# Protomer

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{{Short description|Structural unit of an oligomeric protein}}
{{distinguish|Promoter (disambiguation){{!}}Promoter}}
In [structural biology](/source/structural_biology), a '''protomer''' is the structural unit of an [oligomeric protein](/source/Protein_quaternary_structure). It is the smallest unit composed of at least one protein chain. The protomers associate to form a larger [oligomer](/source/Protein_quaternary_structure) of two or more copies of this unit. Protomers usually arrange in [cyclic symmetry](/source/cyclic_symmetry) to form closed [point group](/source/point_group) [symmetries](/source/symmetries).

The term was introduced by Chetverin<ref>{{cite journal|last1=Chetverin|first1=A.B.|title=Evidence for a diprotomeric structure of Na, K-ATPase: Accurate determination of protein concentration and quantitative end-group analysis|journal=FEBS Lett|date=1986|volume=196|issue=1 |pages=121–125|pmid=3002859|ref=Che-86|doi=10.1016/0014-5793(86)80225-3|bibcode=1986FEBSL.196..121C |doi-access=free}}</ref> to make nomenclature in the [Na/K-ATPase](/source/Na%2B%2FK%2B-ATPase) [enzyme](/source/enzyme) unambiguous. This enzyme is composed of two subunits: a large, catalytic α subunit, and a smaller glycoprotein β subunit (plus a [proteolipid](/source/proteolipid), called γ-subunit). At the time it was unclear how many of each work together. In addition, when people spoke of a [dimer](/source/dimer_(chemistry)), it was unclear whether they were referring to αβ or to (αβ)<sub>2</sub>. Chetverin suggested to call αβ a protomer and (αβ)<sub>2</sub> a diprotomer. Thus, in the work by Chetverin the term protomer was only applied to a [hetero-oligomer](/source/Protein_quaternary_structure) and subsequently used mainly in the context of hetero-oligomers. Following this usage, a protomer consists of a least two different proteins chains. In current literature of structural biology, the term is commonly also applied to the smallest unit of [homo-oligomers](/source/Protein_quaternary_structure), avoiding the term "[monomer](/source/monomer)".

In [chemistry](/source/chemistry), a so-called '''protomer''' is a molecule which displays [tautomerism](/source/tautomer) due to position of a proton.<ref>P. M. Lalli, B. A. Iglesias, H. E. Toma, G. F. de Sa, R. J. Daroda, J. C. Silva Filho, J. E. Szulejko, K. Araki and M. N. Eberlin, J. Mass Spectrom., 2012, 47, 712–719.</ref><ref>C. Lapthorn, T. J. Dines, B. Z. Chowdhry, G. L. Perkins and F. S. Pullen, Rapid Commun. Mass Spectrom., 2013, 27,
2399–2410.</ref>

==Examples==
[Hemoglobin](/source/Hemoglobin) is a [heterotetramer](/source/heterotetramer) consisting of four subunits (two α and two β). However, structurally and functionally hemoglobin is described better as (αβ)<sub>2</sub>, so we call it a dimer of two αβ-protomers, that is, a diprotomer.<ref name=Bux-07>{{cite book|last1=Buxbaum|first1=E.|title=Fundamentals of protein structure and function|date=2007|publisher=Springer|location=New York|isbn=978-0-387-26352-6|pages=105–120}}</ref>

[Aspartate carbamoyltransferase](/source/Aspartate_carbamoyltransferase) has a α<sub>6</sub>β<sub>6</sub> subunit composition. The six αβ-protomers are arranged in D<sub>3</sub> symmetry.

Viral [capsids](/source/capsid) are usually composed of protomers.

[HIV-1 protease](/source/HIV-1_protease) forms a [homodimer](/source/homodimer) consisting of two protomers.

Examples in [chemistry](/source/chemistry) include [tyrosine](/source/tyrosine) and [4-aminobenzoic acid](/source/4-aminobenzoic_acid). The former may be deprotonated to form the carboxylate and phenoxide anions,<ref>J. Am. Chem. Soc., 2009, 131 (3), pp 1174–1181</ref> and the later may be protonated at the amino or carboxyl groups.<ref>J. Phys. Chem. A, 2011, 115 (26), pp 7625–7632</ref>

==References==
{{Reflist}}

==External links==
{{wiktionary}}

Category:Structural biology
Category:Polymer chemistry

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Adapted from the Wikipedia article [Protomer](https://en.wikipedia.org/wiki/Protomer) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Protomer?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.