How sensitive are protein hydration shells to electrolyte concentration and protein composition?

Proteins of obligate halophilic organisms have an unusually high number of acidic amino acids, thought to enable them to function in multimolar KCl environments. Clarifying the molecular scale mechanisms by which this occurs is relevant for biotechnology, to enable enzymatic synthesis of economically important small molecules in salty environments and other environments with low water activity. Previous studies have suggested that acidic amino acids are necessary at high salt concentration to keep the proteins hydrated by competing with the ions in solution for available water (the "solvent-only" model). We use a combination of solvation shell spectroscopy and molecular dynamics simulations for in total 13 proteins, at high and low KCl concentration, to investigate this scenario. We show that the solvation shells of halophilic and mesophilic proteins of widely different amino acid compositions, net charges, sizes, and structure respond similarly, in terms of composition and of hydrogen bond network, to changes in KCl concentration. The results do not support the solvent-only model, and point to other mechanisms behind the acidity of halophilic proteins. Excess acidic amino acids may ensure protein solubility by the combined effects of having particularly favorable electrostatic interactions with the solvent, ensuring very short range protein-protein repulsion, and having smaller hydrophobic solvent accessible surface area than other charged amino acids. Also possible is that highly acidic proteins are well-tolerated-but not necessarily indispensable-in terms of stability and solubility.