What are the four levels of protein structure?

Protein structure has four hierarchical levels. Primary structure is the linear sequence of amino acids connected by peptide bonds, determined by the gene sequence. Secondary structure includes local folding patterns like alpha-helices (right-handed coils stabilized by hydrogen bonds) and beta-sheets (extended strands connected by hydrogen bonds). Tertiary structure is the overall 3D shape of a single polypeptide, stabilized by hydrophobic interactions, disulfide bonds, ionic bonds, and hydrogen bonds. Quaternary structure is the arrangement of multiple polypeptide subunits, found in multi-subunit proteins like hemoglobin.