We implement a numerical algorithm to survey proteins for electron-tunneling pathways. Insight is gained into the nature of the mediation process in long-distance electron-transfer reactions. The dominance of covalent and hydrogen bond pathways is shown. The method predicts the relative electronic couplings in ruthenated myoglobin and cytochrome c consistent with measured electron-transfer rates. It also allows the design of long-range electron-transfer systems. Qualitative differences between pathways arise from the protein secondary structure. Effects of this sort are not predicted from simpler models that neglect various details of the protein electronic structure.