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In structural biology, a protomer is the structural unit of an oligomeric protein. A protomer can be a protein subunit or several different subunits, that assemble in a defined stoichiometry to form an oligomer. The protomer is the smallest subset of different subunits that form the oligomer. The protomers usually arrange in cyclic symmetry to form closed point group symmetries. Protomers are the main subunit in a viral capsid.

Different polypeptide chains (protein subunits) are conventionally designated by Greek letters. In the following examples, the number of each subunit in the oligomeric complex is indicated by the subscript number, similarly to a chemical formula of a compound consisting of elements.


Hemoglobin consists of two α-chains and two β-chains. The oligomer stoichiometry is thus α2β2. Hemoglobin is a heterotetramer consisting of four subunits (two α and two β). It is a dimer of two αβ-protomers.

Aspartate carbamoyltransferase has a α6β6 subunit composition. The six αβ-protomers are arranged in D3 symmetry.

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