AndMotioninBiology

2.4.QuaternaryStructureandSupramolecularAssemblies

Many proteins consist of two or more different polypeptide chains.These are termedoligomericproteins.Thetertiarysubunitsareheldtogetherinthequaternarystruc-ture through hydrogen bonding, salt bridges, and disulfide bonds.The main stabilizingforce, however, is hydrophobic interaction between the different subunits. When a singlepolypeptidefoldsintoatertiarystructurewithitshydrophilicsidechainsexposedandits hydrophobic side chains shielded inside, there may still be some hydrophobic sectionson the outside.In such cases, two or more subunits will assemble themselves so thattheir exposed hydrophobic sections are in contact, thus placing the hydrophobic sectionsintheircombinedinterior.Anexampleofaoligomericproteinishemoglobin,whichisa tetramer consisting of two a and two b subunits.Other proteins may be even morecomplex; for example, ribosomes and replisomes are macromolecules made up of manydifferentproteinsubunits.

3. 3.MotifsandDomains

   1. 3.1.Second-and-a-half-aryStructures:Motifs

Motifs are basic building blocks that occur repeatedly in different proteins. Some exam-plesofmotifsarethehelix-loop-helixmotif,thezinc-fingermotif,thecoiledcoilmotif,
Figure5:Examplesofstructuralmotifs

the beta barrel, and the beta helix (Figure 5).The coiled coil motif is found in fibrousproteins and consists of two alpha-helices wrapped around each other (see Figure 6). Theresidues at positions a and d are buried in the core of the coiled coil. Thus these residuesaretypicallyhydrophobic.Hydrophilicresiduesarelikelytobefoundatpositionseand

g. Another interesting motif is the beta helix, which is only found in prokaryotic proteinsandnoteukaryoticones.Thismayperhapsbeanexampleofdivergentevolution.

2. 3.2.Second-and-2/3-aryStructures:Domains

A domain is a region of protein’s amino acid sequence that has evolutionary, structural,or functional significance. Domains determine the binding sites of proteins. An exam-ple of a domain is the Epidermal Growth Factor (EGF) domain.EGF is generated byproteolytic cleavage of a precursor protein containing multiple EGF domains and a mem-brane spanning domain. Another example of protein domains is the globular domain andfibrousdomaininviralmembraneproteins.

4.