Submolecular cooperativity produces multi-state protein unfolding and refolding.

Hydrogen exchange experiments show that cytochrome c and other proteins under native conditions reversibly unfold in a multi-step manner. The step from one intermediate to the next is determined by the intrinsically cooperative nature of secondary structural elements, which is retained in the native protein. Folding uses the same pathway in the reverse direction, moving from the unfolded to the native state through relatively discrete intermediate forms by the sequential addition of native-like secondary structural units.