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Section 11 - Quaternary Structure

Literally, quaternary is subsequent to tertiary. In the context of biology, quaternary structures, such as enzymes, are assemblies of tertiary structural units, such as proteins. The assembly of bio-molecules into quaternary structures provides enhanced, multiple or novel functional roles. These assemblies may contain as few as two units, as in an enzyme complex, or hundreds, as in a virus. Often quaternary structure is organized symmetrically. This allows the formation of large complexes with only a few different tertiary units. In multiple unit (multimeric) complexes the single units (monomers) form contacts between each other. In some cases, the monomer must change its conformation in order to make these contacts. Insulin illustrates how the tertiary structure can be influenced by the quaternary organizational requirements. Enzymes exemplify the advantages of multimeric complexes that combine different functions. Multiple functionality is enhanced by forming multi-enzyme complexes. Finally, large assemblies play not only functional but structural roles on the cellular level.

While studying this section, bear in mind the interplay between tertiary and quaternary structure. For instance, consider the question of the definition of quaternary structure- is it an arbitrary classification? Also consider the advantages, both functional and structural, that are provided by the assemblies of tertiary structures.

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John Kenney, John Walshaw
Last updated 14th April '97