The site-site Ornstein-Zernike equation with a simple mean spherical closure 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 calcium binding affinities and protein acidity constants are found to be in excellent agreement with the results of computer simulations.