Amino acid which acts as a buffer at the physiological pH of 7.4 is:
**Core Concept**
The question tests the understanding of amino acids acting as buffers at physiological pH. Amino acids can act as buffers due to the presence of amino and carboxyl groups, which can accept or donate protons.
**Why the Correct Answer is Right**
The correct amino acid is one that has a pKa close to physiological pH, allowing it to effectively act as a buffer. Histidine, with its imidazole side chain, has a pKa near 7.4, making it an ideal buffer at physiological pH.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not have a side chain with a pKa near 7.4.
**Option B:** Incorrect as its side chain does not contribute to buffering at physiological pH.
**Option C:** Incorrect because, although it has a carboxyl group, its pKa is not near 7.4.
**Clinical Pearl / High-Yield Fact**
Histidine's unique ability to act as a buffer at physiological pH is crucial in proteins like hemoglobin, where it helps regulate oxygen binding.
**Correct Answer:** D. Histidine