This is the highest level of protein
structure. The quaternary structure describes how proteins assemble together. A protein which stands alone is referred to as a monomer
. Some proteins polymerize
into long chains such as actin
which is involved in some way in just about every cell
process. Some proteins just assemble in pairs (dimers)
or in groups of three (trimers)
. If two different proteins pair up, its a heterodimer
, the same two - a homodimer
, and I'm sure you can imagine the rest.
While the tertiary structure is often sufficient to explain the function of monomeric (no pairing) proteins, many proteins do assemble into higher order clusters which then modulate their function.