The reaction domain on Rhodospirillum rubrum cytochrome c2 and horse cytochrome c for the Rhodospirillum rubrum cytochrome bc1 complex.

The interaction of the Rhodospirillum rubrum cytochrome bc1 complex with R. rubrum cytochrome c2 and horse cytochrome c was studied using specific lysine modification and ionic strength dependence methods. In order to define the reaction domain on cytochrome c2, several fractions consisting of mixtures of singly labeled carboxydintrophenyl-cytochrome c2 derivatives were employed. Fraction A consisted of a mixture of derivatives modified at lysines 58, 81, and 109 on the back of cytochrome c2, while fractions C1, C2, C3, and C4 were mixtures of singly labeled derivatives modified at lysines 9, 13, 75, 86, and 88 on the front of cytochrome c2 surrounding the heme crevice. The rate of the reaction of fraction A was found to be nearly the same as that of native cytochrome c2. However, the rate constants of fractions C1-C4 were found to be more than 20-fold smaller than that of native cytochrome c2. These results indicate that lysine residues surrounding the heme crevice of cytochrome c2 are involved in electrostatic interactions with carboxylate groups at the binding site on the cytochrome bc1 complex. Since the same domain is involved in the reaction with the photosynthetic reaction center, cytochrome c2 must undergo some type of rotational or translational diffusion during electron transport in R. rubrum. The reaction rates of horse heart cytochrome c derivatives modified at single lysine amino groups with trifluoroacetyl or trifluoromethylphenylcarbamoyl were also measured. Modification of lysines 8, 13, 25, 27, 72, 79, and 87 surrounding the heme crevice was found to significantly lower the rate of the reaction, while modification of lysines in other regions had no effect. This indicates that the reaction of horse cytochrome c also involves the heme crevice domain.