Modification of the spectral properties of cytochrome b in mutants of Saccharomyces cerevisiae resistant to 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Mapping at two distinct genetic loci of the split mitochondrial gene of cytochrome b

Investigating the Qn site of the cytochrome bc1 complex in Saccharomyces cerevisiae with mutants resistant to ilicicolin H, a novel Qn site inhibitor

The cytochrome bc1 complex resides in the inner membrane of mitochondria and transfers electrons from ubiquinol to cytochrome c. This electron transfer is coupled to the translocation of protons across the membrane by the protonmotive Q cycle mechanism. This mechanism topographically separates reduction of quinone and reoxidation of quinol at sites on opposite sites of the membrane, referred to as center N (Qn site) and center P (Qp site), respectively. Both are located on cytochrome b, a transmembrane protein of the bc1 complex that is encoded on the mitochondrial genome. To better understand the parameters that affect ligand binding at the Qn site, we applied the Qn site inhibitor ilicicolin H to select for mutations conferring resistance in Saccharomyces cerevisiae. The screen resulted in seven different single amino acid substitutions in cytochrome b rendering the yeast resistant to the inhibitor. Six of the seven mutations have not been previously linked to inhibitor resistance. Ubiquinol-cytochrome c reductase activities of mitochondrial membranes isolated from the mutants confirmed that the differences in sensitivity toward ilicicolin H originated in the cytochrome bc1 complex. Comparative in vivo studies using the known Qn site inhibitors antimycin and funiculosin showed little cross-resistance, indicating different modes of binding of these inhibitors at center N of the bc1 complex.

Sequence analysis of three deficient mutants of cytochrome oxidase subunit I of Saccharomyces cerevisiae and their revertants

Three respiratory-deficient mutants of cytochrome oxidase subunit I in the yeast mitochondrion have been sequenced. They are located in, or near, transmembrane segment VI, the catalytic core of the enzyme. Respiratory-competent revertants have been selected and studied. The mutant V244M was found to revert at the same site in valine (wild-type), isoleucine or threonine. The revertants of the mutant G251R were of three types: glycine (wild-type), serine and threonine at position 251. A search for second-site mutations was carried out but none were found. Among 60 revertants tested, the mutant K265M was found to revert only to the wild-type allele.

Analysis of revertants from respiratory deficient mutants within the center N of cytochrome b in Saccharomyces cerevisiae

Four modified cytochrome b's carrying mononucleotide substitutions affecting center N residues were analysed. The mutant carrying a G33D change does not incorporate heme into the apocytochrome b and fails to grow on non-fermentable carbon sources. Out of 85 genetically independent revertants derived from this mutant, 82 were true back-mutants restoring the wild type sequence (D33G). The remaining three replaced the aspartic acid by an alanine (D33A) indicating that small size residues are best tolerated at this position which is consistent with the perfect conservation of the G33 during evolution. This glycine may be of crucial importance for helix packing around the hemes. The replacement of methionine at position 221 by lysine (M221K) produced a non-functional cytochrome b [(1993) J. Biol. Chem. 268, 15626-15632]. Non-native revertants replacing the lysine 221 by glutamic acid (K221E) or glutamine (K221Q) expressed a selective resistance to antimycin and antimycin derivatives having a modified dilactone ring moiety. Cytochrome b residues in 33 and in 221 seemed to contribute to the quinone reduction (QN) site of the cytochrome bc1 complex. Possible intramolecular interactions between the N-terminal region and the loop connecting helices IV and V of cytochrome b are 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.

Random mutant generation and its utility in uncovering structural and functional features of cytochrome b in Saccharomyces cerevisiae

The generation of random mutations in the mitochondrial cytochrome b gene of Saccharomyces cerevisiae has been used as a most fruitful means of identifying subregions that play a key role in the bc1 complex mechanism, best explained by the protonmotive Q cycle originally proposed by Peter Mitchell. Selection for center i and center o inhibitor resistance mutants, in particular, has yielded much information. The combined approaches of genetics and structural predictions have led to a two-dimensional folding model for cytochrome b that is most compatible with current knowledge of the protonmotive Q cycle. A three-dimensional model is emerging from studies of distant reversions of deficient mutants. Finally, interactions between cytochrome b and the other subunits of the bc1 complex, such as the iron-sulfur protein, can be affected by a single amino acid change.

Cytochrome b of fish mitochondria is strongly resistant to funiculosin, a powerful inhibitor of respiration

We report here some unusual properties of ubiquinol: cytochrome c reductase of eel and other fish mitochondria. The turnover rate of the reductase is clearly higher than in mammalian mitochondria and the binding constant for ubiquinone seems to be larger than in other vertebrates. Additionally, the reductase activity of fish mitochondria is resistant to some powerful inhibitors that bind to cytochrome b, in particular to funiculosin. After sequencing most of the gene of eel cytochrome b and comparing the deduced amino acid sequence with that of other fish and animals, we hypothesize that the decreased binding of funiculosin could be due to a few amino acid replacements in the third and fourth transmembrane helix of the protein. In particular, the presence of methionine instead of alanine at position 125 seems to be largely responsible for the strong resistance to funiculosin and also to the partial resistance to myxothiazol in all fish mitochondria. Correlations between some residue substitutions in cytochrome b and the different effects of funiculosin in different species are also considered.

Isolation and RNA sequence analysis of cytochrome b mutants resistant to funiculosin, a center i inhibitor of the mitochondrial ubiquinol-cytochrome c reductase in Saccharomyces cerevisiae

Funiculosin is a well-known inhibitor of the mitochondrial respiratory chain, probably acting at the ubiquinone reducing site or center i of QH2-cytochrome c reductase. We report here the isolation, mapping and RNA sequence analysis of yeast apo-cytochrome b mutants resistant to this inhibitor. Funiculosin-resistance was found to be conferred, in 4 independent isolates, upon replacement of a leucine residue by phenylalanine in position 198 of the cytochrome b polypeptide chain.

The pathway of electron transfer in the dimeric QH2: cytochrome c oxidoreductase

The experimental data currently available suggest that QH2:cytochrome c oxidoreductase functions according to a Q-cycle type of mechanism. The molecular weight of the enzyme in a natural or artificial phospholipid bilayer or in solution corresponds to that of a dimer. The pre-steady state kinetics of reduction of the prosthetic groups indicate that the enzyme is functionally dimeric. A double Q cycle is proposed, describing the pathway of electron transfer in the dimeric QH2:cytochrome c oxidoreductase. According to this scheme, the two monomeric halves of the enzyme act in a cooperative fashion to complete the catalytic cycle. It is proposed that high-potential cytochrome b-562 and low-potential cytochrome b-562 act cooperatively, viz. as a functional pair, in the antimycin-sensitive reduction of ubiquinone to ubiquinol.

Cytochrome b of cob revertants in yeast. Bioenergetic characterization of revertants with reduced content and shifted maximum absorption wavelength of cytochrome b

22 revertants of Saccharomyces cerevisiae with intragenic suppressors (supa) of cob exon mutations (G. Burger, Mol. Gen. Genet., in the press) were analyzed. They display either a reduced amount of cytochrome b, or a shifted maximum absorption wavelength of total cytochrome b or a reduced growth rate on glycerol. The relationship of physico-chemical properties (content, light absorption and midpoint potential of cytochromes bK and bT) and functional properties (electron transport and energy yield) has been examined. In seven of eight revertants with a shifted maximum absorption wavelength of cytochrome b neither growth rate nor electron transfer activity was affected. In 13 of 14 revertants, reduced content of cytochrome b corresponds to a reduced electron transport rate through the cytochrome bc1 segment. A lower enzymatic activity, which is not due to a quantitative but to a qualitative alteration of cytochrome b was found in two revertants. Two revertants show electron transport rates of wild-type level concomitant with a reduced growth rate on glycerol, obviously due to a less efficient energy coupling. All revertants were shown to contain a high and a low potential cytochrome b, referred to as bK and bT. Those cob-/supa mutations which shift the maximum absorption wavelength or diminish the content of cytochrome b affect both b cytochromes in all cases. The results support that electron transport and energy conservation are catalyzed by the unity of cytochrome bK and bT and that both heme centers are bound to an identical apoenzyme. Comparing electron flow rates of succinate:cytochrome c oxidoreductase and NADH:cytochrome c oxidoreductase in cob- mutants and two revertants provides evidence that ubiquinone does not constitute a homogeneous pool, suggested by the dissimilar interaction of both dehydrogenases with the bc1 segment.

Effect of b-c1-site inhibitors on the midpoint potentials of mitochondrial cytochromes b

Anaerobic potentiometric titrations of b cytochromes have been carried out in beef heart submitochondrial particles in the presence of several specific inhibitors of electron transfer through the b-c1-site of the respiratory chain. Whereas antimycin shows no significant effect on the titration curve of cytochrome b-562, NoHOQnO is found to shift the Em of b-562 by 20-30 mV to the positive. Funiculosin raises the Em of b-562 by greater than 100 mV and also appears to bring about a minor shift of b-566 midpoint potential. In the presence of myxothiazol, both b cytochromes titrate with Em values 15-30 mV more positive than in the control.

Is cytochrome b really the antimycin-binding component of the cytochrome b--c1 complex of yeast mitochondria

1. The antimycin binding sites were found to be independent of cytochrome(s) b synthesis in pseudo-wild-type revertants of cytochrome b mutants. These revertants, whose primary mutation is located in the introns of the cob-box gene of mitochondrial DNA, have modified contents of cytochromes b-562 and b-565 and fully functional respiratory chain. 2. Missense mutations in three genetic loci allocated to the exons of the cytochrome b gene, abolish the strong affinity binding of antimycin. 3. It is proposed that the antimycin-binding component is not cytochrome b itself, but interacts with it in such a way that an alteration of the cytochrome b structure affects the antimycin-binding site.

Genetics and biogenesis of cytochrome b

We have reviewed here the genetic methods used for isolating and manipulating nuclear and mitochondrial mutants of bakers' yeast that affect the function and biogenesis of complex III of the mitochondrial respiratory chain. All the methods have been used with success in the past, and it is hoped that this compilation will aid biochemists in using these techniques to study electron transfer.

Reduction of respiratory-chain cytochrome b by lactate in Saccharomyces cerevisiae

Cytochrome b of yeast mitochondria can be reduced by a part of the electrons resulting from the oxidation of lactate enantiomers. 1. The respiration of D-lactate and L-lactate is 30-40% inhibited by antimycin A. 2. Reduction of cytochrome b is observed in submitochondrial particles in the presence of low concentration of D-lactate and L-lactate (half-optimal concentration of 4.7 mM and 2.4 mM respectively) in the presence of different bc1 inhibitors. 3. Reduction of cytochrome b and c1 occurs in purified complex III of yeast in the presence of L-lactate and added L-lactate: NAD+ oxidoreductase. 4. In the particles obtained from yeast grown in lactate the oxidation of L-lactate involves the reduction of a pigment absorbing at 558 nm.