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1.5 Quaternary structure

This level of protein structure applies only to those proteins that consist of more than one polypeptide chain, termed subunits. In such proteins, sometimes referred to as multisubunit proteins, the same kinds of non-covalent interaction that stabilise the folded polypeptides also specify the assembly of complexes of subunits. Quaternary structure refers to the way in which the subunits of such proteins are assembled in the finished protein.

Multisubunit proteins can have a number of identical (homomeric) or non-identical (heteromeric) subunits. The simplest multisubunit proteins are homodimers – two identical polypeptide chains that are independently folded but held together by non-covalent interactions. An example of a homodimeric protein is the Cro repressor protein from bacteriophage lambda (Figure 19), which turns off expression of specific genes in its bacterial host. Haemoglobin, the red blood cell protein responsible for carrying molecular oxygen, contains two each of two different subunits, termed α and β globin (Figure 20). Note the symmetry of the two subunits in both these quaternary structures.

Figure 19 Bacteriophage lambda Cro repressor protein is a homodimer. It is represented here in three ways. (a) Ribbon format, with the subunits coloured differently. (b) Space-filling format with the subunits coloured as in (a). (c) Ribbon format with the polypeptide backbone coloured according to secondary structure: α helix, red; β sheet, cyan; turn, green; random coil, white. (Based on pdb file 1cop)
Figure 20 Space-filling model of human haemoglobin in its deoxygenated form (deoxyhaemoglobin). The two α and two β subunits are indicated and are coloured differently. The haem complexes (coloured red) that associate with each subunit are only visible in the α1 and β2 subunits in this representation. (Based on pdb file 1a3n)

Some proteins can assemble to form long filaments. Two such proteins are actin and tubulin. These proteins exist in a soluble globular form that can assemble into long helical filaments called microfilaments (actin) and microtubules (tubulin) (Figure 21). Both these proteins are important components of the cytoskeleton, and the filaments that they form can extend from one end of a cell to another (Figure 21c). Dynamic assembly and disassembly of microfilaments and microtubules is integral to the responsive nature of the cytoskeleton during many cell processes such as cell division, intracellular transport, and cell movement and adhesion.

Figure 21 (a) Actin subunits can assemble into helical filaments called microfilaments. (b) Microtubules consist of repeating tubulin heterodimers (α and β subunits). (c) Microtubules (stained red) in a cultured mammalian cell during interphase; chromosomes are stained blue.

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