Characterization of mouse nuclear and mitochondrial mutants with increased resistance to cytochrome b inhibitors

Electrochemical and spectral analysis of the long-range interactions between the Qo and Qi sites and the heme prosthetic groups in ubiquinol-cytochrome c oxidoreductase

The results are presented of an electrochemical and high-resolution spectral analysis of the heme prosthetic groups in the bc1 complex from mouse cells. To study the long-range interactions between the Qo and Qi quinone redox sites and the b heme groups, we analyzed the effects on the proximal and distal b heme groups, and the c1 heme, of inhibitors that tightly and specifically bind to the Qi or Qo redox site. A number of results emerged from these studies. (1) There is inhomogeneous broadening of the b heme alpha band absorption spectra. Furthermore, contrary to the conclusion from low-resolution spectral analysis, the higher energy transition in the split-alpha band spectrum of the bL heme is more intense than the lower energy transition. (2) Inhibitors that bind at the Qi site have significant effects upon the electronic environment of the distal bL heme. Conversely, Qo site inhibitors induced changes in the electronic environment of the distal bH heme. (3) In contrast, inhibitor binding at either site has little effect upon the midpoint potential of the distal heme. (4) Experiments in which both a Qi and a Qo inhibitor are bound at the redox sites indicate that the long-range effects of one inhibitor are not blocked by the second inhibitor; enhanced effects are often observed. (5) In the double-inhibitor titrations involving the Qo inhibitor myxothiazol, there is evidence for two electrochemically and spectrally distinct species of the bL heme group, a phenomenon not observed previously. (6) The high-resolution deconvolutions of alpha band absorption spectra allow an interpretation of these inhibitor-induced changes in terms of homogeneous broadening, inhomogeneous broadening, and changes in x-y degeneracy. The general conclusion from these experiments is that when an inhibitor binds to a quinone redox site of the cytochrome b protein, it produces local conformational changes that, in turn, are transmitted to distal regions of the protein. The ligation of the bH and bL hemes between two parallel transmembrane helices provides a mechanism by which long-distance interactions can be propagated. The lack of long-range effects upon the midpoint potentials of the heme groups suggests, however, that protein conformational changes are unlikely to be a major control mechanism for the transmembrane electron- and proton-transfer steps of the Q cycle.

Glycine-231 residue of the mouse mitochondrial protonmotive cytochrome b: mutation to aspartic acid deranges electron transport

The mouse LA9 HQN-R11 cytochrome b mutant, in which the glycine residue at position 231 is replaced by aspartic acid, has increased resistance to all inhibitors of the Qn redox center. It is shown here that this single amino acid alteration has multiple and unexpectedly diverse effects upon the mitochondrial protonmotive bc1 complex. (1) The specific activities of both succinate- and ubiquinol-cytochrome c oxidoreductases in isolated mitochondria are reduced by approximately 65% in the mutant. The parallel reductions in both oxidoreductase activities are not compatible with simple Q pool kinetics for mitochondrial electron transport. (2) There is also a reduction in the relative concentration of cytochrome b in the mutant when calculated on the basis of mitochondrial protein; this decrease does not account for more than a small portion of the reduced catalytic fluxes. (3) The increased antimycin resistance of the mutant is lost upon solubilization by the detergent dodecyl maltoside of the bc1 complex from mitochondria. (4) In pre-steady-state assays of cytochrome b reduction by quinol, the mutant shows a reduced extent of reduction. It was observed in other experiments that there was less oxidant-induced extrareduction of cytochrome b in the mutant. These results could arise from a lowering of the midpoint potentials of both the cytochrome b-562 and cytochrome b-566 heme groups. Alternatively, these effects may reflect changes at the Qp and Qn quinone/quinol binding sites. (5) An unexplained observation for the mutant is the increased rate of cytochrome c1 reduction in the presence of myxothiazol. (6) These functional alterations in the LA9 HQN-R11 mutant are not accompanied by detectable changes in the spectral properties of the cytochrome b or c1 heme groups.

Nuclearly inherited diuron-resistant mutations conferring a deficiency in the NADH--or succinate--ubiquinone oxidoreductase activity in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, diuron, antimycin and myxothiazol block the respiratory pathway at the bc1 complex level. Nuclearly inherited mutations located at the DIU3 and DIU4 loci confer in vitro resistance to diuron and cross-resistance to antimycin and myxothiazol at the NADH oxidase level. The mutant strains do not exhibit diuron resistance at the quinol-cytochrome-c oxidoreductase level. Thus, the apparent resistance does not seem to be the result of a modification of the inhibitory sites. Instead, the quinone reduction rate was found to be altered in the mutant. The diu3 mutations lead to a deficiency of the NADH--ubiquinone oxidoreductase activity, and the diu4 mutations to a deficiency of the succinate--ubiquinone oxidoreductase activity. On the basis of the model of Kröger and Klingenberg, a decrease of quinone reduction could explain the resistance to the bc1 complex inhibitors. Thus, the apparent resistance to the bc1 complex inhibitors was found to be due to a modification of the electron transfer kinetics.

Two nuclearly inherited loci conferring increased diuron resistance to NADH oxidase in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, diuron blocks the respiration pathway at the level of the bc1 complex. Nuclear diuron-resistant mutations which confer in vitro resistance to mitochondrial NADH oxidase have been identified. Five mutations were found to be clustered at two distinct nuclear loci DIU3 and DIU4. The distance between the two loci was estimated to be about 36.7 cM. These loci do not appear to be centromere-linked and did not show a linkage to any of the genes coding for bc1 complex subunits. DIU3 and DIU4 loci might, therefore code for other components of the respiratory chain.

Myxothiazol resistance in human mitochondria

We have investigated electron transfer activities of respiratory chain complexes in platelet mitochondria of a patient with intermittent ataxia and lactic acidosis who was previously reported to be deficient in the E1 (decarboxylase) component of the pyruvate dehydrogenase complex. Electron transfer from succinate to cytochrome c was normal, but the mitochondria exhibited moderately decreased (63% of control) quinol: cytochrome-c oxidoreductase activity, suggesting a defect in complex III. Consistent with some perturbation in complex III, electron flux through complex III was resistant to inhibition by myxothiazol compared to normal controls. In contrast, titration with antimycin revealed a less abnormal pattern of inhibition. The extreme specificity of myxothiazol binding at or near the quinol oxidase domain of mitochondrial cytochrome b, i.e., b-566, suggests a defect in this region of complex III which may perturb the kinetics or thermodynamics of quinol oxidation in the complex. These data suggest that the patient's illness results from a mutation in the quinol oxidase domain of mitochondrial cytochrome b (b-566).

Mutational analysis of the mouse mitochondrial cytochrome b gene

The protonmotive cytochrome b protein of the mitochondrial bc1 respiratory chain complex contains two reactions centers, designated Qo and Qi, which can be distinguished by the effects of different inhibitors. The nucleotide sequences have been determined of the mitochondrial cytochrome b genes from a series of mouse cell mutants selected for increased inhibitor resistance. Each mutant contains a single nucleotide change which results in an amino acid substitution. When the proximity of the altered amino acid residues to the histidines involved in heme ligation is considered, the results support a model for cytochrome b folding in which there are eight transmembrane domains rather than the nine of the Widger-Saraste model. Replacement of the Gly38 residue by valine results in resistance to the Qi inhibitors antimycin A and funiculosin but not 2-n-heptyl-hydroxyquinoline-N-oxide. Based upon sequence comparisons of mitochondrial and bacterial cytochrome b and chloroplast b6 proteins, the region of the molecule involved in antimycin binding is as highly conserved as those domains involved in heme ligation. It is suggested that the antimycin binding domain of cytochrome b is involved in forming the Qi reaction center. Alterations of the Gly142 and Thr147 residues result in resistance to myxothiazol and stimatellin, respectively. While both inhibitors block the Qo reaction center, the two mutations do not confer cross-resistance to each other. This region of cytochrome b is the most highly conserved during evolution and these inhibitor binding sites probably occur within the protein domain constituting the Qo reaction center. In addition, there is a less conserved region of the protein, defined by the Leu294 residue, which may function in binding the hydrophobic portions of Qo inhibitors.

Mitochondrial chloramphenicol-resistant mutants can have deficiencies in energy metabolism

Three pairs of mouse CAP-R PYR-IND OLI-R mitochondrial mutants, and the corresponding CAP-S parental lines, were assayed to determine if cellular expression of these phenotypes was accompanied by changes in cellular energy metabolism: glycolysis, cellular respiration, citric acid cycle activity, and mitochondrial electron transport. Relative to its parental CAP-S line, the SVT2 CAP-R mutant had no significant deficiencies in any of the pathways analyzed. In contrast, the LA9 and SVA31 CAP-R mutants showed significant reductions in cellular respiration. At the biochemical level, respiration deficiency was accompanied by derangements in mitochondrial electron transport. It was also found that the CAP-R mutants had very high levels of glycolysis when the cells were maintained in the presence of chloramphenicol. The possibility is discussed that the sequence changes in the mitochondrial large rRNA gene which determine chloramphenicol resistance can also result, at least in some cases, in reduced levels of mitochondrial biogenesis, leading to respiration deficiency. The PYR-IND and OLI-R phenotypes, which also appear to be encoded by the CAP-R mutations, may result from a compensatory increase in glycolysis-generated ATP or metabolic intermediates.