Index to Course Material Index to Section 11
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.
1pyk (44Kb) [Bbk|BNL|ExP|Waw|Hal]
E.coli produces a bifunctional enzyme which performs both the isomerisation of phospho-ribosyl anthranilate (residues 256 - 452) AND the synthesis of indole-glyceryl phosphate (residues 1 - 255), two steps in tryptophan biosynthesis. It comprises two very similar eight-stranded alpha/beta barrels, each barrel acting as a separate enzyme.
pdb file 1pii (359Kb) [Bbk|BNL|ExP|Waw|Hal].
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
, by Lauri Kuutti and at the Center for Scientific Computing, Finland.
(You also may want to re-visit the material on the
photoreaction centre.)
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.
tryptophan synthase 1wsy (406Kb)[Bbk|BNL|ExP|Waw|Hal]
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.
Allostery in haemoglobin will be described in the following chapter on Protein Interactions.
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.
Allostery in haemoglobin will be described in the following chapter on Protein Interactions.
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.
(Diagrams and further information can be found at Judith Murray-Rust's Neurotrophic Factors page.)
We usually find that hetero and especially homo-multimers exhibit symmetry, a subject worthy of study in itself; covered elsewhere in this section.
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/; or access ftp://pdb.pdb.bnl.gov/user_group/biological_units/ with your WWW browser. 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 RasMol by entering the appropriate 4-letter code (in place of ****) into a URL of the form
http://www.pdb.bnl.gov/cgi-bin/get-pdb-bu?id=****&type=view&encoding=none
There are other examples where dimerisation is necessary to actually create the active site of the enzyme in question. Instances of this include, for example,
Fiberous proteins are very elongated and interact to form long strong fibers. The extracellular matrix is an example of a fiberous protein (collagen) complex.
Filamentous proteins are globular and assemble into long tubules or filaments. The cell cytoskeleton controls cell morphology and intra-cellular transport and contains two such assemblies: microtubules and actin filaments.
Viruses are large assemblies of proteins, nucleic acids and sometimes lipids. They perform a wide range of functions from replication to membrane fusion which are made possible only by their diverse multimeric nature.
Last updated 28th Jun '96