In past work the direct electron transfer reactions of bovine cytochrome c oxidase in an electrode-supported lipid bilayer membrane have been studied. Its reaction with cytochrome c in solution was also studied and found to be consistent with previous solution studies. In this work it is shown that the electron transfer reactions of cytochrome c oxidase in this electrode-supported lipid bilayer membrane depend on the source of cytochrome c. This property has also been widely studied for solution samples. The differences in the electron transfer reaction rates correlate with the differences in the amino acid sequence for the cytochrome c molecules studied. Electrochemical results suggest that the dissociation of the cytochrome c/cytochrome c oxidase reaction complex is the rate-controlling step in this electron transfer mechanism for cytochrome c from some sources. Moreover, the electron transfer reaction mechanism exhibits biphasic reaction kinetics, which is consistent with earlier work on reactions between solubilized cytochrome c oxidase/cytochrome c samples. These results indicate that the cytochrome c oxidase modified electrodes described herein could be used to distinguish amino acid sequence variations in proteins such as cytochrome c, and this has potential relevance as a diagnostic for disease states.