Life on the edge
Meet nature’s extremophiles
Proteins are generally sensitive to heat, but in recent decades bacteria and other organisms that survive in extreme conditions have been found. These ‘extremophiles’ need specially adapted enzymes so they can carry out essential reactions. Thermophiles thrive in high temperatures. One example is the bacterium Thermus aquaticus, first isolated from the scalding hot springs of Yellowstone Park. Its enzymes work normally at the temperature of a hot cup of tea.
The polymerase chain reaction (PCR), which is widely used in laboratories to amplify small samples of DNA, uses cycles of heating and cooling to separate and rejoin DNA double helices. The synthesis of new DNA, which produces the amplification, needs a DNA polymerase enzyme that can withstand high temperatures. The answer was to use Taq polymerase from Thermus aquaticus.
A microscopic animal called the water bear (Milnesium tardigradum) can withstand prolonged drying out and is a psychrophile – it can withstand the freezing temperatures of the Antarctic. Experiments by NASA showed it is also able to tolerate the high vacuum and intense radiation of space. The water bears survive thanks to an unusually large repertoire of heat shock proteins – so named because they were first discovered as part of cells’ response to heat stress. Most organisms produce some of these, and they have a general role in protecting other proteins. Many are chaperone proteins, which help to stabilise newly formed protein chains.
Closer to home, our stomach contents are kept at a pH low enough to kill most cells and denature enzymes, but our digestive proteases operate happily in this acidic soup.