The behavior of HIV-1 protease in aqueous NaCl and KCl solutions is investigated by
kinetic measurements and molecular dynamics simulations. Experiments show cation-specific
effects on enzymatic activity. The initial velocity of peptide substrate hydrolysis increases
with salt concentration more dramatically in potassium than in sodium chloride solutions.
Furthermore, significantly higher catalytic efficiencies (kcat/KM) are observed in the presence
of K+ compared to Na+ at comparable salt concentrations. Molecular dynamics simulations
provide insight into this ion-specific behavior. Sodium is attracted more strongly than
potassium to the protein surface primarily due to stronger interactions with carboxylate side
chain groups of aspartates and glutamates. These effects are of particular importance for
acidic amino acid residues at or near the active site of the enzyme, including a pair of
aspartates at the entrance to the reaction cavity. We infer that the presence of more Na+ than
K+ at the active site leads to a lower increase in enzymatic activity with increasing salt
concentration in the presence of Na+, likely due to the ability of the alkali cations at the active
site to lower the efficiency of substrate binding.