Editing Protein quaternary structure (section)

==Nomenclature==
[[File:1axc tricolor.png|thumb|The quaternary structure of this protein complex would be described as a homo-trimer because it is composed of three identical smaller protein subunits (also designated as monomers or protomers).]]
The number of subunits in an [[oligomer]]ic complex is described using names that end in -mer (Greek for "part, subunit").  Formal and Greco-Latinate names are generally used for the first ten types and can be used for up to twenty subunits, whereas higher order complexes are usually described by the number of subunits, followed by -meric.

{| border="1"
|-----
| valign="top" |
* 1 = [[protein subunit|monomer]]
* 2 = [[protein dimer|dimer]]
* 3 = [[protein trimer|trimer]]
* 4 = [[tetramer protein|tetramer]]
* 5 = [[Pentameric protein|pentamer]]
* 6 = hexamer
| valign="top" |
* 7 = heptamer     
* 8 = [[octamer]]  
* 9 = nonamer
* 10 = decamer
* 11 = undecamer
* 12 = [[dodecamer]]
| valign="top" |
* 13 = tridecamer
* 14 = tetradecamer
* 15 = pentadecamer*
* 16 = hexadecamer
* 17 = heptadecamer*
* 18 = octadecamer
| valign="top" |
* 19 = nonadecamer
* 20 = [[eicosamer]]
* 21 = 21-mer
* 22 = 22-mer
* 23 = 23-mer*
* etc.
|}

:<nowiki>*</nowiki>''No known examples''
The smallest unit forming a homo-oligomer, i.e. one protein chain or [[protein subunit|subunit]], is designated as a monomer, subunit or [[Protomer (structural biology)|protomer]]. The  latter term was originally devised to specify the smallest unit of hetero-oligomeric proteins, but is also applied to homo-oligomeric proteins in current literature. The subunits usually arrange in [[cyclic symmetry]] to form closed [[point group]] [[symmetries]].

Although complexes higher than octamers are rarely observed for most proteins, there are some important exceptions.  [[Capsid|Viral capsids]] are often composed of multiples of 60 proteins.  Several [[molecular machine]]s are also found in the cell, such as the [[proteasome]] (four heptameric rings = 28 subunits), the transcription complex and the [[spliceosome]]. The [[ribosome]] is probably the largest molecular machine, and is composed of many RNA and protein molecules.

In some cases, proteins form complexes that then assemble into even larger complexes.  In such cases, one uses the nomenclature, e.g., "dimer of dimers" or "trimer of dimers". This may suggest that the complex might dissociate into smaller sub-complexes before dissociating into monomers. This usually implies that the complex consists of different oligomerisation interfaces. For example, a tetrameric protein may have one four-fold rotation axis, i.e. point group symmetry 4 or ''C''<sub>4</sub>. In this case the four interfaces between the subunits are identical. It may also have point group symmetry 222 or ''D''<sub>2</sub>. This tetramer has different interfaces and the tetramer can dissociate into two identical homodimers. Tetramers of 222 symmetry are "dimer of dimers". Hexamers of 32 point group symmetry are "trimer of dimers" or "dimer of trimers". Thus, the nomenclature "dimer of dimers" is used to specify the point group symmetry or arrangement of the oligomer, independent of information relating to its dissociation properties. 

Another distinction often made when referring to [[oligomer]]s is whether they are homomeric or heteromeric, referring to whether the smaller protein subunits that come together to make the protein complex are the same (homomeric) or different (heteromeric) from each other. For example, two identical protein monomers would come together to form a homo-dimer, whereas two different protein monomers would create a hetero-dimer.