Figure 1. Schematic of protein denaturation showing protein cartoons of conformational states and varying degrees of cooperativity in the transitions between them. Protein compactness is plotted versus denaturation stress (e.g. increasing concentration of denaturant (GuHCl, urea, guanidinium isothiocyanate) or increasing temperature or decreasing pH). The denaturation curve is shown in red. Blue cartoons represent conformational states of protein molecules in native (N), intermediate (I) (including molten globules), compact unfolded (U_c) and expanded unfolded states (U_e). The transitions between these conformational states are numbered 1 to 3, in decreasing order of cooperativity. Clearly, the most cooperative transition is 1 between N and I, followed by the unfolding of I to U_c. Transition 3 describes the noncooperative unfolding of the compact unfolded state.

Figure 2. Energy landscape of protein denaturation shown as a ‘funnel’. The width of the funnel represents the conformational heterogeneity of the protein and the energy represents the height of the funnel under folding-permissive conditions (e.g. physiological conditions). The conformational states of Figure 1 are placed at various positions inside the funnel. The most ordered conformation, having the lowest energy (most stable) under physiological conditions, is the well-defined structure of the native state (N). The most heterogeneous states are represented by the expanded unfolded states (U_e). Reproduced with permission from Dill KA and Chan HS (1997) From Levinthal to pathways to funnels. Nature Structural Biology 4: 10–19.