The available experimental data on the electron donor (D)-acceptor (A) coupling (H-DA) for electron-transfer (ET) reactions in proteins are re-examined. In spite of their structural and energetic similarities, the photosynthetic reaction center and other ET protein systems exhibit a marked difference of the exponential decay of H-DA with the distance separating D and A. A numerical study shows that this difference is explained in terms of very small variations of the energetics between these two classes of proteins.