Final answer:
The amino acid that is not found on alpha helices and is considered a 'helix breaker' is proline (option 2).
Step-by-step explanation:
The amino acid that is not found on alpha helices and is considered a 'helix breaker' is proline (option 2).
In the secondary structure of proteins, alpha-helices and beta-pleated sheets are commonly found. Alpha helices are formed by a coiled arrangement of amino acid residues, while beta pleated sheets consist of hydrogen bonds between adjacent segments of the polypeptide chain. Proline, due to its unique structure, disrupts the formation of alpha helices. Instead of having a flexible backbone, the R group of proline is linked to the amino group, forming a ring-like structure. This rigid structure introduces a kink in the helical conformation, hindering the formation of continuous alpha helices.