ENZYMES FROM PSYCHROPHILIC ORGANISMS

Psychrophilic organisms such as micro-organisms and other ectothermic species living in polar, deep- sea or any constantly low temperature e nvironments, produce enzymes adapted to function at low temperature. T hese enzymes are characterized by a high catalytic efficiency at low a nd moderate temperatures but are rather thermolabile. Due to their hig h specific activity and their rapid inactivation at temperatures as lo w as 30 degrees C, they offer, along with the producing micro-organism s, a great potential in biotechnology. The molecular basis of the adap tation of cold cu-amylase, subtilisin, triose phosphate isomerase from Antarctic bacteria and of trypsin from fish living in North Atlantic and in Antarctic sea waters have been studied. The comparison of the 3 D structures obtained either by protein modelling or by X-ray crystall ography (North Atlantic trypsin) with those of their mesophilic counte rparts indicates that the molecular changes tend to increase the flexi bility of the structure by a weakening of the intramolecular interacti ons and by an increase of the interactions with the solvent. For each enzyme, the most appropriate strategy enabling it to accommodate the s ubstrate at a low energy cost is selected. There is a price to pay in terms of thermosensibility because the selective pressure is essential ly oriented towards the harmonization of the specific activity with am bient thermal conditions. However, as demonstrated by site-directed mu tagenesis experiments carried out on the Antarctic subtilisin, the pos sibility remains to stabilize the structure of these enzymes without a ffecting their high catalytic efficiency.