Enzymes which exhibit high catalytic efficiency at low temperatures are called cold-adapted enzymes. Indeed, cold-adapted enzymes have been isolated from cold-adapted organisms including psychrotrophic and psychrophilic bacteria. While cold-active enzymes are characterized by a high catalytic efficiency at a low temperature, they may behave differently at moderate temperatures: some of them exhibit a high catalytic efficiency at moderate temperatures but are rather thermo-labile, others inactivate rapidly at a moderate temperature (Feller et al. 1996). However, not all enzymes found in psychroactive organisms are cold-adapted. Many enzymes of psychrophiles show comparable thermostability and catalytic efficiency to the counterparts of mesophilic organisms (Brenchley 1996). In general, rates of biochemical reactions are reduced under low-temperature conditions. However, since levels of the growth rates of psychrophilic bacteria are comparable to those of homologous organisms living at a moderate temperature, relatively similar metabolic rates must be maintained in psychrophilic bacterial cells. For achieving metabolic rate compensation, two enzymatic mechanisms have been proposed: (1) alterations in the concentration of enzymes present in the cells, and (2) changes in the catalytic efficiencies of enzymes (Hochachka and Somero 1984). For instance, an increase of enzyme concentration and activity in the Lactococcus lactis has been reported during cold adaptation (Wouters et al. 2000). Overexpression of polynucleotide phosphorylase has been detected in E. coli at low temperatures (Mathy et al. 2001).
E. sibiricum 255-15 is able to grow efficiently at temperatures down to -6°C (Vishnivetskaya et al. 2007); therefore, clearly, this organism has found mechanisms of temperature compensation in order to cope with the reduction of chemical reaction rates induced by low temperatures. A proteomics study of cold-adapted cells of E. sibiricum 255-15 showed that 28 out of 39 identified CAPs were enzymes. The higher levels of triosephosphate isomerase, acetolactate decarboxylase and cyclohydrolase have been detected in cells of E. sibiricum 255-15 grown at low temperature (Qiu et al. 2006). Cold-adapted enzymes in psychrophilic organisms may catalyze rate-limiting steps in metabolism, and play essential roles in survival at a low temperature. Another mechanism for survival is to express enzymes with temperature-independent reaction rates. This is the case of perfectly evolved enzymes, where such enzymes are relatively rare: typical examples are carbonic anhydrase, acetylcholinesterase, and triosephosphate isomerase. Perfectly evolved enzymes, apparently, do not need to be adapted to low temperatures from a kinetic point of view, therefore they could be extremely useful to probe the various hypotheses related to enzyme adaptation. It may be suggested that the possible role of these enzymes involves maintenance of the bacterial metabolism enabling the cells to adapt to cold temperatures.
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