subunit?

• A. 0.5
• B. 1
• C. 2
• D. 4

Answer: (B)

To determine the ratio of cationic to anionic functional groups in the NqrD subunit at pH 8.50, it's important to understand the behavior of amino acids and their side chains under different pH conditions.

At pH 8.50, a number of amino acid side chains will be in their deprotonated forms, resulting in the formation of anions, while others that have basic characteristics will be protonated, thus becoming cationic. Specifically, the amino acids that can contribute to cationic and anionic functional groups include aspartate and glutamate (anionic) and lysine, arginine, and histidine (cationic).

At this slightly alkaline pH, aspartate and glutamate will typically be negatively charged, while lysine remains positively charged. Arginine also maintains a positive charge, while histidine can be either positively charged or neutral depending on its side-chain pKa and the overall solution pH.

When the pH is above the pKa of the carboxyl groups in aspartate and glutamate (around 4.0), those groups will remain anionic. Conversely, the basic side chains like lysine and arginine remain cationic