We present a formalism for the calculation of interfacial dielectric constant of hydration shells of proteins from configurations produced by atomistic numerical simulations. The theoretical approach is applied to classical molecular dynamics simulations of hydrated cytochrome c protein in the range of temperatures from 280 to 330 K. The interfacial dielectric constant was found to be equal to 2-4 depending on temperature. This dielectric constant reflects constraints imposed by the protein on the hydration waters and their low ability to polarize in response to an external field.