Although many proteins are complete and fully functional as a single polypeptide chain with a tertiary structure, there are many other proteins that are composed of two or more polypeptide chains or subunits with a tertiary structure that come together to form a higher-order quaternary structure. In the case of a multi-subunit protein, the activity of the complex depends on the quaternary structure formed by the combination of the various tertiary structures.

In a protein with quaternary structure, the polypeptide subunits may be identical or different (Fig. 4.10). Fig. 4.10a shows an example of a protein produced by HIV that consists of two identical polypeptide subunits. By contrast, many proteins, such as hemoglobin (shown in Fig. 4.10b), are composed of different subunits. In either case, the subunits can influence each other in subtle ways and influence their function. For example, the hemoglobin in red blood cells that carries oxygen has four subunits. When one of these binds oxygen, a slight change in its structure is transmitted to the other subunits, making it easier for them to take up oxygen. In this way, oxygen transport from the lungs to the tissues is improved.