Which is the only amino acid with good buffering capacity at physiological pH?
**Core Concept**
Histidine is a unique amino acid with a side chain that contains an imidazole ring, which has a pKa value close to physiological pH (7.4). This property allows histidine to act as a buffer and help maintain pH homeostasis in the body.
**Why the Correct Answer is Right**
Histidine's buffering capacity is due to the imidazole ring, which has a pKa of approximately 6.0. At physiological pH, the imidazole group is partially protonated, allowing histidine to accept or donate a proton and thus act as a buffer. This unique property is essential for maintaining proper pH levels in various bodily fluids and tissues. The imidazole ring is also involved in the activity of several enzymes, including histidine decarboxylase and histamine methyltransferase.
**Why Each Wrong Option is Incorrect**
**Option A:** Arginine is a basic amino acid, but its side chain pKa is too high (12.1) to contribute significantly to buffering capacity at physiological pH.
**Option B:** Glutamic acid is an acidic amino acid, but its side chain pKa is too low (4.25) to be effective as a buffer at physiological pH.
**Option D:** Valine is a nonpolar, neutral amino acid that lacks the imidazole ring and therefore does not have significant buffering capacity.
**Clinical Pearl / High-Yield Fact**
Histidine is also a precursor to histamine, a biogenic amine involved in allergic reactions, immune responses, and smooth muscle contraction. Understanding the unique properties of histidine is crucial for appreciating its role in various physiological and pathological processes.
**✓ Correct Answer: C. Histidine**