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.
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.
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.
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.
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 :
- Fibres of various sorts
- Gap Junctions