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:
The correct answer is C. They are stabilized by hydrogen bonds on the peptide backbone.
Alpha-helices and beta-sheets are two common types of secondary structures found in proteins. Both structures are formed by patterns of hydrogen bonding between the backbone atoms of the polypeptide chain, specifically between the carbonyl oxygen of one amino acid and the amide hydrogen of another. These hydrogen bonds help stabilize the regular folding patterns, giving the protein its specific shape.
An alpha-helix is a coiled structure where the backbone twists into a right-handed spiral. Hydrogen bonds form between every fourth amino acid, creating a stable helical shape. In contrast, beta-sheets consist of strands of polypeptides that lie side by side. These strands can be oriented in parallel or antiparallel directions, with hydrogen bonds forming between the backbone atoms of adjacent strands, resulting in a sheet-like arrangement.
Option A is incorrect because alpha-helices and beta-sheets are not only found in enzymes but in nearly all types of proteins, including structural, transport, and signaling proteins. Option B is incorrect because while alpha-helices are coiled, beta-sheets are more extended and flat, not coil-shaped. Option D is incorrect because alpha-helices and beta-sheets represent local folded regions within a protein, which constitute secondary structure. The overall shape of a complete protein, including how these elements fold and interact, is called the tertiary structure.
Understanding the role of hydrogen bonds in stabilizing these secondary structures is crucial for comprehending how proteins achieve their functional forms. The regular patterns of bonding allow the protein to fold efficiently and maintain stability under physiological conditions. This folding is essential for the protein’s biological activity, as the shape often determines the protein’s ability to interact with other molecules.