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 heterooligomer by association of two or more copies of this unit.
The term was introduced by Chetverin to make nomenclature in Na/K-ATPase unambiguous. Na/K-consists of an α- and a β-subunit (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.
Protomers usually arrange in cyclic symmetry to form closed point group symmetries.
Examples
Hemoglobin is a heterotetramer consisting of four subunits (two α and two β). However, structurally and functionally hemoglobin is described better as (αβ)2, we say it is 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 often are made from protomers.