Advanced Placement (AP) Biology Practice Exam

Which level of protein structure is characterized by alpha helices and beta pleated sheets?

1. Primary structure

2. Secondary structure

3. Tertiary structure

4. Quaternary structure

The correct answer is: Secondary structure

The presence of alpha helices and beta pleated sheets is a defining characteristic of secondary protein structure. This level of structure refers to the folding patterns that form within a polypeptide chain due to hydrogen bonding between the backbone atoms of the amino acids. The alpha helix is a coiled structure stabilized by hydrogen bonds between every fourth amino acid, while beta sheets consist of strands connected laterally by hydrogen bonds, forming a sheet-like arrangement. This structural organization is crucial because it contributes to the overall stability of proteins and can influence their function. The secondary structure is distinct from primary structure, which refers to the linear sequence of amino acids, tertiary structure that involves the three-dimensional folding due to interactions among R groups, and quaternary structure that pertains to the association of multiple polypeptide chains in a larger complex.