Proline isomerization in unfolded ribonuclease A. The equilibrium between fast-folding and slow-folding species is independent of temperature.

Unfolded ribonuclease A (RNaseA) consists of a mixture of fast refolding (UF) and slow-refolding (Us) species. The slow UF in equilibrium Us equilibration reaction is rate-limited by proline peptide bond isomerization. Investigations of the dependence on temperature of the UF in equilibrium Us equilibrium have led to conflicting results and different molecular interpretations. Here the dependence on temperature of the UF:US ratio was reinvestigated by using a new assay for the fast-folding molecules UF. Between 0 degrees C and 60 degrees C the proportion of UF present in unfolded RNase A at 6 M guanidine . HCl was found to be independent of temperature. Consequently, no conclusions can be drawn regarding the role and importance of particular prolines in the UF in equilibrium US transition solely from these results.