Which structure best illustrates the secondary structure of a protein (specifically an alpha helix)?

• A. An assembly of multiple polypeptide subunits into a functional complex (quaternary structure)
• B. A specific linear sequence of amino acids connected by peptide bonds (primary structure)
• C. A right-handed helical coil stabilized by hydrogen bonds between backbone $C=O$ of residue $i$ and backbone $N-H$ of residue $i+4$
• D. A complete three-dimensional fold of a single polypeptide chain into domains (tertiary structure)

Answer: (C)

Verified step by step guidance

1. Understand that protein structure is organized into four levels: primary, secondary, tertiary, and quaternary.
2. Recall that the primary structure is the linear sequence of amino acids connected by peptide bonds.
3. Recognize that the secondary structure refers to local folded structures stabilized mainly by hydrogen bonds between backbone atoms, such as alpha helices and beta sheets.
4. Identify that an alpha helix is a right-handed helical coil stabilized by hydrogen bonds between the backbone carbonyl oxygen (C=O) of residue i and the backbone amide hydrogen (N-H) of residue i+4.
5. Distinguish this from tertiary structure, which is the overall 3D folding of a single polypeptide, and quaternary structure, which is the assembly of multiple polypeptide subunits.