Interleukin-1β Folding between pH 5 and 7: Experimental Evidence for Three-State Folding Behavior and Robust Transition State Positions Late in Folding

Publisher Website
Google Scholar
Abstract
The thermodynamic stability and folding kinetics of the all β-sheet protein interleukin-1β were measured between 0 and 4 M GdmCl concentrations and pH 5−7. Native interleukin-1β undergoes a 3.5 kcal/mol decrease in thermodynamic stability, Δ , as pH is increased from 5 to 7. The native state parameter mNU, measuring protein destabilization/[GdmCl], remains constant between pH 5 and 7, indicating that the solvent-exposed surface area difference between the native state and unfolded ensemble is unchanged across this pH range. Similarly, pH changes between 5 and 7 decrease only the thermodynamic stability, ΔGH2O, and not the m-values, of the kinetic intermediate and transition states. This finding is shown to be consistent with transition state configurations which continue to be the high-energy configurations of the transition state in the face of changing stability conditions. A three-state folding mechanism U ⇄ I ⇄ N is shown to be sufficient in characterizing IL-1β folding under all conditions studied. The m-values of refolding transitions are much larger than the m-values of unfolding transitions, indicating that that the fast, T2 (U ⇄ I), and slow, T1 (I ⇄ N), transition states are highly similar to the intermediate I and native state N, respectively. Many of the folding properties of interleukin-1β are shared among other members of the β-trefoil protein family, although clear differences can exist.