Industrial-Scale Production of Thermostable Enzymes: The Model-System of the β-Glycosidase from Sulfolobus Solfataricus

Abstract

The current relevance of enzymes in biotechnology is well known and many examples exist of applications of these catalysts in different fields (i.e. food and chemicals industries, agriculture, medicine). Since industrial use often revealed to be denaturing for these molecules, several techniques have been used to improve the stability of enzymes in the operative conditions (protein engineering, protein modification and immobilization). However, since this approach resulted some times unsuccessful for the intrinsic fragility of the molecules, particular attention has been focused on enzymes from extreme thermophilic bacteria which grow between 70°C and 100°C and over. All the micro-organisms living at temperatures over 70°C belong to the new taxonomic unit of Archaea, recognised as a super-kingdom (Woese et al., 1990). Whereas conventional enzymes are irreversibly inactivated by heat, the enzymes from extremophiles not only show great stability but also enhanced activity in the presence of common protein denaturants such as heat, detergents, organic solvents and proteolytic enzymes. As a consequence, these molecules have considerable industrial potentialities giving better results at “extreme” operational conditions. However, despite the enthusiasm induced by the use of thermophilic enzymes in biotechnology in middle-eighties, in recent years it has been demonstrated that their properties resulted to be sometimes too “extreme” to allow an effective application in industrial processes. In fact, some practical adversities (modification of pre-existing plants, high temperature-working reactors, incompatibility of reagents and products with high temperatures) can represent a serious limitation for their application. For these reasons these enzymes can be an ideal and unique answer to specific problems, but cannot be considered a general solution in biotechnology (Cowan, 1992). A further restriction to their direct use in biotechnology is the moderated amount available of these proteins, since their purification from thermophilic sources is expensive and time consuming. Hence, the gene cloning and expression in heterologous mesophilic hosts such as Escherichia coli, Bacillus subtilis and yeast is advisable.