Quaternary structure
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.
- See also : primary structure -- secondary structure -- tertiary structure -- translation