Final answer:
Proline is known as the helix breaker because it has a cyclic structure (option B) that disrupts the regular alpha-helix formation, due to its distinctive ring-shaped side chain that binds to the backbone nitrogen and prevents necessary hydrogen bonding.
Step-by-step explanation:
Proline is referred to as the helix breaker because (B) It has a cyclic structure. The unique characteristic of proline is its ring-shaped side chain that links to the nitrogen in its backbone, causing a kink in the polypeptide chain and disrupting the regularity of the alpha-helix. Unlike proline, other amino acids have side chains (R groups) that allow the alpha-helix structure to form through hydrogen bonding without significant distortion.
The alpha-helix is a common secondary structure in proteins characterized by a right-handed coil with hydrogen bonds stabilizing the arrangement. However, proline lacks a hydrogen on its backbone amide nitrogen, which precludes it from forming the necessary hydrogen bonds for alpha-helix stability. This interruption leads to a bend that can disrupt the helical structure, hence why proline is known as the helix breaker.