The conformation of cytochrome c in solution. Localization of a conformational difference between ferri- and ferrocytochrome c on the surface of the molecule.

The conformation of cytochrome c in solution is believed to change depending on the oxidation-reduction state of the heme iron, since ferri- and ferrocytochrome c exhibit several different physicochemical properties, but so far it is unknown if the conformational difference(s) is (are) confined to a particular part or domain of the molecule. We have therefore applied the method of differential chemical modification (Bosshard, H. R. (1979) Methods Biochem. Anal. 25, 273-304) to compare the chemical reactivity toward acetic anhydride of the 19 lysine residues of ferri- and ferrocytochrome c from horse heart. The epsilon-amino groups of the spatially related residues 39, 53, and 55 were significantly less reactive in ferrocytochrome c as compared to their reactivity in ferricytochrome c. The difference of reactivity was pH-dependent and was shown to be due to an increase of the pK values of the three epsilon-amino groups in ferrocytochrome c. These results indicate a local conformational change on the surface of the cytochrome c molecule in an area to the lower left of and below the heme cleft (standard front view of the molecule facing the exposed edge of the prosthetic group). The local conformational change might be instrumental in the cytochrome c-mediated electron shuttle between cytochrome c1 and a in the final segment of the mitochondrial electron transport chain.