= Protein quaternary structure
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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 $2\times12=24$ residues and $2\times11=22$ <peptide bonds>. Assembly alone adds no <peptide bond> joining their ends. Stable functional assembly should also be distinguished from <protein aggregation>. See https://www.ncbi.nlm.nih.gov/books/NBK26830/[Alberts et al.: proteins as assemblies of polypeptide subunits].
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