Alpha-helix is found in which level of protein structure ?

The hierarchy of protein structure includes four distinct levels:

1. Primary structure: the linear sequence of amino acids linked by peptide bonds.

2. Secondary structure: regular, localized folding patterns of the polypeptide backbone, primarily stabilized by hydrogen bonds. The most common secondary structures are the α‑helix and the β‑sheet.

3. Tertiary structure: the overall three‑dimensional shape of a single polypeptide chain, arising from interactions among secondary‑structure elements.

4. Quaternary structure: the arrangement of multiple polypeptide subunits into a functional protein complex.

Since an α‑helix is a regular, hydrogen‑bonded coiling of the backbone that occurs locally within a single polypeptide chain, it is classified as a secondary structure element.