The protein quaternary structure is the arrangement of multiple polypeptide chains in a functional assembly. A single-chain protein has protein tertiary structure without requiring protein quaternary structure. For an exact bookkeeping example, two intact model chains of twelve amino acid residues each contain residues and peptide bonds. Assembly alone adds no peptide bond joining their ends. Stable functional assembly should also be distinguished from protein aggregation. See Alberts et al.: proteins as assemblies of polypeptide subunits.
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Protein quaternary structure refers to the highest level of structural organization in proteins, where two or more polypeptide chains, known as subunits, come together to form a larger, functional protein complex. Each subunit in a quaternary structure can consist of one or more polypeptide chains, and these subunits can be identical (homomeric) or different (heteromeric).