Which amino acids are commonly known as helix breakers?

Helix breakers are amino acids that disrupt the stability of alpha helices in protein structures. Proline plays a key role in this regard because its unique cyclic structure does not allow for the standard backbone angles associated with alpha-helical structures. It introduces a kink or bend in the peptide chain, making it difficult for residues upstream and downstream to maintain the helical conformation. Glycine, on the other hand, has a very small side chain (just a hydrogen atom), which allows for greater flexibility and movement in the backbone, making it less conducive to stabilizing a helical structure.

In contrast, the other options contain amino acids that either stabilize helical structures or do not significantly disrupt their formation. For example, alanine and valine are more commonly found in helices due to their hydrophobic side chains that favor the interactions necessary for helix formation. Therefore, proline and glycine are recognized for their ability to interrupt the helical structure, making them the correct answer as helix breakers.