The cytochrome bc1 complex (also known as ubiquinol:cytochrome c oxidoreductase, ubiquinol:ferricytochrome c oxidoreductase, and respiratory complex III) is a highly conserved enzyme of the mitochondrial respiratory chain (reviewed in 4). In S. cerevisiae it consists of three catalytic subunits, Cobp, Rip1p, and Cyt1p, plus seven additional subunits: Cor1p, Qcr2p, Qcr6p, Qcr7p, Qcr8p, Qcr9p, and Qcr10p (4, 5). The crystal structure of the complex shows that two functional units, each containing these ten subunits, associate with each other in the mitochondrial inner membrane (6). Assembly of a functional complex requires two proteins, Cbp3p and Cbp4p, that are not components of the complex but may associate with it during assembly (7). It also requires Bcs1p, an AAA-family ATPase that interacts with a precursor of the complex to mediate incorporation of the Rip1p and Qcr10p subunits (8). The mechanism of energy transfer by the complex, known as the protonmotive Q cycle, has been studied in detail (reviewed in 4). The net result of the Q cycle is the stepwise transfer of an electron through the complex from ubiquinol to cytochrome c (Cyc1p), coupled with the translocation of a proton across the mitochondrial inner membrane (4). The function of the cytochrome bc1 complex is essential to the energy-generating process of oxidative phosphorylation, which is carried out by the enzyme complexes of the mitochondrial respiratory chain.

The Qcr8p subunit is essential to activity of the cytochrome bc1 complex: the qcr8 complete deletion, and certain missense mutations, completely block respiratory growth (9). Substitution of a proline residue with a positively charged amino acid in the single transmembrane helix of Qcr8p, which constitutes part of the ubiquinone binding site, causes instability and reduced function of the complex and partial loss of cytochrome b (Cobp) (9). Certain qcr8 mutations affect the activity of the succinate-ubiquinone reductase complex (complex II), suggesting that Qcr8p may mediate an interaction between these two mitochondrial respiratory chain complexes (10). Qcr8p is conserved across eukaryotes (3, 11, 12), and its bovine counterpart can partially substitute for the function of yeast Qcr8p (13).

Last updated: 2007-07-26