Editing Protein quaternary structure (section)

==Description and examples==
 
Many proteins are actually assemblies of multiple [[polypeptide]] chains. The quaternary structure refers to the number and arrangement of the [[protein subunit]]s with respect to one another.<ref name="Predicting protein quaternary struc">{{cite journal | vauthors = Chou KC, Cai YD | title = Predicting protein quaternary structure by pseudo amino acid composition | journal = Proteins | volume = 53 | issue = 2 | pages = 282–289 | date = November 2003 | pmid = 14517979 | doi = 10.1002/prot.10500 | s2cid = 23979933 }}</ref> Examples of proteins with quaternary structure include [[hemoglobin]], [[DNA polymerase]], [[ribosome]]s, [[Antibody|antibodies]], and [[ion channel]]s.

[[Enzyme]]s composed of subunits with diverse functions are sometimes called [[holoenzyme]]s, in which some parts may be known as regulatory subunits and the functional core is known as the catalytic subunit. Other assemblies referred to instead as [[multiprotein complex]]es also possess quaternary structure. Examples include [[nucleosome]]s and [[microtubule]]s. Changes in quaternary structure can occur through [[Protein conformation|conformational changes]] within individual subunits or through reorientation of the subunits relative to each other. It is through such changes, which underlie [[cooperative binding|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 distinction 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 complex]]es.