Structural and Mutational Studies of the Cytochrome bc 1 Complex

Abstract

The ubihydroquinone:cytochrome c oxidoreductase, or the cytochrome bc 1, is a widespread multi-subunit, multi-cofactor-bearing, membrane-integral enzyme complex of crucial importance both for photosynthesis and respiration. It is a major contributor of proton motive force that is subsequently used depending on the growth conditions for multiple cellular tasks, including ATP production via photo- or oxidative-phosphorylation. The simplest form of the cytochrome bc 1 is generally found in prokaryotes, and that of the purple nonsulfur anoxygenic photosynthetic bacteria of Rhodobacter species is amenable to diverse multidisciplinary approaches. This chapter is focused on recent progress related to structural and mutational studies on the cytochrome bc 1. Detailed information derived from the three-dimensional (3-D) structures of the bacterial enzyme, which consists of the iron-sulfur protein, cytochrome b and cytochrome c 1 subunits, is integrated with molecular genetic studies that yielded invaluable mutant variants. In particular, the roles of various structural components of the cytochrome bc 1 that affect the unique mobility of the iron-sulfur subunit during catalysis are emphasized. Clearly, the nature of specific protein-protein interactions between the surface loops of the iron-sulfur and the cytochrome b subunits is of importance for this mobility and the steady-state activity of the enzyme. Recent progress illustrates that the cytochrome bc 1 provides an invaluable model system for studying the mechanism of redox driven proton translocation across energy transducing membranes.