(The) study of the ionic strength effect on the folding of the trp-cage miniprotein

Abstract

The small size and stability of Trp-cage make it an ideal choice for simulation and experimental studies of protein folding. Experimentally, folding kinetics of miniproteins is commonly described by simple two-state transitions. From a theoretical point of view, however, it has been proposed that true two-state protein folding is fundamentally impossible. Trp-cage is the one of miniproteins which is still under dispute about two-state folding mechanism. Even though many groups including Neidigh et al. who designed Trp-cage mentioned that this has two-state folding property, many of them have token the focus with its intermediate or transition state and showed that the salt-bridge of Trp-cage is the essential structure of intermediate or transition state. In this study, the ionic strength effects concerning interaction with salt bridge of Trp-cage on the folding kinetics and equilibrium were studied in various concentration of NaCl. The results of salt effects can be explained by Debye screening effects at low concentration of NaCl and Hofmeister effects at high concentration. The ionic strength effects can affect not only the equilibrium of folding but also the kinetics by influence on rate-determining step which is intermediate state or transition states. These results imply that Trp-cage is not a simple two-state folding protein.