Reaction of C-type cytochromes with the iron hexacyanides. Mechanistic implications

The reaction of c-cytochromes with iron hexacyanides is similar in mechanism to the interaction of cytochromes with their physiological oxidants and reductants in that the formation of complexes precedes electron transfer. Analysis of the kinetics of oxidation and reduction of a number of c-cytochromes by solving the simultaneous differential equations defining the mechanism is possible, and allows assignment of all six rate constants describing a minimum three-step mechanism [cyto(Fe(+3)) + Fe(+2) right harpoon over left harpoon cyto (Fe(+3)) - Fe(+2) right harpoon over left harpoon cyto(Fe(+2)) - Fe(+3) right harpoon over left harpoon cyto(Fe(+2)) + Fe(+3)]. We find that the usual steady-state approximations are not valid. Furthermore, the ratio of first-order rate constants for electron transfer was approximately 1.0, and no correlation was found between any of the six rate constants and the differences in oxidation-reduction potential of the iron-hexacyanides and different cytochromes c. However, it was found that the ratio of the rate constants for complex formation between ferricytochrome c and potassium ferrocyanide and ferrocytochrome c and potassium ferricyanide was proportional to the difference in oxidation-reduction potentials. Thus the minimum three-step mechanism given above accurately describes the observed kinetic data. However, this mechanism leads to a number of conceptual difficulties. Specifically, the mechanism requires that the collision complexes formed [cyto(Fe(+3)) - Fe(CN)(6) (-4) and cyto(Fe(+2)) - Fe(CN)(6) (-3)] have very different equilibrium constants, and further requires that formation of the collision complexes be accompanied by "chemistry" to make the intermediates isoenergetic. A more complex five-step mechanism which requires that the reactants [Fe(CN)(6) (-4) and ferricytochrome c or Fe(CN)(6) (-3) and ferrocytochrome c] form a collision complex followed by a first-order process before electron transfer, was found to yield results similar to those of the three-step mechanism. However, describing the formation of the collision complex in terms of a rapid equilibrium circumvents conceptual difficulties and leads to a physically reasonable mechanism. In this mechanism the reactants are in rapid equilibrium with the collision complexes and the rate constants for complex formation are controlled by diffusion and accessibility. The collision complexes then rearrange, possibly through conformational changes and/or solvent reorganization, to yield isoenergetic intermediates that can undergo rapid reversible electron transfer. The five-step mechanism can be described by the same rate constants obtained from the three-step mechanism with the appropriate adjustments to account for rapid equilibrium. This more complex analysis associates the oxidation-reduction potential of a particular cytochrome with the relative magnitude of the first-order conversion of the oxidant and reductant collision complexes to their respective intermediates. Thus the cytochromes c control their oxidation-reduction potential by chemical and/or structural alterations. This mechanism appears to be general in that it is consistent with the observed kinetics of 11 different cytochromes c from a wide variety of sources with a range of oxidation-reduction potentials.