We present a three-dimensional model system which allows for a detailed investigation of the superexchange coupling (V-S) between distant redox centers in large electron-transfer systems. The system (bridge) is described hv a tight-binding Hamiltonian for an aperiodic array of sites interacting through an exponentially decaying function of distance. The dependence of V-S on the donor-acceptor distance, the density of sites in the medium, and the energy separation between the bridge and the redox sites is studied numerically, We present an analytical calculation, based on a continuous-medium approximation, which accounts fur most of the numerical results. Evidence is given for the existence of a unique control parameter (E/Gamma), expressed by the ratio of the energy gap to the half-bandwidth of the medium. Due to interferences between the large number of pathways, it is found that the sign of (E/Gamma), which distinguishes electron from hole transfers, is of particular importance. Depending on this sign, the effective coupling differs by many orders of magnitude and, while presenting a monotonic variation for the case E/Gamma < 0, shows an oscillatory behavior for the case E/Gamma > 0. These results are beyond the usual McConnell or WKB approximations.