Enzyme Catalysis in Psychrophiles

Abstract

Cold-active enzymes are produced by organisms, known as psychrophiles, adapted to permanently cold habitats. Low temperatures have an exponential deleterious effct on reaction rates, and thus psychrophilic enzymes have to be adapted to secure appropriate reaction rates in their environment. These enzymes have a high specific activity at low temperatures, in any case higher than that of their mesophilic and thermophilic counterparts, and display a shift of the apparent optimum temperature for activity towards low temperatures as well as a reduced thermal stability and increased flexibility. The increased flexibility may be global, involving the overall edifice, or local, involving only those zones crucial for activity, be they near or distant from the active site. The reduced thermodynamic stability of cold-adapted enzymes is illustrated by a significantly lower stabilisation energy as compared to that of their mesophilic and thermophilic counterparts, yet maximum stability occurs at similar temperatures in all cases. The comparison of their three-dimensional structures with higher temperature-adapted homologues, in conjunction with various mutagenesis studies, has shown that their high activity results from rather discrete molecular changes that tend to decrease the stability of the molecular edifice. Each cold-adapted enzyme however adopts a specific strategy. There is apparently a continuum in the adaptation, with some enzymes showing extremely acute cold adaptation, as illustrated by a severe shift of the activity towards low temperatures, whereas others appear to cover a broader range of temperatures. This probably depends on the specific evolutionary history of the organisms which produce them.