The response functions of two proteins, separated by a fixed distance and coupled via a screened electrostatic potential, imply a fluctuation-driven interaction in the standard second order perturbation theory. The dominant contribution comes from monopole charge fluctuations and the subdominant term from the dipole charge fluctuations. For two proteins located at r 1,2 , the excess electrostatic free energy as a functional of the local electrostatic potential configuration can be written as