The transition state for folding of a peripheral subunit-binding domain contains robust and ionic-strength dependent characteristics

The denaturant dependencies of the folding and unfolding kinetics were used to characterize the structure of the transition state for folding of E3BD, a peripheral subunit-binding domain. For the majority of E3BD mutants, the Phi-values calculated at 298 K from the analysis of chevron plots were in good agreement with those previously determined at 325 K using Arrhenius analysis. This agreement further demonstrates the general robustness of Phi-value analyses, since different experiments, methods of denaturation and thermodynamic assumptions were used to determine each set of Phi(F) values. The structure of the transition state for folding was grossly conserved at 298 K and 325 K, with residues in Helix I playing a lesser role in folding than those located in the 3(10) helix, disordered loop and Helix II. However, the energetic contributions of a cluster of basic residues close to the N-terminus and Helix I, which are an integral part of the ligand-binding site, were susceptible to ionic strength effects because of electrostatic strain in native and transition states of E3BD at low ionic strength. We found no evidence of the downhill folding previously proposed for E3BD, even though the conditions employed in this study significantly increased the energetic bias towards the native state.