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