What constitutes the secondary structure of a protein?

• A. Alpha helix and beta pleated sheet, hydrogen bonds
• B. Hydrophobic core interactions
• C. Disulfide cross-links between distant chains
• D. Primary amino acid sequence

Answer: (A)

The secondary structure is defined by regular local folding patterns that arise from hydrogen bonds between the peptide backbone, not from side chains. These backbone hydrogen bonds create the two classic motifs: alpha helices, a right-handed coil stabilized by bonds between the N–H of one residue and the C=O of the residue four places ahead, and beta sheets, where strands align side-by-side and are held together by hydrogen bonds between backbone carbonyl and amide groups on adjacent strands. Because these structures depend on the arrangement of the backbone, they are determined by the pattern of the chain itself rather than the specific side chains.

This is why the correct description emphasizes alpha helix and beta pleated sheet with hydrogen bonds. Hydrophobic core interactions involve mainly the interior packing of side chains and relate to the overall three-dimensional shape (tertiary structure), not the local backbone patterns. Disulfide cross-links between cysteines are covalent bonds that stabilize tertiary or quaternary structure, and the primary amino acid sequence refers to the linear order of residues before folding.