An alpha helix of a protein is most likely to be disrupted if a missense mutation introduces the following amino acid within the alpha-helical structure –
**Core Concept**
An alpha helix is a type of secondary protein structure characterized by a spiral conformation, stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another, four residues away. The regular spacing and hydrogen bonding in alpha helices require specific properties of the amino acids involved.
**Why the Correct Answer is Right**
The correct answer is **D. Proline**. This is because proline is a unique amino acid with a cyclic side chain that disrupts the alpha-helical structure. Proline's side chain is bonded to the nitrogen atom of the amino group, making it unable to participate in the hydrogen bonding necessary for alpha-helix stability. As a result, proline introduces a kink or a bend in the alpha helix, making it the most likely amino acid to disrupt the structure.
**Why Each Wrong Option is Incorrect**
* **Option A:** Glycine is the smallest amino acid and has a flexible side chain, but it does not disrupt alpha-helical structures in the same way proline does.
* **Option B:** Alanine is a non-polar, uncharged amino acid that does not interfere with alpha-helical stability.
* **Option C:** Valine is a non-polar, branched amino acid that does not disrupt the regular spacing and hydrogen bonding of alpha helices.
**Clinical Pearl / High-Yield Fact**
When evaluating the effects of missense mutations on protein structure and function, consider the properties of the introduced amino acid, particularly its charge, size, and ability to participate in hydrogen bonding.
**Correct Answer:** D. Proline.