Which amino acid can protonate & deprotonate at neutral pH
**Core Concept**
The question is testing the student's understanding of the chemical properties of amino acids, specifically their ability to act as buffers at physiological pH (neutral pH). Amino acids with this property can donate and accept protons (H+ ions), making them zwitterionic molecules.
**Why the Correct Answer is Right**
The correct answer is a histidine, an amino acid with a side chain that can donate a proton (pKa around 6-7) and accept a proton (pKa around 9-10). This unique property allows histidine to act as a buffer at neutral pH, making it an essential component of many biological molecules, including enzymes and proteins. The imidazole ring of histidine is responsible for its buffering capacity.
**Why Each Wrong Option is Incorrect**
* **Option A:** Glycine is the simplest amino acid and lacks a side chain with buffering capacity. It does not have the necessary pKa values to act as a buffer at neutral pH.
* **Option B:** Alanine is a non-polar amino acid with a side chain that does not have the ability to donate or accept protons. It does not have the necessary chemical properties to act as a buffer at neutral pH.
* **Option C:** Valine is a non-polar amino acid with a side chain that does not have the ability to donate or accept protons. It does not have the necessary chemical properties to act as a buffer at neutral pH.
**Clinical Pearl / High-Yield Fact**
Histidine is an essential amino acid, meaning it cannot be synthesized by the human body and must be obtained through the diet. A deficiency in histidine can lead to impaired protein synthesis and function, particularly in enzymes and proteins that require histidine as a cofactor.
**Correct Answer: C**. Histidine.