quaternary structure

n.
The structure formed by the noncovalent interaction of two or more macromolecules, such as that formed by four globin protein molecules to make hemoglobin or that formed by histones interacting with DNA to make a nucleosome.

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the fourth order of complexity of structural organization exhibited by protein, nucleic acid, and nucleoprotein molecules. It refers to the arrangement in space of the subunits of a multimeric macromolecule, and the ensemble of its intersubunit contacts and interactions, without regard to the internal geometry of the subunits. Hence, it is possessed only when the molecule in question is made up of at least two (identical or non-identical) subunits that are (at least potentially) separable, i.e. are not linked by covalent bonds.

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