Structure and function of the bacterial bc1 complex:: Domain movement, subunit interactions, and emerging rationale engineering attempts

The ubiquinol: cytochrome c oxidoreductase, or the bc(1) complex, is a key component of both respiratory and photosynthetic: electron transfer and contributes to the formation of an electrochemical gradient necessary for ATP synthesis. Numerous bacteria harbor a bc(1) complex comprised of three redox-active subunits, which bear two b-type hemes, one c-type heme, and one [2Fe-2S] cluster as prosthetic groups. Photosynthetic bacteria like Rhodobacter species provide powerful models for studying the function and structure of this enzyme and are being widely used. In recent years, extensive use of spontaneous and site-directed mutants and their revertants, new inhibitors, discovery of natural variants of this enzyme in various species, and engineering of novel bc(1) complexes in species amenable to genetic manipulations have provided us with a wealth of information on the mechanism of function, nature of subunit interactions, and assembly of this important enzyme. The recent resolution of the structure of various mitochondrial bc(1) complexes in different crystallographic forms has consolidated previous findings, added atomic-scale precision to our knowledge, and raised new issues, such as the possible movement of the Rieske Fe-S protein subunit during Q(o) site catalysis. Here, studies performed during the last few years using bacterial bc(1) complexes are reviewed briefly and ongoing investigations and future challenges of this exciting field are mentioned.