Definition of the interaction domain for cytochrome c on the cytochrome bc(1) complex. Steady-state and rapid kinetic analysis of electron transfer between cytochrome c and Rhodobacter sphaeroides cytochrome bc(1) surface mutants

The interaction domain for cytochrome c on the cytochrome bc(1) complex was studied using a series of Rhodobacter sphaeroides cytochrome bc(1) mutants in which acidic residues on the surface of cytochrome c(1) were substituted with neutral or basic residues. Intracomplex electron transfer was studied using a cytochrome c derivative labeled with ruthenium trisbipyridine at lysine 72 (Ru-72-Cc). Flash photolysis of a 1:1 complex between Ru-72-Cc and cytochrome bc(1) at low ionic strength resulted in electron transfer from photoreduced heme c to cytochrome c(1) with a rate constant of k(et) = 6 x 10(4) s(-1). Compared with the wild-type enzyme, the mutants substituted at Glu-74, Glu-101, Asp-102, Glu-104, Asp-109, Glu-162, Glu-163, and Glu-168 have significantly lower k(et) values as well as significantly higher equilibrium dissociation constants and steady-state K(m) values. Mutations at acidic residues 56, 79, 82, 83, 97, 98, 213, 214, 217, 220, and 223 have no significant effect on either rapid kinetics or steady-state kinetics. These studies indicate that acidic residues on opposite sides of the heme crevice of cytochrome c(1) are involved in binding positively charged cytochrome c. These acidic residues on the intramembrane surface of cytochrome c(1) direct the diffusion and binding of cytochrome c from the intramembrane space.