One of the largest proteins, pyruvate dehydrogenase found in mitochondria, contains 72 polypeptide chains! The individual polypeptides are subunits of the larger protein assembly.

Quaternary structures are held together by the same types of chemical bonds that are found in tertiary structure, including a variety of weak bonds and disulfide bridges.

A fully functional alkaline phosphatase molecule, shown above, contains two polypeptide chains, each with an active site capable of phosphate hydrolysis. This is an example of a homomultimer, where both subunits are identical. Proteins with quaternary structure may also be heteromultimers, as in the case of hemoglobin, whose subunits are not all identical.

Do not confuse the term "polypeptide" with the term "protein." Polypeptide refers to the structure of a single chain. Every polypeptide has one free amino group (the N-terminus) and one free carboxyl group (the C-terminus). Protein refers to the overall functional assembly, created when one or more polypeptides fold up and become functional units. Some proteins consist of only a single folded polypeptide chain, but many proteins contain multiple polypeptides, and frequently inorganic atoms as well, such as zinc, iron, and magnesium.