Which statement correctly identifies the two most common secondary structures of proteins?

Which statement correctly identifies the two most common secondary structures of proteins?

• A. Alpha helices and beta pleated sheets
• B. Alpha helices and random coils
• C. Beta sheets and gamma turns
• D. Alpha helices and beta turns

Answer: (A)

Secondary structure comes from regular patterns of hydrogen bonding in the protein backbone. The two most common motifs are the alpha helix and the beta pleated sheet. In an alpha helix, the chain coils into a right-handed spiral, with every backbone carbonyl oxygen hydrogen-bonded to the amide hydrogen four residues ahead, which stabilizes a tight, repetitive helix along the length of the chain. In a beta sheet, extended strands align side-by-side and hydrogen bonds form between adjacent strands, creating a sheet that can run parallel or antiparallel to its neighbors. These arrangements maximize stable hydrogen bonding and provide versatile, stable building blocks used throughout many proteins, which is why they appear so frequently. In contrast, random coils are irregular, nonrepeating segments lacking a regular hydrogen-bond network, and turns are short loops that connect helices and sheets rather than representing major, prevalent secondary elements. That combination is why alpha helices and beta sheets are counted as the primary, most common secondary structures.

Secondary structure comes from regular patterns of hydrogen bonding in the protein backbone. The two most common motifs are the alpha helix and the beta pleated sheet. In an alpha helix, the chain coils into a right-handed spiral, with every backbone carbonyl oxygen hydrogen-bonded to the amide hydrogen four residues ahead, which stabilizes a tight, repetitive helix along the length of the chain. In a beta sheet, extended strands align side-by-side and hydrogen bonds form between adjacent strands, creating a sheet that can run parallel or antiparallel to its neighbors. These arrangements maximize stable hydrogen bonding and provide versatile, stable building blocks used throughout many proteins, which is why they appear so frequently. In contrast, random coils are irregular, nonrepeating segments lacking a regular hydrogen-bond network, and turns are short loops that connect helices and sheets rather than representing major, prevalent secondary elements. That combination is why alpha helices and beta sheets are counted as the primary, most common secondary structures.