| Abstract: | The association and reduction reactions of ten different 4-carboxy-2,6-dinitrophenyl (CDNP) horse heart cytochromes c, singly modified at lysines 8, 13, 27, 39, 60, 72, 73, 86, 87, and 99, with Saccharomyces cerevisiae cytochrome b2 were studied to determine the region of cytochrome c interacting with cytochrome b2. In the presence of higher ratios of free cytochrome c to cytochrome b2, native cytochrome c, and the CDNP-lysine 39, 60, and 99 derivatives associated with cytochrome b2 with a binding stoichiometry close to 2:1, while CDNP-cytochromes c modified at lysines 8, 13, 27, 72, 73, 86, and 87 formed only 1:1 complexes. In the presence of lower ratios of free cytochrome c, modifications of lysines 8, 27, 86, and 87 had more inhibitory effects on the association of cytochrome c with cytochrome b2 than modifications of lysines 13, 39, 60, 72, 73, and 99. This tendency was similar to that on removal of free cytochrome c, except in the case of CDNP-lysine 13 and 73 derivatives. The rate of reduction of cytochrome c by cytochrome b2 was decreased by carboxydinitrophenylation of lysines 8, 13, 27, 72, 73, 86, and 87. In contrast, the rate of reduction of cytochrome c was not affected by modifications of lysines 39, 60, and 99. Since lysines 8, 13, 27, 72, 73, 86, and 87 are located on the front surface and lysines 39, 60, and 99 on the back side, and since different effects of modifying lysine residues located on the front surface may be interpreted in terms of effects on the complementary interaction of cytochrome c and cytochrome b2, these results indicate that the region of cytochrome c interacting with cytochrome b2 is located on the front surface of the cytochrome c molecule containing the exposed heme edge. |
