The overall 3D structure of a protein which is made up of multiple polypeptide chains (subunits), including how those subunits fit together, is the quaternary structure.

The shape of a protein is very important to its function and there are four levels of protein structure: primary, secondary, tertiary, and quaternary. Some proteins are made up of a single polypeptide chain and have only three levels of structure, however, some proteins are made up of multiple polypeptide chains, or subunits. When these subunits come together, they give the protein its quaternary structure. In general, the same types of interactions that contribute to tertiary structure (mostly weak interactions, such as hydrogen bonding and London dispersion forces) also hold the subunits together to give quaternary structure.