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Biochim Biophys Acta. 2000 Nov 30;1543(1):1-10.

Psychrophilic enzymes: revisiting the thermodynamic parameters of activation may explain local flexibility.

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Laboratory of Biochemistry, Institute of Chemistry B6, University of Liège, B-4000, Liège, Belgium.


Basic theoretical and practical aspects of activation parameters are briefly reviewed in the context of cold-adaptation. In order to reduce the error impact inherent to the transition state theory on the absolute values of the free energy (DeltaG(#)), enthalpy (DeltaH(#)) and entropy (DeltaS(#)) of activation, it is proposed to compare the variation of these parameters between psychrophilic and mesophilic enzymes, namely Delta(DeltaG(#))(p-m), Delta(DeltaH(#))(p-m) and Delta(DeltaS(#))(p-m). Calculation of these parameters from the available literature shows that the main adaptation of psychrophilic enzymes lies in a significant decrease of DeltaH(#), therefore leading to a higher k(cat), especially at low temperatures. Moreover, in all cases including cold-blooded animals, DeltaS(#) exerts an opposite and negative effect on the gain in k(cat). It is argued that the magnitude of this counter-effect of DeltaS(#) can be reduced by keeping some stable domains, while increasing the flexibility of the structures required to improve catalysis at low temperature, as demonstrated in several cold-active enzymes. This enthalpic-entropic balance provides a new approach explaining the two types of conformational stability detected by recent microcalorimetric experiments on psychrophilic enzymes.

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