Hyperthermophiles: Taking the Heat and Loving It

Hyperthermophiles, a recently discovered group of microorganisms, are operationally defined as having an optimum growth temperature of at least 80°C and a maximum growth temperature of over 90°C. Most of the enzymes isolated so far from such organisms exhibit correspondingly enhanced thermostability. This property has been used both for investigation of fundamental biological questions of protein structure and stability, and for the development of technological applications that require protein stability at high temperatures. Examples of such applications of thermostable proteins include their use in biocatalysts, in various materials and in crystallization methods. The use of DNA polymerases from hyperthermophilic microorganisms in the polymerase chain reaction (PCR) is another of the more practical examples of the recent impact on biochemistry and molecular biology of enzymes isolated from hyperthermophilic organisms. To date, the maximum growth temperatures observed for a hyperthermophilic organism is about 110°C. Whether this is the upper limit for life is unknown; some workers estimate that the maximum growth temperature for, as yet, uncultured organisms may approach, or even exceed 150°C [1]. In any event, it is clear that the surface has barely been scratched in the study of extremely thermostable proteins from known hyperthermophiles. Two basic questions often come to mind when considering these enzymes: the practically motivated, how can I get some of these enzymes and the more fundamental question, how do these enzymes achieve extreme thermostability? Aspects of both of these issues are briefly discussed below. More detailed discussions of hyperthermophiles and their proteins may be found in review articles [2], [3], [4] and [5]