Abstract
Cold-adapted enzymes produced by microorganisms living in permanently cold habitats are characterized by a high specific activity at low and moderate temperatures. Typically they display a high thermosensitivity related to a high flexibility of a part or of the whole molecular edifice. Indeed, this flexibility is apparently essential in counteracting the freezing effect of the low temperature environment on the three dimensional structure, thereby securing an appropriate plasticity as required for catalysis. One of the main strategies selected by extracellular enzymes interacting with large size substrates (α-amylase, proteases etc..) consists of increasing the overall flexibility of the molecular structure by weakening of numerous intra-molecular interactions in all domains of the molecule. However, in the case of enzymes which interact with small size substrates such as for example, phosphoglycerate kinase and chitobiase, only one part of the protein, presumably that related directly or indirectly to the catalytic site, appears to be flexible, whereas the other parts are even more rigid than their mesophilic counterparts, as demonstrated by microcalorimetry and quenching of intrinsic fluorescence using acrylamide. In the case of a cold-adapted cellulase, isolated from an Antarctic bacterium, recent investigations have demonstrated that the flexibility, required for a good accommodation of the large size substrate at low temperatures, is mainly provided by an unusually long and flexible linker connecting the catalytic and cellulose binding modules. This has been demonstrated by x-ray crystallography and small angle x-ray scattering coupled to protein engineering experiments leading to a reduction of the length and to an increased rigidity of the linker. Cold-adapted enzymes such as β-galactosidase and xylanase have been found to be very efficient in milk and baking industry respectively.
