This is SWISS-MODEL Beta server. Modelling hetero-oligomers is now possible!


You can input multiple target sequences for a single modelling job.
Enter the first sequence the normal way with copy and paste, or add a UniProtKB AC, then add another - and another - and another!
You can revert to the stable SWISS-MODEL website clicking here

5 Quaternary Structure


Single protein strands are seldom the functional unit. Quaternary structure describes how subunits of a protein complexes aggregate. If all subunits are of the same type they are called homomultimers, otherwise heteromultimers.
Below are a few examples to illustrate different quaternary structures.

5.1 Covalently-connected tertiary domains

In this class of protein, domains are usually formed as modules covalently "strung together" on a single polypeptide chain. The individual chains of antibodies are like this, strings of immunoglobulin domains. However, light and heavy chains then combine to produce hetero-multimers, which may even associate into higher complexes, as with IgM.

In the case of the single polypeptide chain of pyruvate kinase there are four domains; the central TIM-barrel is the catalytic domain, whereas the other three play no direct role in enzymatic activity. However, the small N-terminal domain of 42 residues is involved in intersubunit contacts when four copies associate to form a homo-tetramer.

E.coli produces a bifunctional enzyme which performs both the isomerisation of phosphoribosyl anthranilate and the synthesis of indole-glycerol phosphate, two steps in tryptophan biosynthesis. It comprises two very similar eight-stranded alpha/beta barrels, each barrel acting as a separate enzyme.

5.2 Hetero-multimers


In this case we see different tertiary domains aggregating together to form a unit. Hemoglobin consists of two alpha (green) and two beta (yellow) subunits.

Cartoon view of Hemoglobin
Cartoon view of Hemoglobin. The two alpha subunits are shown in green, the two beta subunits in yellow.

Sometimes, several domains are found in a single enzyme complex, either in a single polypeptide chain, or as an association of separate chains.

Often the domains have related functions, for instance, where one domain will be responsible for binding, another for regulation, and a third for enzymatic activity. Cellobiohydrolase provides an example of such a protein. It is not uncommon to find more than once the same chain in a protein complex.

5.3 Homo-multimers


It is far more common to find copies of the same tertiary domain associating non-covalently. Such complexes are usually, though not always symmetrical. Because proteins are inherently asymmetrical objects, the multimers almost always exhibit rotational symmetry about one or more axes. Below are four examples of homo-tetramers.

Homo-multimer structures of proteins
Four examples of homo-multimer forming proteins. Top left: Homo-dimer (pdb: 2O99), top right: homo-trimer (pdb: 1GU9), bottom left: homo-tetramer (pdb: 5KKC), bottom right homo-14-mer (the protein forms two homo-heptameric rings which sit on top of each other, it forms a dimer of heptamer.) pdb:1GRL.

5.4 Larger Structures

The molecular machinery of the cell and indeed of assemblies of cells, rely on components made from multimeric assemblies of proteins, nucleic acids, and sugars. A few examples include :

Cartoon view of l1 protein of papillomavirus 16
Cartoon view of l1 protein papillomavirus 16 (homo-60-mer)