Dramatic shape changes occur as Cytochrome c folds

Extensive experimental studies on the folding of Cytochrome c (Cyt c) make this small protein an ideal target for atomic detailed simulations for the purposes of quantitatively characterizing the structural transitions and the associated time scales for folding to the native state from an ensemble of unfolded states. We use previously generated atomically detailed folding trajectories by the Stochastic Difference Equation in Length (SDEL) to calculate the time-dependent changes in the Small Angle X-ray scattering (SAXS) profiles. Excellent agreement is obtained between experiments and simulations for the time dependent SAXS spectra, allowing us to identify the structures of the folding intermediates, which shows that Cyt c reaches the native state by a sequential folding mechanism. Using the ensembles of structures along the folding pathways we show that compaction and the sphericity of Cyt c change dramatically from the prolate ellipsoid shape in the unfolded state to the spherical native state. Our data, which provides unprecedented quantitative agreement with all aspects of time-resolved SAXS experiments, shows that hydrophobic collapse and amide group protection coincide on the 100 microseconds time scale, which is in accord with ultrafast Hydrogen/Deuterium exchange studies. Based on these results we propose that compaction of polypeptide chains, accompanied by dramatic shape changes, is a universal characteristic of globular proteins, regardless of the underlying folding mechanism.