In structural biology

Structural biology

Structural biology is a branch of molecular biology, biochemistry, and biophysics concerned with the molecular structure of biological macromolecules, especially proteins and nucleic acids, how they acquire the structures they have, and how alterations in their structures affect their function...

, a protomer is the structural unit of an oligomeric protein. A protomer can be a protein subunit

Protein subunit

In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...

 or several different subunits, that assemble in a defined stoichiometry

Stoichiometry

Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...

 to form an oligomer

Oligomer

In chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...

. 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

Point group

In geometry, a point group is a group of geometric symmetries that keep at least one point fixed. Point groups can exist in a Euclidean space with any dimension, and every point group in dimension d is a subgroup of the orthogonal group O...

 symmetries. Protomers are the main subunit in a viral capsid

Capsid

A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...

.


Examples:

Different polypeptide chains (protein 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

Hemoglobin

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 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

Aspartate carbamoyltransferase

Aspartate carbamoyltransferase catalyzes the first step in the pyrimidine biosynthetic pathway .In E. coli, the enzyme is a multi-subunit protein complex composed of 12 subunits...

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

Animal Acetyl-CoA

Acetyl-CoA

Acetyl coenzyme A or acetyl-CoA is an important molecule in metabolism, used in many biochemical reactions. Its main function is to convey the carbon atoms within the acetyl group to the citric acid cycle to be oxidized for energy production. In chemical structure, acetyl-CoA is the thioester...

 Carboxylase