The interaction between mitochondrial NADH-ubiquinone oxidoreductase and ubiquinol-cytochrome c oxidoreductase. Restoration of ubiquinone-pool behaviour

New concepts on the role of ubiquinone in the mitochondrial respiratory chain

Ubiquinone participates in the oxidation-reduction reactions of the mitochondrial respiratory chain. In addition, this molecule possesses the necessary properties to function as a hydrogen carrier, thereby stoichiometrically coupling proton translocation to respiration by a direct chemiosmotic mechanism. This review discusses recent experimental evidence and new concepts relating to ubiquinone function in the mitochondrial respiratory chain. Emphasis is placed on possible protonmotive mechanisms of ubiquinone function, recent evidence implicating stable forms of ubisemiquinone in the respiratory chain, and properties of the ubiquinone molecule which may relate to its biological function.

Studies on the stabilized ubisemiquinone species in the succinate-cytochrome c reductase segment of the intact mitochondrial membrane system

1. Evidence is presented for the presence of a stable ubisemiquinone pair in the vicinity of iron-sulphur centre S-3, based on its thermodynamic and spin relaxation properties. 2. These semiquinones are coupled by dipolar interaction; quantitative analysis of the signals of the spin-coupled semiquinones (at pH 7.4) gives midpoint redox potentials E1 (oxidized to semiquinone state) and E2 (semiquinone to fully reduced state) of 140 and 80mV, respectively, for individual ubiquinones. 3. Values of pKS (pK of the semiquinone form) below 6.5 and pKR (pK of the fully reduced ubiquinone) of about 8.0 or above were estimated from the pH-dependence of the midpoint potentials of the spin coupled signals. Thus the ubisemiquinone associated with succinate dehydrogenase (designated as SQS) functions mostly in the anionic form of the physiological pH range. 4. Theonyltrifluoroacetone, a specific inhibitor of the succinate-ubiquinone reductase segment of the respiratory chain, destabilized the intermediate redox state; thus it quenches both the g = 2.00 signal and ubisemiquinone (SQS) and split signals from the spin coupled pair. This inhibitor has no significant effect on another bound ubisemiquinone species present in the cytochrome bc1 region (designated as SQC). 5. The possible function and location of these stabilized ubisemiquinone species were discussed in connection with Site-II energy transduction.

The reconstitution of L-3-glycerophosphate-cytochrome c oxidoreductase from L-3-glycerophosphate dehydrogenase, ubiquinone-10 and ubiquinol-cytochrome c oxidoreductase

Purified L-3-glycerophosphate dehydrogenase from pig brain mitochondria interacts with ubiquinone-10 and ubiquinol-cytochrome c oxidoreductase (Complex III) from bovine heart mitochondria to reconstitute antimycin-sensitive L-3-glycerophosphate- cytochrome c oxidoreductase. This activity is completely dependent on the two enzymes and largely dependent on ubiquinone-10. Reconstitution requires that the two enzymes should be simultaneously present in the same membranous aggregate produced by removal of detergent from the enzymes. Reconstitution by removing detergent by dialysis or dilution is inefficient because of self-aggregation of the dehydrogenase. Highly efficient reconstitution can be achieved if the enzymes are co-precipitated by addition of ethanol. The rate with reconstituted enzyme approaches that expected from the turnover of the dehydrogenase with ubiquinone-1 as acceptor. The behaviour of the reconstituted system shows some of the characteristics expected for a stoicheiometric association of one molecule of dehydrogenase with one molecule of Complex III. On raising the phospholipid/protein ratio, the dehydrogenase and Complex III appear to operate as independent enzymes acting in sequence. These effects are very similar to those observed for the interaction of NADH dehydrogenase and Complex III and are explained in terms of the model proposed by Heron, Ragan & Trumpower [(1978) biochem. J. 174, 791-800].

The effects of lipid phase transitions on the interaction of mitochondrial NADH--ubiquinone oxidoreductase with ubiquinol--cytochrome c oxidoreductase

1. The endogenous phosphatidylcholine and phosphatidylethanolamine of Complexes I and III from bovine heart mitochondria may be completely replaced with 1,2-ditetradecanoyl-sn-glycero-3-phosphocholine with at least partial retention of activity. 2. The lipid-replaced enzymes associate in 1:1 molar ratio to give a Complex I--III unit catalysing NADH-cytochrome c oxidoreductase activity. 3. On increasing the concentration of ubiquinone-10 and the synthetic phospholipid, the lipid-replaced Complexes appear to operate independently of each other as in the natural membrane. Thus the lipid-replaced enzymes associate in exactly the same ways as the enzymes containing natural phospholipids. 4. Arrhenius plots of NADH--cytochrome c oxidoreductase activity reconstituted from lipid-replaced Complexes I and III exhibit changes in slope at 24 degrees C. When the concentrations of phospholipid and ubiquinone-10 are increased, the Arrhenius plots show discontinuities at 24 degrees C as well as changes in slope. 5. The kinetics of cytochrome b reduction by NADH were measured in mixtures containing 2 mol of Complex III/mol of Complex I. When the enzymes contained natural phospholipids. the reduction kinetics were biphasic. When the enzymes had been supplemented with further phospholipid and ubiquinone-10 the kinetics were monophasic. When lipid-replaced enzymes were supplemented with 1,2-ditetradecanoyl-sn-glycero-3-phosphocholine and ubiquinone-10, reduction of cytochrome b was monophasic above the phase-transition temperature of the lipid but biphasic below it. 6. These findings are interpreted in terms of the model for the interaction of Complexes in the natural membrane proposed by Heron, Ragan & Trum-power [(1978) Biochem. J. 174, 791--800].

The interaction between mitochondrial NADH-ubiquinone oxidoreductase and ubiquinol-cytochrome c oxidoreductase. Evidence for stoicheiometric association

1. The NADH-ubiquinone oxidoreductase complex (Complex I) and the ubiquinol-cytochrome c oxidoreductase complex (Complex III) combine in a 1:1 molar ratio to give NADH-cytochrome c oxidoreductase (Complex I-Complex III). 2. Experiments on the inhibition of the NADH-cytochrome c oxidoreductase activity of mixtures of Complexes I and III by rotenone and antimycin indicate that electron transfer between a unit of Complex I-Complex III and extra molecules of Complexes I or III does not contribute to the overall rate of cytochrome c reduction. 3. The reduction by NADH of the cytochrome b of mixtures of Complexes I and III is biphasic. The extents of the fast and slow phases of reduction are determined by the proportion of the total Complex III specifically associated with Complex I. 4. Activation-energy measurements suggest that the structural features of the Complex I-Complex III unit promote oxidoreduction of endogenous ubiquinone-10.