Protein folding rates and stability: how much is there beyond size?

An intriguing feature of protein folding is that the overall behavior obeys simple physical rules, but the finer details show a great deal of complexity. The scaling of thermodynamic and kinetic properties with protein size is one such rule. However, it is not clear to what extent biologically relevant folding properties (i.e., rates and stabilities) depend on size and/or on other factors such as structure and amino acid sequence. Here we address this question analyzing experimental data on 52 nonmultistate folding proteins with a simple theoretical model. We find that size scaling is the primary factor in determining folding rates, and more surprisingly also protein stability. Furthermore, our analysis reveals that the experimental deviations from size predictions are due to minute differences in the fundamental parameters (e.g., less than 2% for the stability). Folding is thus highly sensitive to little changes in protein energetics, but at the same time the folding properties of natural proteins are remarkably homogeneous. These results suggest that evolution has selected a small subset of possibilities from the physically plausible folding catalog and highlight the need for highly accurate protein force fields to predict rates and stabilities beyond general trends.