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Extremophiles as a source for novel enzymes
Bertus van den Burg
Microbial life does not seem to be limited to specific environments. During the past few decades it has become clear that microbial communities can be found in the most diverse conditions, including extremes of temperature, pressure, salinity and pH. These microorganisms, called extremophiles, produce biocatalysts that arefunctional under extreme conditions. Consequently, the unique properties of these biocatalysts have resulted in several novel applications of enzymes in industrial processes. At present, only a minor fraction of the microorganisms on Earth have been exploited. Novel developments in the cultivation and production of extremophiles, but also developments related to the cloning and expression of theirgenes in heterologous hosts, will increase the number of enzyme-driven transformations in chemical, food, pharmaceutical and other industrial applications.
Addresses IMEnz Bioengineering BV, Kerklaan 30, PO Box 14, 9750 AA Haren, The Netherlands e-mail: burgb@biol.rug.nl

chemical extremes such as salinity and pH (Table 1). Most of the extremophiles that have been identified to date belong to thedomain of the Archaea. However, many extremophiles from the eubacterial and eukaryotic kingdoms have also been recently identified and characterized. The notion that extremophiles are capable of surviving under non-standard conditions in non-conventional environments has led to the assumption that the properties of their enzymes have been optimized for these conditions. Indeed, data for aconsiderable fraction of the enzymes that have been isolated and functionally characterized from extremophiles support this assumption. In this review, I present and discuss recent examples of the discovery, isolation and application of enzymes from extremophiles.

Extremophiles
As illustrated in Table 1, the classification of ‘extreme environments’ refers to a wide variety of different conditions towhich microorganisms have adapted. The biocatalysts obtained from these microorganisms could be applicable in similarly diverse conditions (Table 1). For the degradation of polymers such as chitin, cellulose or starch, enzymes that are active at and resistant to high temperatures are often preferred. Under these conditions the solubility and, consequently, the accessibility of the substrate isimproved. Alternatively, if one needs to perform a stereospecific modification of a compound for the synthesis of a pharmaceutically relevant product in organic solvents, very different prerequisites apply to the biocatalysts. As salt is known to reduce water activity, enzymes from halophilic microorganisms could be the most suitable choice for application in nonaqueous media [4,5]. This diversity ofenvironments to which different extremophiles have adapted offers many exciting opportunities for a variety of applications.

Current Opinion in Microbiology 2003, 6:213–218 This review comes from a themed issue on Ecology and industrial microbiology Edited by Bernard Witholt and Eugene Rosenberg 1369-5274/03/$ – see front matter ß 2003 Elsevier Science Ltd. All rights reserved. DOI10.1016/S1369-5274(03)00060-2

Introduction
Driven by increasing industrial demands for biocatalysts that can cope with industrial process conditions, considerable efforts have been devoted to the search for such enzymes. Compared with organic synthesis, biocatalysts often have far better chemical precision, which can lead to more efficient production of single stereoisomers, fewer side reactions and a lowerenvironmental burden [1]. Despite the fact that to date more than 3000 different enzymes have been identified and many of these have found their way into biotechnological and industrial applications, the present enzyme toolbox is still not sufficient to meet all demands. A major cause for this is the fact that many available enzymes do not withstand industrial reaction conditions. As a result, the...