Quaternary structure

Quaternary structure is the fourth and highest level of protein structure, and only proteins built from more than one polypeptide chain have it. Each chain first folds into its own three-dimensional shape and is then called a subunit. Quaternary structure describes how many subunits a protein has, which kinds they are, and how they pack against one another. Two identical chains form a homodimer and two different chains a heterodimer; proteins with several subunits in general are called oligomers. A single-chain protein such as myoglobin has no quaternary structure.

The subunits are held together mostly by weak, noncovalent attractions where their surfaces meet: hydrogen bonds, ionic bonds between charged side chains, and hydrophobic contacts, where water-avoiding side chains cluster together. In some proteins, such as antibodies, covalent disulfide bonds also link the chains. Because the subunits touch, a shape change in one can spread to the others. This allows cooperative binding and allosteric regulation, in which binding at one site alters activity at another.

Hemoglobin is the classic example: a pair of alpha chains and a pair of beta chains, each holding an iron-containing heme group. Once one subunit binds oxygen, the rest take it up more easily, which gives hemoglobin its S-shaped oxygen-binding curve. Virus capsids are another example, built from many copies of a few proteins.