Structural changes in the transition state of protein folding: alternative interpretations of curved chevron plots

The interpretation of folding rates is often rationalized within the context of transition state theory. This means that the reaction rate is linked to an activation barrier, the height of which is determined by the free energy difference between a ground state (the starting point) and an apparent transition state. Changes in the folding kinetics are thus caused by effects on either the ground state, the transition state, or both. However, structural changes of the transition state are rarely discussed in connection with experimental data, and kinetic anomalies are commonly ascribed to ground state effects alone, e.g., depletion or accumulation of structural intermediates upon addition of denaturant. In this study, we present kinetic data which are best described by transition state changes. We also show that ground state effects and transition state effects are in general difficult to distinguish kinetically. The analysis is based on the structurally homologous proteins U1A and S6. Both proteins display two-state behavior, but there is a marked difference in their kinetics. S6 exhibits a classical V-shaped chevron plot (log observed rate constant vs denaturant concentration), whereas U1A's chevron plot is symmetrically curved, like an inverted bell curve. However, S6 is readily mutated to display U1A-like kinetics. The seemingly drastic effects of these mutations are readily ascribed to transition state movements where large kinetic differences result from relatively small alterations of a common free energy profile and broad activation barriers.