What are the four levels of protein structure?

The four levels of protein structure are: Primary structure - the linear sequence of amino acids in the polypeptide chain, determined by the gene sequence. Secondary structure - local folding patterns stabilized by hydrogen bonds, including alpha-helices, beta-sheets, turns, and loops. Tertiary structure - the overall 3D shape of a single polypeptide, determined by interactions between side chains (hydrophobic, ionic, disulfide bonds, hydrogen bonds). Quaternary structure - the arrangement of multiple polypeptide subunits into a functional complex, present only in multi-subunit proteins like hemoglobin.