Understanding the voltage-dependence of molecular conductance is essential to characterize molecular electronics devices. We have reproducibly measured current-voltage characteristics of individual redox-active proteins with a scanning tunneling microscope (STM) under potentiostatic control in both tunneling and wired configurations. Based on these results, the transition voltage spectra (TVS) of individual redox molecules can be calculated and analyzed statistically, adding a new dimension to the conductance measurements. The transition voltage (TV) is discussed in terms of the two-step electron transfer (ET) mechanism. Azurin displays the lowest TV measured to date (0.4V), consistent with the previously reported distance decay factor. This low TV may be advantageous to fabricate and operate molecular electronic devices for different applications. Our measurements support TVS as a helpful tool for single molecule ET measurements, and suggest a mechanism of gating ET between partner redox proteins.