A minimal model of three-state folding dynamics of helical proteins.

A diffusion-collision-like model is proposed for helical proteins with three-state folding dynamics. The model generalizes a previous scheme based on the dynamics of putatively essential parts of the protein (foldons) that was successfully tested on proteins with two-state folding. We show that the extended model, unlike the original one, allows satisfactory calculation of the folding rate and reconstruction of the salient steps of the folding pathway of two proteins with three-state folding (Im7 and p16). The dramatic reduction of variables achieved by focusing on the foldons makes our model a good candidate for a minimal description of the folding process also for three-state folders. Finally, the applicability of the foldon diffusion-collision model to two-state and three-state folders suggests that different folding mechanisms are amenable to conceptually homogeneous descriptions. The implications for a unification of the variety of folding theories so far proposed for helical proteins are discussed in the final discussion.