What are the four levels of protein structure?

Proteins are organized into four distinct levels of structure, which describe the arrangement of their peptide chains in space:1

• Primary structure: This is the linear sequence of amino acids within the peptide chain. It is determined by the specific order of residues linked by peptide bonds.12
• Secondary structure: This level refers to the spatial arrangement of the main peptide chain, such as the α-helix or the pleated sheet (β-structure), formed by hydrogen bonds between carbonyl and imino groups.13
• Tertiary structure: This describes the three-dimensional folding of the peptide chain, determined by interactions between side chains (R groups) and adjacent segments of the main chain. These interactions include hydrophobic interactions, salt bridges, and disulfide bonds.14
• Quaternary structure: This level involves the arrangement of multiple separate peptide chains, or subunits, into a single large protein complex, such as hemoglobin.15