Quaternary structure is the arrangement of the chains in a protein that contains more than one peptide chain. The types of interactions that hold quaternary structure are the same as for tertiary structure, but they occur between two different chains. The different chains within a protein with quaternary structure are called subunits.

The structure of hemoglobin, an oxygen transport protein, is shown below. Hemoglobin is a protein that contains a total of four subunits (shown as different colors). Try turning each of the subunits on and off using the controls below.

Subunit	A	B	C	D
Display	Ribbons	Sticks	Heme Groups

Notice how each of the subunits looks very much like the others. This type of symmetry is a feature found in many proteins. Each of the subunits contains one iron heme unit which is responsible for binding the oxygen.