We report the results of Molecular Dynamics simulations of electron transfer activation parameters of plastocyanin metalloprotein involved as electron carrier in natural photosynthesis. We have discovered that slow, non-ergodic conformational fluctuations of the protein, coupled to hydrating water, result in a very broad distribution of donor-acceptor energy gaps far exceeding that observed for commonly studied inorganic and organic donor-acceptor complexes. The Stokes shift is not affected by these fluctuations and can be calculated from solvation models in terms of the response of the solvent dipolar polarization. The non-ergodic character of large-amplitude protein/water mobility breaks the strong link between the Stokes shift and reorganization energy characteristic of equilibrium (ergodic) theories of electron transfer. This mechanism might be responsible for low activation barriers in natural electron transfer proteins characterized by low reaction free energy.