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Back to Main Index . . . last updated 7th.April'95
To best appreciate the content of this chapter you need to INTERACTIVELY EXAMINE a number of protein structures. If you haven't done so already, we STRONGLY URGE you to install the FREE molecular viewer software, RasMol and configure it to be automatically invoked by chemical/MIME. See our Technology Page to find out how.

Quaternary Structure

is that level of form in which units of tertiary structure aggregate to form homo- or hetero- multimers. This is found to be remarkably common, especially in the case of enzymes. The prokaryotic biosynthesis of tryptophan provides interesting examples which fall into each of the categories below. (See also Branden & Tooze `Introduction to Protein Structure' pub.Garland)

Covalently-Connected Tertiary Domains

The mosaic proteins are an example we've already come across, where the domains are usually formed as modules covalently `strung together' on a single polypeptide chain. The individual chains of antibodies are like this, strings of immuno-globulin 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 inter-subunit contacts when four copies associate to form a homo-tetramer.

E.coli produces a bifunctional enzyme which performs both the isomerisation of phospho-ribosyl anthranylate AND the synthesis of indole-glyceryl phosphate, two steps in tryptophan biosynthesis. It comprises two very similar eight-stranded alpha/beta barrels, each barrel acting as a separate enzyme. Examine the text-head of the pdb file.


In this case we see different tertiary domains aggregating together to form a unit. We've already seen an example in the case of the membrane-embedded photoreaction centre.

Sometimes, we find that 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. Take a glimpse at a delightful study of cellobiohydrolase.

Two (further) steps in the biosynthetic pathway of tryptophan (in S.typhimirium) are catalysed by tryptophan synthase which consists of two separate chains, designated alpha and beta, each of which is effectively a distinct enzyme. The biologically active unit is a hetero-tetramer comprised of 2 alpha and 2 beta units. Again you will recognise the TIM-barrel fold. Study the text-head of the pdb file.

We sometimes find slightly different versions of the same protein associating. Thus, haemoglobin has both an A-chain and a B-chain, which come together to form a hetero-dimer. Two copies of this then associate to form the normal haemoglobin tetramer. Which is equivalent to an A-dimer associating with a B-dimer. Or is it?

Student Exercise

In a naturally occurring mutant haemoglobin, we find a higher level of association, where fibres of the tetramer grow to distort the red blood cell (erythrocyte). See what the OMIM database has to say about Sickle-cell anaemia. Find out why it has a higher prevalence amongst ethnic groups of African origin. Identify the mutation in the structure and develop the causal explanation.


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. The majority of the enzymes of the metabolic pathways seem to aggregate in this way, forming dimers, trimers, tetramers, pentamers, hexamers, octamers, decamers, dodecamers, (or even tetradecamers in the case of the chaperonin GroEl).

The reason for this is now thought to be the allosteric cooperativity that results in increased catalytic efficiency, effectively a `sharing' of the small conformational changes that accompany substrate binding and catalytic activity. A good well-studied example is the `breathing motion' observed in the haemoglobin tetramer.

Another interesting case study is found with the growth factors where we see dimers formed in 3 different ways, corresponding to two-fold axes in different directions.

We usually find that hetero and especially homo-multimers exhibit symmetry, a subject worthy of study in itself.

Examples of symmetrical enzyme multimers in the form usually found in cells have been especially prepared and archived at Brookhaven in a directory dedicated to these biological units. It may be accessed by anonymous ftp to ftp.pdb.bnl.gov and going to directory /user_group/biological_units/ The contents and associated README are worth a look.

The entries in the PDB biological units directory may be arranged to MIME-invoke a molecular viewer such as HIV protease, the viral (aspartic) protease responsible for excising the separate proteins from the single polyprotein that the virus produces once inside the cell;

  • glutathione reductase 3GRS (monomer only), a crucially important enzyme in maintaining the reductive environment of the cytosol.

    Student Exercise

    Use whatever means at your disposal to recreate the dimer of glutathione reductase. Look at the PDB file header. Examine the active site that is formed. Learn about the enzyme mechanism. Find out about trypanothione reductase, and do the same. Compare and contrast. Think about the global implications for rational drug design.

    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 :-

    If you have MAGE installed and configured for Chemical MIME, you can view the Branden and Tooze, Protein Science and Protein Tourist Kinemages, some of which are relevant to this material. Again, refer to the Technology Page if necessary. There are some links to appropriate Kinemages within this chapter.

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    John Walshaw & Alan Mills, Birkbeck College, Apr'95