What structures are included in the secondary structure?

• A. Random coils
• B. Alpha helices and beta pleated sheets
• C. Only alpha helices
• D. Only beta sheets

Answer: (B)

Secondary structure is the local folding pattern of the protein backbone stabilized by hydrogen bonds between backbone atoms. The two main forms are alpha helices and beta sheets (beta-pleated sheets). Alpha helices are right-handed coils held together by hydrogen bonds between the backbone carbonyl of one residue and the amide hydrogen four residues away. Beta sheets consist of beta strands aligned side by side, with interstrand hydrogen bonds; they can be parallel or antiparallel.

Random coils are unstructured segments that don’t adopt a regular hydrogen-bonding pattern, so they aren’t considered secondary structure. Therefore, the structures included in secondary structure are alpha helices and beta sheets.