In structural biology, a protomer is the structural unit of an oligomeric protein. It is the smallest unit composed of at least two different protein chains that form a larger hetero-oligomer by association of two or more copies of this unit.
The term was introduced by Chetverin  to make nomenclature in the Na/K-ATPase enzyme unambiguous. This enzyme is composed of two subunits, a large catalytic subunit, alpha and a smaller glycoprotein subunit, beta (plus a proteolipid, called γ-subunit). At the time it was unclear how many of each work together. In addition, when people spoke of a dimer, did they refer to αβ or to (αβ)2? Chetverin suggested to call αβ a protomer and (αβ)2 a diprotomer.
Hemoglobin is a heterotetramer consisting of four subunits (two α and two β). However, structurally and functionally hemoglobin is described better as (αβ)2, so we call it a dimer of two αβ-protomers, that is, a diprotomer.
Aspartate carbamoyltransferase has a α6β6 subunit composition. The six αβ-protomers are arranged in D3 symmetry.
Viral capsid are often composed of protomers.
Examples in chemistry include Tyrosine and 4-aminobenzoic acid. The former may be deprotonated to form the carboxylate and phenoxide anions, and the later may be protonated at the amino or carboxyl groups.
- Chetverin, A.B. (1986). "Evidence for a diprotomeric structure of Na, K-ATPase: Accurate determination of protein concentration and quantitative end-group analysis". FEBS Lett. 196: 121–125. doi:10.1016/0014-5793(86)80225-3. PMID 3002859.
- 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.
- C. Lapthorn, T. J. Dines, B. Z. Chowdhry, G. L. Perkins and F. S. Pullen, Rapid Commun. Mass Spectrom., 2013, 27, 2399–2410.
- Buxbaum, E. (2007). Fundamentals of protein structure and function. New York: Springer. pp. 105–120. ISBN 978-0-387-26352-6.
- J. Am. Chem. Soc., 2009, 131 (3), pp 1174–1181
- J. Phys. Chem. A, 2011, 115 (26), pp 7625–7632
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