Movement of the Intermediate and Rate Determining Transition State of Barnase on the Energy Landscape with Changing Temperature

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Abstract
Barnase folds cooperatively via an intermediate, followed by a rate-limiting transition state. We have probed possible movements of the intermediate and transition state on the energy landscape with changing temperature, from the temperature dependence of φ-values. These measure interaction energies at the level of individual residues. The results suggest that single destabilizing mutations can redistribute the structures in each ensemble on the energy landscape as the temperature is varied. The results were also analyzed in terms of the bulk properties of each ensemble and their movements on the energy landscape. These movements can be described in terms of the “new view” or equivalently in terms of the classical “Hammond” or “anti-Hammond” effects, observed previously for the transition states of barnase at 7.25 M urea and chymotrypsin inhibitor 2 (CI2) at 0.3 and 6 M GdmCl. The results presented here are under more relevant physiological conditions, free of chemical denaturants. The “average” structures of the intermediate and the transition state do not appear to move on the energy landscape as the temperature is varied. However, there are small rearrangements in the major α-helix of the transition state, its average structure moving closer to the native state as the temperature is increased, in agreement with the Hammond effect observed previously.