Salt Effects on Folding of a Helical Mini Protein Villin Headpiece Subdomain HP36 Studied by Generalized-Ensemble Simulations

Abstract

Additives dissolved in solvent are important factors that affect proteins' stability and/or folding. In this study we investigated effects of salt ions in solvent on folding events of a helical mini protein HP36. Addition of low concentrations of ions should alter electrostatic interactions among charged groups, so that populations for conformational substates of proteins should be changed. Here we compared two data sets of folding simulations of HP36 with explicit water solvent. For efficient sampling of conformational space of the protein, multicanonical replica-exchange method was adopted.Results of the present analyses suggest that addition of ions reduces the number of nonnative, nonlocal salt bridges in the protein molecule at later stages of folding at room temperature. Especially, nonnative salt bridges between Glu5 and Arg15 and/or another between Asp4 and Lys30 have been kept in the near-native conformations in pure water. Because dehydration of the hydrophobic core of HP36 is completed only at the latest stage of folding where correct hydrophobic-core packing becomes formed, these salt bridges can prevent folding into the fully native structure of HP36 at room temperature.