The site-site Ornstein-Zernike equation with a simple mean spherical c
losure is used to study electrostatic interactions of proteins. Using
a Debye-Huckel approximation for the correlation functions of the bulk
electrolyte and a simple basis expansion for the protein-salt direct
correlation functions, we obtain a very simple variational expression
for the electrostatic component of the excess chemical potential of a
protein in an electrolyte solution. The predictions of the theory are
tested on a model of the protein calbindin Dg,. Our calculations for c
alcium binding affinities and protein acidity constants are found to b
e in excellent agreement with the results of computer simulations. (C)
1996 American Institute of Physics.