Dynamical properties of α-amylase in the folded and unfolded state:: the role of thermal equilibrium fluctuations for conformational entropy and protein stabilisation

A comparative analysis of thermal equilibrium fluctuations occurring in a mesophilic and in a thermophilic alpha -amylase was performed to study the effect of structural fluctuations on thermostability. The thermal fluctuations determining the conformational entropy of both enzymes have been characterised for the folded (at 30 degreesC and 60 degreesC) and for the unfolded state by applying neutron spectroscopy (at 30 degreesC). The folded state shows a higher structural flexibility for the thermophilic protein as compared to the mesophilic homologue. In contrast, the unfolded state of both enzymes is rather similar with respect to the structural fluctuations. On the basis of this result, a mechanism characterised by entropic stabilisation (i.e., smaller DeltaS for the unfolding transition of thermophilic a-amylase) can be assumed to be responsible for the higher thermostability of the thermophilic enzyme. (C) 2001 Elsevier Science B.V. All rights reserved.