**Core Concept**
Protein structure and function are crucial for various cellular processes. The alpha helix is a fundamental secondary structure of proteins, stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another. This structure is essential for maintaining the native conformation of proteins.

**Why the Correct Answer is Right**
Proline is a unique amino acid that disrupts the alpha helix due to its cyclic side chain, which prevents the formation of the characteristic hydrogen bonds. This is because proline's side chain is covalently bonded to the main chain nitrogen, making it impossible for it to participate in hydrogen bonding. As a result, proline is often found at the end of alpha helices or in regions where the helix is disrupted.

**Why Each Wrong Option is Incorrect**
**Option A:** Glycine is a small amino acid that can participate in alpha helix formation, although its flexibility can sometimes disrupt the structure.
**Option B:** Glutamic acid is a charged amino acid that can participate in alpha helix formation, and its side chain can interact with other residues in the helix.
**Option C:** Leucine is a non-polar amino acid that can participate in alpha helix formation, and its side chain can be buried within the hydrophobic core of the helix.

**Clinical Pearl / High-Yield Fact**
Proline's inability to participate in alpha helix formation makes it an important residue for protein function and stability. Its presence can disrupt protein-protein interactions or alter the activity of enzymes.

**Correct Answer: D. Proline**