Mapping of the interaction domain for purified cytochrome c1 on cytochrome c

Abstract

The steady-state kinetics of the reaction between ubiquinone cytochrome c oxidoreductase and cytochrome c have been studied [l] with the isolated reductase complex (cytochrome bcr, complex III) and several singly-substituted CDNP-lysine cytochromes c. This study demonstrated that the interaction domain on cytochrome c is located on the front surface of the molecule containing the exposed edge of the heme prosthetic group in the vicinity of lysines 13,72,86, 87 and 27 [1,2]. This is essentially the same as the binding domain observed in [3] from the differential chemical reactivities of the lysines of cytochrome c in the presence and absence of cytochrome bq and in [4] with singly-modified cytochromes c and succinate-cytochrome c reductase. Although it has been assumed generally that cytochrome cr is the direct reaction partner for cytochrome c [5,6], the possibility that the latter interacts with the iron-sulfur cluster of complex III has not been excluded. However, with the availability of highly purified beef cytochrome cr [7] it has become possible to study the reaction directly. As shown below, the interaction domain on cytochrome c for cytochrome cr, defined from a study of the presteady-state kinetics with CDNP-lysine 7, 13,25,27, 60,72,73,86 and 87 horse cytochromes c, is essen-