Cyclic voltammetry experiments were carried out on native Saccharomyces cerevisiae iso-1-cytochrome c and its C102T/N62C variant immobilized on bare polycrystalline gold electrode through the S-Au bond formed by a surface cysteine. Experiments were carried out at different temperatures (5-65 degrees C) and pH values (1.5-7). The E-o' value at pH 7 (+370 mV vs SHE) is approximately 100 mV higher than that for the protein in solution. This difference is enthalpic in origin and is proposed to be the result of the electrostatic repulsion among the densely packed molecules onto the electrode surface. Two additional electrochemical waves are observed upon lowering the pH below 5 (E-o' = +182 mV) and 3 (E-o', = +71 mV), which are attributed to two conformers (referred to as "intermediate" and "acidic", respectively) featuring an altered heme axial ligation. This is the first determination of the reduction potential for low-pH conformers of cytochrome c in the absence of denaturants. Since the native form of cytochrome c can be restored, bringing back the pH to neutrality, the possibility offered by this transition to reversibly modulate the redox potential of cytochrome c is appealing for bioelectronic applications. The immobilized C102T/N62C variant, which differs from the native protein in the orientation of the heme group with respect to the electrode, shows very similar reduction thermodynamics. For both species, the rate constant for electron transfer between the heme and the electrode increases for the acidic conformer, which is also found to act as a biocatalytic interface for dioxygen reduction.