Protomer
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In structural biology, a protomer consists of one single 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.
Examples:
Different polypeptide chains (subunits) are conventionally designated by Greek letters. The number of each subunit in the oligomeric complex is indicated by the subscript number, similar to a chemical formula 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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