In biochemistry, many proteins are actually assemblies of more than one protein molecule, which in the context of the larger assemblage are known as protein subunits. In addition to the tertiary structure of the subunits, multiple-subunit proteins possess a quaternary structure, which is the arrangement into which the subunits assemble. Enzymes composed of subunits with diverse functions are sometimes called holoenzymes, in which some parts may be known as regulatory subunits and the core is often called the catalytic subunit . Examples of proteins with quaternary structure include hemoglobin, DNA polymerase, and ion channels. Other assemblies referred to instead as multiprotein complexes also possess quaternary structure. Examples include nucleosomes and microtubules. Changes in quaternary structure are called conformational changes. It is through such changes, which underlie cooperativity and allostery in "multimeric" enzymes, that many proteins undergo regulation and perform their physiological function.
The above definition follows a classical approach to biochemistry, established at times when the disctinction between a protein and a functional, proteinaceous unit was difficult to elucidate. More recently, people refer to protein-protein interaction when discussing quaternary structure of proteins and consider all assemblies of proteins as protein complexes.
See also: primary structure -- secondary structure -- tertiary structure -- structural biology -- translation