Instructural biology, aprotomer is the structural unit of anoligomeric protein. It is the smallest unit composed of at least one protein chain. The protomers associate to form a largeroligomer of two or more copies of this unit. Protomers usually arrange incyclic symmetry to form closedpoint groupsymmetries.
The term was introduced by Chetverin[1] to make nomenclature in theNa/K-ATPaseenzyme unambiguous. This enzyme is composed of two subunits: a large, catalytic α subunit, and a smaller glycoprotein β subunit (plus aproteolipid, called γ-subunit). At the time it was unclear how many of each work together. In addition, when people spoke of adimer, it was unclear whether they were referring to αβ or to (αβ)2. Chetverin suggested to call αβ a protomer and (αβ)2 a diprotomer. Thus, in the work by Chetverin the term protomer was only applied to ahetero-oligomer and subsequently used mainly in the context of hetero-oligomers. Following this usage, a protomer consists of a least two different proteins chains. In current literature of structural biology, the term is commonly also applied to the smallest unit ofhomo-oligomers, avoiding the term "monomer".
Inchemistry, a so-calledprotomer is a molecule which displaystautomerism due to position of a proton.[2][3]
Hemoglobin is aheterotetramer 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.[4]
Aspartate carbamoyltransferase has a α6β6 subunit composition. The six αβ-protomers are arranged in D3 symmetry.
Viralcapsids are usually composed of protomers.
HIV-1 protease forms ahomodimer consisting of two protomers.
Examples inchemistry includetyrosine and4-aminobenzoic acid. The former may be deprotonated to form the carboxylate and phenoxide anions,[5] and the later may be protonated at the amino or carboxyl groups.[6]
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