This is a wiki article on quaternary structure, it represents my views on the definition.
Many proteins are actually assemblies of multiple polypeptide [protein] chains. The quaternary structure refers to the number and arrangement of the protein subunits.
Unfortunately the word “protein” and polypeptide are not clearly delineated in the literature and are used interchangeably. But with respect to a TYPICAL helicase there are
6 identical copies of the helicases protein/polypeptide that are coded from a single helicase gene. The 6 copies must them be assembled by a machine called a ring loader that puts the 6 copies into the complex in the diagram (there are some variants of six copies, like 12 copies, etc.).
Then when helicase is done doing it’s job, there is a ring breaking machine.
Some enzymes like topoisomerase will spontaneously form quaternary structures in vitro in solution to look like this complex made of two topoisomerase polypeptides/proteins:
There are some functional enzymes with quaternary structures that I think violate expectation from random amino acid sequence. I’m presently exploring how difficult it is for random sequences to create functional quaternary structures. This is problem in bio-physics.
In a pre-biotic scenario, like Sidney Fox’s proteinoids, for sure, homo-meric quaternary structures of any length above 100 is astronomically remote as a matter of principle, unless we have a trivial senario of only one species of amino acid in the soup! This is because we don’t expect random sequences to be duplicated.
Just that duplication alone in a pre-biotic context, would be a violation of the law of large numbers, if the assembly were random.
Homo Sapien helicase polypeptide is 1270 residues long, forming a complex (like that shown above) would require 7620 residues.
If even 100 or so residues are critical to making the fold and the connection interfaces such that they can fit snugly around DNA this would be an astronomically remote event, perhaps comparable to a house of cards forming spontaneously by gust of wind.
For the reader’s benefit, here is a 2-minute animation of Helicase in action: