Secondary structure in proteins is formed by hydrogen bonds between amino acids in close proximity. Which structures are formed?

• A. Alpha helices
• B. Beta pleated sheets
• C. Alpha helices and beta pleated sheets
• D. Tertiary structure

Answer: (C)

Secondary structure arises from hydrogen bonds between the backbone atoms of a polypeptide, creating stable local shapes. The two classic motifs formed this way are alpha helices and beta pleated sheets. In an alpha helix, the backbone carbonyl of one residue hydrogen bonds with the amide hydrogen of another four residues away, producing a right-handed coil. In beta sheets, hydrogen bonds form between backbone groups of neighboring strands, which can be parallel or antiparallel, giving a pleated sheet. These structures are local, defined by backbone interactions, while tertiary structure refers to the overall 3D folding of the entire chain, driven by interactions among side chains and other forces.