Studies on the CoQH2-cytochrome c reductase segment of the respiratory chain of yeast mitochondria, using mutants of the cytochrome b split gene

Redox components of cytochrome bc-type enzymes in acidophilic prokaryotes. I. Characterization of the cytochrome bc1-type complex of the acidophilic ferrous ion-oxidizing bacterium Thiobacillus ferrooxidans

The redox components of the cytochrome bc1 complex from the acidophilic chemolithotrophic organism Thiobacillus ferrooxidans were investigated by potentiometric and spectroscopic techniques. Optical redox titrations demonstrated the presence of two b-type hemes with differing redox midpoint potentials at pH 7.4 (-169 and + 20 mV for bL and bH, respectively). At pH 3.5, by contrast, both hemes appeared to titrate at about +20 mV. Antimycin A, 2-heptyl-4-hydroxyquinoline N-oxide, and stigmatellin induced distinguishable shifts of the b hemes' alpha-bands, providing evidence for the binding of antimycin A and 2-heptyl-4-hydroxyquinoline N-oxide near heme bH (located on the cytosolic side of the membrane) and of stigmatellin near heme bL (located on the periplasmic side of the membrane). The inhibitors stigmatellin, 5-(n-undecyl)-6-hydroxy-4,7-dioxobenzothiazole, and 2, 5-dibromo-3-methyl-6-isopropyl-p-benzoquinone affected the EPR spectrum of the Rieske iron-sulfur center in a way that differs from what has been observed for cytochrome bc1 or b6f complexes. The results obtained demonstrate that the T. ferrooxidans complex, although showing most of the features characteristic for bc1 complexes, contains unique properties that are most probably related to the chemolithotrophicity and/or acidophilicity of its parent organism. A speculative model for reverse electron transfer through the T. ferrooxidans complex is proposed.

Mitochondrial cytochrome b: evolution and structure of the protein

Cytochrome b is the central redox catalytic subunit of the quinol: cytochrome c or plastocyanin oxidoreductases. It is involved in the binding of the quinone substrate and it is responsible for the transmembrane electron transfer by which redox energy is converted into a protonmotive force. Cytochrome b also contains the sites to which various inhibitors and quinone antagonists bind and, consequently, inhibit the oxidoreductase. Ten partial primary sequences of cytochrome b are presented here and they are compared with sequence data from over 800 species for a detailed analysis of the natural variation in the protein. This sequence information has been used to predict some aspects of the structure of the protein, in particular the folding of the transmembrane helices and the location of the quinone- and heme-binding pockets. We have observed that inhibitor sensitivity varies greatly among species. The comparison of inhibition titrations in combination with the analysis of the primary structures has enabled us to identify amino acid residues in cytochrome b that may be involved in the binding of the inhibitors and, by extrapolation, quinone/quinol. The information on the quinone-binding sites obtained in this way is expected to be both complementary and supplementary to that which will be obtained in the future by mutagenesis and X-ray crystallography.