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2-8 Quaternary structure is the total complex of a functional protein
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[Prev] | [Next]- Proteins can contain several polypeptides that form a functional unit. These protein complexes can be composed of several copies of the same polypeptide or different polypeptides.
- Glycoproteins are proteins with attached sugars.
Many proteins are actually complexes of several polypeptides. The arrangement of more than a single polypeptide into one protein is termed the quaternary structure of that protein. Protein complexes might contain two or more copies of the same protein or they may consist of any number of different polypeptides in various ratios. Such complexes are certainly not random, but reflect precise interactions among the protein subunits based on the same sorts of interactions (e.g. hydrophobic, hydrogen bonds, etc) described above.
Figure 2-16 shows the catabolite activator protein, an example of a protein containing identical subunits. In this case is has two subunits, so it is termed a dimer. Each subunit has a cyclic adenosine monophosphate molecule bound that activates the protein. In the active state this protein binds DNA and activates various genes in the cell.
Figure 2-16 Catabolite Activator Protein structure
Two depictions of the catabolite activator protein (CAP) of E. coli are shown. The left panel shows a ribbon depiction, with the two identical subunits shown in red and blue. CAP senses the presence of cyclic AMP (cAMP) in the cell and a molecule of cAMP bound to each monomer as indicated. The dimer is actually symmetrical, but that is not obvious here because the model is rotated slightly. The right panel shows a ball-and-stick representation of the same dimer and gives a better sense of the overall shape. Note that the bound cAMP molecules are buried within the protein (so how do they ever come and go?).
Dinitrogenase is an example of a protein containing non-identical subunits. As shown in Figure 2-17, the active protein has two copies of one protein (termed α) and two copies of a second protein (termed β), and therefore is also referred to as a α2β2 tetramer. This protein is responsible for the reduction of N2 gas to ammonia and is critical to global nitrogen cycling. The importance of the quaternary structure of dinitrogenase is only partially clear. The α subunits contain the metal cluster where N2 is actually reduced (or "fixed") and the β subunit helps form another metal center that helps transfer electrons to that active site. We can therefore understand why there might need to be an αβ dimer. However, it is not clear why there are two such αβ dimers hooked together in the tetramer found in nature. There is, for example, no apparent communication between the two active sites. Quite possibly, the fact that dinitrogenase is a tetramer is simply a relic of evolution; perhaps a precursor to dinitrogenase really did have a need to be a tetramer and there is simply no disadvantage to the protein retaining that organization.
Figure 2-17 Nitrogenase structure
Nitrogenase is made up of four protein chains, with two copies each of different proteins, termed the α and β subunits. Though not apparent in this view, the two α subunits are identical to each other, as are the two β subunits. The quaternary structure is referred to as an α2β2 tetramer.
Glycoproteins are proteins with attached sugars that are typically important for the proper function of the protein. In most cases the sugars are added to the protein after it has been translated. Proteins that encounter the outside environment of the cell are sometimes glycosylated to stabilize the protein to attack from degradative enzymes and destructive physical forces. Glycolipids are combinations of lipids and polysaccharides. In these cases the third glycerol hydroxyl attaches to a polysaccharide instead of a small polar molecule. These polysaccharides can be quite long, extending several microns into the outside environment. Lipopolysaccharides (one type of glycolipid and usually abbreviated LPS) impart several unique properties to the surface of gram-negative bacteria and we will have more to say about them when examining the cell wall.
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