Electrostatic Effects in Highly Charged Proteins:  Salt Sensitivity of pK a Values of Histidines in Staphylococcal Nuclease

The pK a values of most histidines in small peptides and in myoglobin increase on average by 0.30 unit between 0.02 and 1.5 M NaCl [Kao et al. (2000) Biophys. J. 79, 1637]. The ΔpK a values reflect primarily the ionic strength dependence of the solvation energy; screening of Coulombic interactions contributes only in a minor way. This implies that Coulombic interactions are weak, or that attractive and repulsive contributions to the pK a values are balanced. To distinguish experimentally between these two possibilities, and to further characterize the magnitude and salt sensitivity of surface electrostatic interactions in proteins, the salt dependence of pK a values of histidines in staphylococcal nuclease was measured by 1H NMR spectroscopy. Three of the four histidines titrated with significantly depressed pK a values, and the salt sensitivity of all histidine pK a values was substantial. In three cases, the pK a values increased by a full unit between 0.01 and 1.5 M KCl. Anion-specific effects were found; the pK a values measured under equivalent ionic strengths in SCN- and SO4 2- were higher than in Cl-; the order of the sensitivity of pK a values to anions was SCN- > Cl- > SO4 2-. Structure-based pK a calculations with continuum methods were performed to interpret the measured effects structurally and to test their ability to capture the experimental behavior. Calculations in which the protein interior was treated empirically with a dielectric constant of 20 reproduced the pK a values and their dependence on the concentration of Cl-. According to the calculations, the pK a values are depressed because of unfavorable self-energies and repulsive Coulombic interactions. Their striking salt sensitivity reflects screening of weak, repulsive, Coulombic interactions among charges separated by more than 10 Å. Long-range Coulombic interactions on the surfaces of proteins are weak, but they can add up to produce substantial electrostatic effects when positive and negative charges are not balanced.