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.

Cyt1p, more commonly known as cytochrome c1, is one of the catalytic subunits of the cytochrome bc1 complex and is essential for electron transfer within the complex and for respiratory growth (9, 4). Maturation and mitochondrial import of Cyt1p involve proteolytic cleavages of the N- and C-terminal regions as well as insertion of a heme prosthetic group, which is mediated by cyctochrome c1 heme lyase (Cyt2p) (10, 11, 12). CYT1 is conserved across both prokaryotes and eukaryotes (13); its human homolog is CYC1 (OMIM; note that the S. cerevisiae gene named CYC1 encodes a distinct protein, cytochrome c).

Last updated: 2007-07-26