Inhibition of the cytochrome c/cytochrome c oxidase system by cytochrome c derivatives and related fragments

Spectroscopic analysis of the cytochrome c oxidase-cytochrome c complex: circular dichroism and magnetic circular dichroism measurements reveal change of cytochrome c heme geometry imposed by complex formation

Binding of cytochrome c to cytochrome c oxidase induces a conformational change in both proteins as well as a change of the electronic structure of the heme of cytochrome c, indicating an altered heme c-protein interaction. This follows from the observation that the induced circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of the oxidase-cytochrome c complex in the Soret region differ from the summed spectra of oxidase plus cytochrome c. Spectral changes occur in the complex composed of either the two ferric or the two ferrous hemoproteins. The difference CD and MCD signals saturate at a ratio of 1 heme c per heme aa3. The difference spectra are specific to the cognate complex. The results are interpreted to reflect a direct relationship between the recognition/binding step and the electron-transfer reaction. The conformational rearrangement induced in cytochrome c by cytochrome c oxidase consists of a structural rearrangement of the heme environment and possibly a change of the geometry of the heme iron-methionine-80 sulfur axial bond. This rearrangement may decrease the reorganizational free energy of electron transfer by adjusting the heme c geometry to a state between that of ferri- and ferrocytochrome c.

The effect of trifluoroacetyl-cytochrome c on the cytochrome c/cytochrome c oxidase reaction

The importance of electrostatic interactions to the reaction between cytochrome c and cytochrome c oxidase is indicated most directly by the rapid increase in Km as ionic strength is increased. However, Chessa et al. (1980, Hoppe-Seyler's Z. Physiol. Chem. 361, 1077--1091) have recently found that a cytochrome c derivative trifluoroacetylated at all 19 lysine amino groups decreased the reaction rate between ferrocytochrome c and cytochrome c oxidase nearly as much as native ferricytochrome c, a known inhibitor that binds cytochrome c oxidase. They suggested that this was due to the formation of an inhibitory complex between trifluoroacetyl-cytochrome c and cytochrome c oxidase, which would argue against the importance of electrostatic interactions to the native complex. We have demonstrated that this apparent inhibition is in fact caused by a rapid electron transfer between native ferrocytochrome c and trifluoroacetyl-ferricytochrome c which decreases the concentration of native ferrocytochrome c, thus decreasing the reaction rate.