Role of specific lysine residues in the reaction of Rhodobacter sphaeroides cytochrome c2 with the cytochrome bc1 complex

The reaction of Rhodobacter sphaeroides cytochrome c2 with the Rb. sphaeroides cytochrome bc1 complex was studied by using singly labeled cytochrome c2 derivatives. Cytochrome c2 was treated with chlorodinitrobenzoic acid to modify lysine amino groups to negatively charged carboxydinitrophenyllysines and separated into eight different fractions by ion-exchange chromatography on a Whatman SE 53 (sulfoxyethyl)cellulose column. Peptide mapping studies indicated that six of these fractions were modified at single lysine amino groups. Each of the derivatives had the same Vmax value as native cytochrome c2 in the steady-state reaction with the Rb. sphaeroides cytochrome bc1 complex. However, the Km values of the cytochrome c2 derivatives modified at lysines 10, 55, 95, 97, 99, and 106 were found to be larger than that of native cytochrome c2 by factors of 6, 2, 3, 32, 13, and 8, respectively. These results indicate that lysines located in the sequence 97-106 on the left side of the heme crevice have the greatest involvement in binding the cytochrome bc1 complex. The involvement of lysine 97 is especially significant because it is located in an extra loop comprising residues 89-98 that is not present in eukaryotic cytochrome c.