Abstract: The structure of the complex between cytochrome c (CYC) and the cytochrome bc(1) complex (QCR) from yeast crystallized with an antibody fragment has been recently determined at 2.97 A resolution [Proc. Natl. Acad. Sci. U. S. A. 99 (2002) 2800]. CYC binds to subunit cytochrome c(1) of the enzyme stabilized by hydrophobic interactions surrounding the heme crevices creating a small, compact contact site. A central cation-pi interaction is an important and conserved feature of CYC binding. Peripheral patches with highly conserved complementary charges further stabilize the enzyme-substrate complex by long-range electrostatic forces and may affect the orientation of the substrate. Size and characteristics of the contact site are optimal for a transient electron transfer complex. Kinetic data show a bell-shaped ionic strength dependence of the cytochrome c reduction with a maximum activity near physiological ionic strength. The dependence is less pronounced in yeast compared to horse heart CYC indicating less impact of electrostatic interactions in the yeast system. Interestingly, a local QCR activity minimum is found for both substrates at 120-140 mM ionic strength. The architecture of the complex results in close distance of both c-type heme groups allowing the rapid reduction of cytochrome c by QCR via direct heme-to-heme electron transfer. Remarkably, CYC binds only to one of the two possible binding sites of the homodimeric complex and binding appears to be coordinated with the presence of ubiquinone at the Q(i) site. Regulatory aspects of CYC reduction are discussed.