Editing Protein structure (section)

===Quaternary structure===
{{Main|Protein quaternary structure}}
Quaternary structure is the three-dimensional structure consisting of the aggregation of two or more individual polypeptide chains (subunits) that operate as a single functional unit ([[multimer]]). The resulting multimer is stabilized by the same [[non-covalent interaction]]s and disulfide bonds as in tertiary structure. There are many possible quaternary structure organisations.<ref name="pmid19059267">{{cite journal | vauthors = Moutevelis E, Woolfson DN | title = A periodic table of coiled-coil protein structures | journal = Journal of Molecular Biology | volume = 385 | issue = 3 | pages = 726–732 | date = January 2009 | pmid = 19059267 | doi = 10.1016/j.jmb.2008.11.028 }}</ref> Complexes of two or more polypeptides (i.e. multiple subunits) are called [[multimer]]s. Specifically it would be called a [[dimer (chemistry)|dimer]] if it contains two subunits, a [[trimer (chemistry)|trimer]] if it contains three subunits, a [[tetramer]] if it contains four subunits, and a [[pentamer]] if it contains five subunits, and so forth. The subunits are frequently related to one another by [[symmetry group|symmetry operations]], such as a 2-fold axis in a dimer. Multimers made up of identical subunits are referred to with a prefix of "homo-" and those made up of different subunits are referred to with a prefix of "hetero-", for example, a heterotetramer, such as the two alpha and two beta chains of [[hemoglobin]].