The Reduction Potential of Cytochrome b₅ Is Modulated by Its Exposed Heme Edge^†

When the reduction potential of cytochrome b₅ is measured with the aid of several different surface-modified electrodes that function on the basis of electrostatic interactions with the protein, the resultant values have been consistently more positive (40−100 mV) than the reduction potentials measured with potentiometric methods. In this paper, we report that the heme edge containing the exposed heme propionate, a heme methyl, and a heme vinyl, and which constitutes part of the surface of cytochrome b₅, modulates its reduction potential. The positive shifts observed in the voltammetric measurements appear to originate from the formation of a complex between cytochrome b₅ and the modified electrode surface which (a) neutralizes the charge on the heme propionate located on the exposed heme edge and (b) lowers the dielectric of the exposed heme microenvironment by excluding water from the complex interface, factors which result in the destabilization of the positive charge on the ferric heme with respect to the neutral ferrous heme. The observed positive shift, which is induced by complexation at the electrode surface, may indicate that similar shifts in the reduction potential of cytochrome b₅ occur when it forms a complex with physiological partners, prior to electron transfer. The effect of the value of the dielectric constant on the reduction potential of cytochrome b₅ was corroborated by preparing the V45L/V61L double mutant whose reduction potential was measured to be 50 mV more negative than the value measured for the wild type protein. The negative shift in the reduction potential of the mutant protein was explained by the increased accessibility of water to the heme binding site, as observed in its X-ray crystal structure.