What do alpha-helices and beta-sheets have in common?
A. They are only found in enzymes.
B.They are both shaped like a coil.
C.They are stabilized by hydrogen bonds on the peptide backbone
D.They represent the overall shape of a complete protein.
The correct answer and explanation is:
Correct Answer: C. They are stabilized by hydrogen bonds on the peptide backbone
Alpha-helices and beta-sheets are two common types of secondary structures in proteins. These structures form as a result of hydrogen bonding between the components of the polypeptide backbone, specifically between the carbonyl oxygen of one amino acid and the amide hydrogen of another. This hydrogen bonding gives stability and a distinct shape to these structures.
In an alpha-helix, the polypeptide chain twists into a right-handed coil, and hydrogen bonds form between every fourth amino acid. This regular pattern keeps the helix tightly wound and stable. The side chains of the amino acids extend outward from the helical backbone, minimizing crowding and making this structure very efficient for packing inside proteins.
In contrast, a beta-sheet is formed when segments of the polypeptide chain align next to each other, either in the same direction (parallel) or in opposite directions (antiparallel). Hydrogen bonds form between the strands, stabilizing the sheet. The side chains in beta-sheets alternate above and below the plane of the sheet, creating a flattened and pleated appearance.
Although alpha-helices and beta-sheets differ in shape and orientation, what they have in common is that they are both stabilized by hydrogen bonds within the peptide backbone. These interactions are independent of the side chains, meaning the backbone structure itself determines the formation of these secondary structures.
This shared feature is essential in protein folding, where secondary structures serve as building blocks that eventually lead to a protein’s tertiary and quaternary structure. Without the stabilizing effect of hydrogen bonds in the backbone, proteins would not maintain their proper shapes or perform their biological functions efficiently.