hguri 4.4 The lipids of the membranes of bacterial and eukaryotic cells (top) form a bilayer, while those of archaeal cells (bottom) form a monolayer. Only the lipid component of the membrane is shown.
tally different from that of bacterial and eukaryotic cells. In the latter two groups, their membranes consist of two layers of lipid, with associated proteins and carbohydrates. In the archaea, the lipid component of the membrane consists of a single layer (Figure 4.4). A lipid mono-layer is less likely to split apart than is a bilayer and hence is more heat stable. This membrane stability contributes to the ability of hyperther-mophilic archaea to survive higher temperatures than do thermophilic eubacteria or eukaryotes. As well as their membranes having a different structure, the lipids of archaeal membranes also have a different chemical composition from those of bacterial and eukaryotic cells. The lipids of thermophilic eubacteria are also different from those of bacteria which grow at more normal temperatures. Their membrane lipids are rich in saturated fatty acids (one of the components of lipids). Saturated fatty acids form stronger bonds between their molecules than do unsaturated fatty acids. The lipid composition of thermophiles thus makes their membranes more stable at high temperatures. However, in order to fulfil its function, a biological membrane must have a degree of fluidity. This allows control over the exchange of substances between the cell and its environment. The composition of the membranes of thermophiles means that their membranes have the required fluidity at high temperatures, without them disintegrating. However, if the temperature falls, the membrane solidifies, loses its fluidity and the cell can no longer exchange materials with its environment. Growth will then cease and the cell becomes dormant or dies.
The enzymes, and other proteins, of thermophiles are not only heat stable but they function best at high temperatures. This thermal stability and high optimum temperature is achieved by only a few changes in the amino acid sequence which makes up the structure of the protein. This causes the protein to fold in a way which increases its resistance to denaturation by heat. An increased number of bonds or bridges between the different parts of the protein molecule contributes to this stability, as does a more tightly packed interior which resists unfolding. An enzyme needs a degree of flexibility in order to interact with the molecules involved in the reaction which it catalyses. The increased thermal stability of the enzymes of thermophiles means that they have the required flexibility, and hence function best, at high temperatures. At lower temperatures, they become too inflexible to function and the growth of cells will cease.
Hyperthermophiles also need to prevent their genes from melting. The DNA of most organisms would melt at temperatures above 90°C. The DNA of some hyperthermophiles is made more heat stable by adjusting the proportions of some of the components (bases) which make up its structure. The DNA molecule of some hyperthermophiles is supercoiled, a configuration in which the double-stranded DNA molecules are further twisted. This presumably increases its heat stability.
Repair and protection mechanisms
As well as having heat stable molecules and membranes, thermophiles produce substances which protect their cells or which repair heat-
induced damage. The production of some low molecular weight compounds, such as trehalose and 2,3-diphosphoglycerate, is associated with thermotolerance in some organisms. Molecular chaperones are produced by thermophiles, as they are by all organisms, and help to stabilise and refold proteins as they start to denature near the upper limit of the organism's temperature range. Proteins which bind to DNA (such as histones) may contribute to thermal stability.
The extreme thermal stability of the enzymes of hypertheromophiles has earned them the name 'extremozymes'. For example, ferredoxin from Pyrococcus furiosus is most active at 118 °C and does not denature until 140 °C. A wide variety of extremozymes have been isolated from various hyperthermophiles. These are of interest as biological catalysts for a variety of industrial applications involving biological processes which function best at high temperatures - from laundry and dry cleaning through to food processing and pharmaceutical manufacture.
One example of an extremozyme which is of great practical importance is the DNA polymerase isolated from Thermus aquaticus (known as Taq polymerase). This is used in the automation of the repetitive steps in the polymerase chain reaction (PCR) which multiplies or amplifies specific DNA sequences for applications such as DNA fingerprinting. The PCR technique involves several cycles of exposure to high temperatures to dissociate the DNA before allowing it to cool and repolymerise. The use of a heat-sensitive DNA polymerase would require it to be added after each heating step, since it would be denatured by the heat. Since Taq polymerase is stable at high temperatures, it can catalyse DNA polymerisation through the several cycles of heating involved in the PCR without fresh enzyme having to be added at each cycle - making the process simpler and cheaper. The major revolution in molecular biology which occurred during the 1990s would not have been possible without this contribution from a humble extremophile.
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