Preorganized secondary structure as an important determinant of fast protein folding.

The folding and unfolding kinetics of the B-domain of staphylococcal protein A, a small three-helix bundle protein, were probed by NMR. The lineshape of a single histidine resonance was fit as a function of denaturant to give folding and unfolding rate constants. The B-domain folds extremely rapidly in a two-state manner, with a folding rate constant of 120,000 s-1, making it one of the fastest-folding proteins known. Diffusion-collision theory predicts folding and unfolding rate constants that are in good agreement with the experimental values. The apparent rate constant as a function of denaturant ('chevron plot') is predicted within an order of magnitude. Our results are consistent with a model whereby fast-folding proteins utilize a diffusion-collision mechanism, with the preorganization of one or more elements of secondary structure in the unfolded protein.