Quaternary Structure
The arrangement of two or more folded polypeptide chains into a functional protein complex.
Unit
Dimensionless
In depth
The subunits are held together by the same non-covalent forces that stabilise tertiary structure. Multi-subunit proteins can show cooperativity, in which binding at one site alters the affinity of the others — a form of regulation impossible for a single chain.
Examples in the real world
Haemoglobin's four subunits binding oxygen cooperatively to give its sigmoidal saturation curve.