Quaternary (4˚) Structure

Quaternary structure in proteins is the most intricate degree of organization still considered a single molecule. To be considered to have quaternary structure, a protein must have two or more peptide chains forming subunits. The subunits can be different or identical, and in most cases they are arranged symmetrically. In general, a protein with two subunits is called a dimer; one with three subunits a trimer; and one with four subunits a tetramer.

350px-DNA_polymerase.gif Changes in quaternary structure can occur through conformational changes within individual subunits or through reorientation of the subunits relative to each other. It is through such changes, which underlie cooperativity and allostery in "multimeric" enzymes, that many proteins undergo regulation and perform their physiological function. A good example would be a DNA polymerase (see image) and ion channels. Subunits are held together by the same types of interactions that stabilize the tertiary structure of proteins.

There is debate as to whether quaternary structure should be defined to include peptides linked by covalent (disulfide) bonds. In CMB, we will us quaternary structure to refer only to arrangement of subunits that are not covalently linked, although covalent disulfide bonds may occur within the individual subunits.