Introduction

Research during the last two decades has increasingly demonstrated that various microorganisms grow and live in environments that previously were considered unimagineable for the support of life. These habitats are characterized by extreme conditions with respect to temperature, pH, salinity, and pressure. At present the apparent limits for these biologically relevant physical parameters in the biosphere are -40°C to 115°C for temperature, pressures up to 120 MPa (corresponding to hydrostatic pressure in the deep sea), water activity (ow) of ~0.6, (as in salt lakes), and pH values between 1 and 11 (1). In order to survive and to proliferate in such hostile environments, organisms had to adapt their metabolic and other cellular functions to the persisting extreme condition. Especially important is the adaptation of stability and activity of enzymes and other proteins that have to be optimized to function under the extreme conditions. As a result of the requirement for molecular adaption, the properties of proteins, especially enzymes, from extremophiles have been a subject of much interest both for basic studies in biology and chemistry, as well as for potential use as biocatalysts in industrial applications.

Since the discovery of Archaea as a new branch on the phylogenetic tree (2), the number and variety of explored extremophilic organisms have increased enormously, as well as research on the biochemical aspects of extremophilic adaptation. An understanding of the molecular principles involved in extremophilic adaptive strategies could also be of considerable commeri-cal interest, as the same principles could be used for example in designing industrial processes involving biocatalysts that have to withstand extreme conditions with respect to temperature and pH. Given that there are constantly expanding sources of novel and unusually stable biocatalysts from extremophiles, the prospects of expanding the limits of biocatalysis in both novel, as well as in more conventional processes, could be realized (3). Enzymes with temperature optima ranging from close to the freezing point of water and to that in excess of its boiling point have been isolated, as well as enzymes that function in extreme salinity, over a wide range of pHs, pressures, and in essentially nonaqueous solvents (3).

High thermal stability is a highly desirable trait in many industrial enzymes, as processes involving enzymes are frequently carried out at high temperatures. There are some distinct advantages in running biotechnological processes at elevated temperatures. In fact, most such industrial processes are operated at temperatures >50°C. Benefits of higher operating temperatures include decreased viscosity, increased diffusion rates, and solubility of most organic compounds, and thus an increase in reaction rates. Higher temperatures also reduce the risks of microbial contaminations in the enzyme reactors (4-6). For these reasons, enzymes from hyperthermophiles have received great attention, as these enzymes are generally found to be very thermostable. This also makes them ideal experimental models for answering fundamental questions regarding the molecular basis for protein stability.

Enzymes from cold-adapted organisms, or psychro-philes, have gained increasing attention by researchers during the last several years, both as subjects of basic studies on molecular origins of cold-adaptation, and as potential industrial enzymes (7, 8). These enzymes are usually found to have higher specific activities at low temperatures than their counterparts from meso- and thermophiles. That property may be beneficial in several industrial applications, such as in the processing of sensitive biological materials, including foodstuffs, that have to be carried out under chilled or refrigerated conditions. Enzymes from cold-adapted organisms are usually found to be comparatively thermolabile, which may be beneficial in operations when enzyme treatment has to be terminated rapidly, without excessive treatment of the raw material (7).

The molecular basis for function and stability of proteins from organisms living at the extremes of temperature in the biosphere is still not well understood despite extensive research effort. However, understanding which structural principles direct extreme temperature adaptation of proteins are fundamental to our general understanding of their structural stability and function. Such information on structure-function relationships in these enzymes also forms the basis for their utilization as industrial biocatalysts under different, and perhaps novel, sets of conditions.

In this chapter we will attempt to give an overview of the current status of knowledge about enzymes that originate from organisms that are adapted to extremes of temperature.

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