Protein molecules are capable of coming together and forming complex arrangements or structures that are composed of more than one polypeptide subunit. Quite often, these larger structures also contain extra, non-protein material such as carbohydrate, lipid or even polynucleotides.
These giant, "super-molecules", can take the form of multicomponent enzyme complexes, strong filaments that hold the contents of cells in place or are specialized for such roles as contraction (in muscles), ribosomes, membranes and hybrid structures such as viruses.
Hemoglobin, for example, is an important oxygen carrying protein that consists of four polypeptides (two alpha-chains and two beta-chains) and four non-protein, iron containing, ring-like structures where the oxygen molecules are bound and carried.
Such proteins are said to have quaternary structure, and these larger polypeptide arrangements can sometimes reach very large sizes. Probably the record quaternary structure protein is found in the bacterium Escherichia coli, where a cytoplasmic enzyme called pyruvate dehydrogenase is composed of 88 polypeptide subunits organized into three massive groups of catalytic enzymes.
Cells and organisms take advantage of the fact that it is both easier, and needs less genetic information, to assemble very large complexes from smaller, easier to make subunits. For example, only one gene coding for a single polypeptide can produce enough small subunits to assemble into a very large protein filament that can stretch all the way across the interior of a cell. This is a great saving in the amount of genetic information which must be stored.
When the cell needs a particular structure, it is easy to assemble one from the component subunits, and then dissemble those same subunits when the structure is no longer needed. This needs less energy than any other method.
If a protein subunit has a region on its surface that is complementary to a different region on the same subunit, then a collection of these subunits will spontaneously unit and form larger and larger structures, without any external direction or control.
Some protein subunits are capable of assembling into flat sheets of material, others into tubes, yet others into spheres, but the champions of self assembly must be those tiny particles of disease and destruction known as viruses.