Anion binding and pH-dependent electrostatic effects in ribonuclease.

The solvent-accessibility-modified, Tanford-Kirkwood, discrete charge model for electrostatic effects is applied to both ribonuclease A and ribonuclease S. The behavior of individual titratable sites and the pH-dependent free energy of denaturation are correctly predicted. The use of the solvent-accessibility factor in reducing charge-site interactions introduces a higher Coulombic shielding for solvent-exposed sites. This shielding is interpreted as a higher local strength or alternatively a higher effective dielectric constant. Specific anion binding sites are determined by locating areas of high positive electrostatic potential at the protein solvent interface. The potential and thus the anion affinity of a given site are calculated and shown to vary with the pH-dependent charge array. pH-dependent anion binding constants are calculated for the ribonuclease S active site. These binding constants and the predicted response of the active-site histidine pK1/2 values to anion binding are shown to agree with experimental determinations.