Amino acid with dissociation constant closest to physiological pH is:
**Core Concept**
The question is testing the knowledge of amino acids and their pKa values, specifically their dissociation constants in relation to physiological pH (approximately 7.4). Physiological pH is the pH at which the concentration of hydrogen ions is at its lowest, and it is the pH at which most enzymes and proteins function optimally.
**Why the Correct Answer is Right**
The amino acid with a dissociation constant closest to physiological pH is Histidine. Histidine has a pKa value of approximately 6.0 for its imidazole group, which is close to the physiological pH. This allows Histidine to act as a buffer, maintaining a stable pH in the body. The imidazole group of Histidine is essential for its role in various biological processes, including enzyme activity and protein structure.
**Why Each Wrong Option is Incorrect**
* **Option A:** Glutamic acid has a pKa value of approximately 4.2, which is far from physiological pH.
* **Option B:** Aspartic acid has a pKa value of approximately 3.9, also far from physiological pH.
* **Option C:** Arginine has a pKa value of approximately 12.1, which is much higher than physiological pH.
**Clinical Pearl / High-Yield Fact**
Histidine is an essential amino acid, meaning it cannot be synthesized by the body and must be obtained through the diet. It plays a crucial role in the production of histamine, a neurotransmitter involved in immune responses and allergic reactions.
**Correct Answer: C. Arginine.**