What is "protomer"

Etymology 1 n. (context chemistry English) Any prototropic tautomer. Etymology 2

n. (context biochemistry English) Any of the subunits that constitute an oligomeric protein.

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)? Chetverin suggested to call αβ a protomer and (αβ) a diprotomer.

Protomers usually arrange in cyclic symmetry to form closed point group symmetries.