The binding domain on horse cytochrome c and Rhodobacter sphaeroides cytochrome c2 for the Rhodobacter sphaeroides cytochrome bc1 complex

by Hall, Joan; Zha, Xiaohui; Yu, Linda; Yu, Chang An; Millett, Francis

The interaction of the R. sphaeroides cytochrome bc1 complex with R. sphaeroides cytochrome c2 and horse cytochrome c was studied by using specific lysine modification and ionic strength dependence methods. The rate of the reactions with both cytochrome c and cytochrome c2 decreased rapidly with increasing ionic strength at >0.2M NaCl. The ionic strength dependence suggested that electrostatic interactions were equally important to the reactions of the 2 cytochromes, even through they have opposite net charges at pH 7.0. In order to define the interaction domain on horse cytochrome c, the reaction rates of derivatives modified at single lysine amino groups with trifluoroacetyl or trifluoromethylphenylcarbamoyl were measured. The modification of lysine-8, -13, -27, -72, -79, and -87 surrounding the heme crevice was found to significantly lowered the rate of the reaction, whereas modification of lysines in other regions had no effect. This result indicated that lysines surrounding the heme crevice of horse cytochrome c are involved in electrostatic interactions with carboxylate groups at the binding site on the cytochrome bc1 complex. In order to define the reaction domain on cytochrome c2, a fraction consisting of a mixture of singly labeled 4-carboxy-2,6-dinitrophenyl-cytochrome c2 derivatives modified at lysine-35, -88, -95, -97, and -105 and several unidentified lysines were prepared Although it was not possible to resolve these derivatives, all the identified lysines were located on the front surface of cytochrome c2 near the heme crevice. The rate of reaction of this fraction was significantly smaller than that of native cytochrome c2, suggesting that the binding domain on cytochrome c2 is also located at the heme crevice. 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. sphaeroides.

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1520-4995; 0006-2960