Nonlinear Impact of Electrolyte Solutions on Protein Dynamics

Halophilic organisms have adapted to multi-molar salt concentrations, their cytoplasmic proteins functioning despite stronger attraction between hydrophobic groups. These proteins, of interest in biotechnology because of decreasing fresh-water resources, have excess acidic amino acids. It has been suggested that conformational fluctuations - critical for protein function - decrease in the presence of a stronger hydrophobic effect, and that an acidic proteome would counteract this decrease. However, our understanding of the salt- and acidic amino acid dependency of enzymatic activity is limited. Here, using solution NMR relaxation and molecular dynamics simulations for in total 14 proteins, we show that salt concentration has a limited and moreover non-monotonic impact on protein dynamics. The results speak against the conformational-fluctuations model, instead indicating that maintaining protein dynamics to ensure protein function is not an evolutionary driving force behind the acidic proteome of halophilic proteins.