MODEL FOR ION-BINDING AND CHARGE EFFECTS IN GLOBULAR-PROTEINS

A statistical mechanical model for describing the binding of ions, including the effects of charge on the stability of a globular protein in ionization equilibrium with a solution of a known pH and ionic strength, is formulated on the basis of a synthesis of the Copeland and Andersen model for the binding of protons to phospholipids with the Ornstein-Zernike-like approach for semipermeable membranes by Zhou and Stell. We obtain an expression far the ion association constant that is valid for folded and unfolded states and acidic or basic ionic groups and that allows for the possibility of charge separation due to selective diffusion of ions between different regions of the molecule. We show that the free-energy change due to the addition of protons to the surface of an unbound protein, in both the folded and the unfolded states, can be obtained via theories of liquid state. In the mean spherical approximation, and assuming the binding sites are acidic, we calculate various quantities of interest such as the potential and charge-density profiles, the binding fraction or the degree of ionization for the folded and the unfolded states, and the electrical free-energy for charging the protein, a quantity that is of direct experimental relevance.