PREDICTING THE EQUILIBRIUM PROTEIN-FOLDING PATHWAY - STRUCTURE-BASED ANALYSIS OF STAPHYLOCOCCAL NUCLEASE

The equilibrium folding pathway of staphylococcal nuclease (SNase) has been approximated using a statistical thermodynamic formalism that ut ilizes the high-resolution structure of the native state as a template to generate a large ensemble of partially folded states, Close to 400 ,000 different states ranging from the native to the completely unfold ed states were included in the analysis, The probability of each state was estimated using an empirical structural parametrization of the fo lding energetics, It is shown that this formalism predicts accurately the stability of the protein, the cooperativity of the folding/unfoldi ng transition observed by differential scanning calorimetry (DSC) or u rea denaturation and the thermodynamic parameters for unfolding. More importantly, this formalism provides a quantitative account of the exp erimental hydrogen exchange protection factors measured under native c onditions for SNase, These results suggest that the computer-generated distribution of states approximates well the ensemble of conformation s existing in solution, Furthermore, this formalism represents the fir st model capable of quantitatively predicting within a unified framewo rk the probability distribution of states seen under native conditions and its change upon unfolding. (C) 1997 Wiley-Liss, Inc.