On the localization of ubiquinone in phosphatidylcholine bilayers

The location of ubiquinone-10 in phospholipid bilayers was analyzed using a variety of physical techniques. Specifically, we examined the hypothesis that ubiquinone localizes at the geometric center of phospholipid bilayers. Light microscopy of dipalmitoylphosphatidylcholine at room temperature in the presence of 0.05–0.5 mol fraction ubiquinone showed two separate phases, one birefringent lamellar phase and one phase that consisted of isotropic liquid droplets. The isotropic phase had a distinct yellow color, characteristic of melted ubiquinone. [¹³C]NMR spectroscopy of phosphatidylcholine liposomes containing added ubiquinone indicated a marked effect on the ¹³C-spin lattice relaxation times of the lipid hydrocarbon chain atoms near the polar head region of the bilayer, but almost no effect on those atoms nearest the center of the bilayer. X-ray diffraction experiments showed that for phosphatidylcholine bilayers, both in the gel and liquid-crystal-line phases, the presence of ubiquinone did not change either the lamellar repeat period or the wide-angle reflections from the lipid hydrocarbon chains. In electron micrographs, the hydrophobic freeze-fracture surfaces of bilayers in the rippled (Pβ′) phase were also unmodified by the presence of ubiquinone. These results indicate that the ubiquinone which does partition into the bilayer is not localized preferentially between the monolayers, and that an appreciable fraction of the ubiquinone forms a separate phase located outside the lipid bilayer.     

On the localization of ubiquinone in phosphatidylcholine bilayers

The location of ubiquinone-10 in phospholipid bilayers was analyzed using a variety of physical techniques. Specifically, we examined the hypothesis that ubiquinone localizes at the geometric center of phospholipid bilayers. Light microscopy of dipalmitoylphosphatidylcholine at room temperature in the presence of 0.05-0.5 mol fraction ubiquinone showed two separate phases, one birefringent lamellar phase and one phase that consisted of isotropic liquid droplets. The isotropic phase had a distinct yellow color, characteristic of melted ubiquinone. [13C]NMR spectroscopy of phosphatidylcholine liposomes containing added ubiquinone indicated a marked effect on the 13C-spin lattice relaxation times of the lipid hydrocarbon chain atoms near the polar head region of the bilayer, but almost no effect on those atoms nearest the center of the bilayer. X-ray diffraction experiments showed that for phosphatidylcholine bilayers, both in the gel and liquid-crystal-line phases, the presence of ubiquinone did not change either the lamellar repeat period or the wide-angle reflections from the lipid hydrocarbon chains. In electron micrographs, the hydrophobic freeze-fracture surfaces of bilayers in the rippled (P beta') phase were also unmodified by the presence of ubiquinone. These results indicate that the ubiquinone which does partition into the bilayer is not localized preferentially between the monolayers, and that an appreciable fraction of the ubiquinone forms a separate phase located outside the lipid bilayer.     

Antioxidative effect of ubiquinones on mitochondrial membranes.

Peroxidation of mitochondria occurs extensively in ubiquinone-depleted membranes. Reincorporation into the membranes of either the physiological ubiquinone or a short-chain homologue protects mitochondria against peroxidation. The ability to prevent this phenomenon is more evident in mitochondria that have incorporated ubiquinone-3 and might be ascribed to an ordering structural effect on the lipid bilayer.     

弱氧化葡糖杆菌ddsA基因在大肠杆菌不同宿主菌中的表达

泛醌（辅酶Q）在生物体氧化呼吸链中作为重要的质子和电子传递物质。聚十异戊烯焦磷酸合成酶催化辅助酶Q10的侧链的生物合成。为了获得高产辅助酶Q10的菌株,将选择了10种不同大肠杆菌宿主菌用于弱氧化葡糖杆菌的聚十异戊烯焦磷酸合成酶基因ddsA的表达,通过产物分析证实该基因能在大肠杆菌中表达出有活性的聚十异戊烯焦磷酸合成酶,使大肠杆菌合成了辅酶Q10。此外,还发现在Escherichia coli HB101这一菌株中,ddsA的表达使辅酶Q10的产量略超过了在野生型中占主导地位的辅酶Q8的产量。该结果证明了利用大肠杆菌大规模发酵生产辅酶Q10的可能性。     

The proton-pumping NADH:ubiquinone oxidoreductase (complex I) of Aquifex aeolicus

The proton-pumping NADH:ubiquinone oxidoreductase, also called complex I, is the first energy-transducing complex of many respiratory chains. Homologues of complex I are present in the three domains of life. Here, we report the properties of complex I in membranes of the hyperthermophilic bacterium Aquifex aeolicus. The complex reacted with NADH but not with NADPH and F(420)H(2) as electron donors. Short-chain analogues of ubiquinone like decyl-ubiquinone and ubiquinone-2 were suitable electron acceptors. The affinities towards NADH and ubiquinone-2 were comparable to the ones obtained with the Escherichia coli complex I. The reaction was inhibited by piericidin A at the same concentration as in E. coli. The complex showed an unusual pH optimum at pH 9 and a maximal rate at 80 degrees C. We found no evidence for the presence of an alternative, single subunit NADH dehydrogenase in A. aeolicus membranes. The NADH:ferricyanide reductase activity of detergent extracts of A. aeolicus membranes sedimented as a protein with a molecular mass of approximately 550 kDa. From the data we concluded that A. aeolicus contains a NADH:ubiquinone oxidoreductase resembling complex I of mesophilic bacteria.     

Antioxidant action of ubiquinol homologues with different isoprenoid chain length in biomembranes

Ubiquinones (CoQn) are intrinsic lipid components of many membranes. Besides their role in electron-transfer reactions they may act as free radical scavengers, yet their antioxidant function has received relatively little study. The efficiency of ubiquinols of varying isoprenoid chain length (from Q0 to Q10) in preventing (Fe2+ + ascorbate)-dependent or (Fe2+ + NADPH)-dependent lipid peroxidation was investigated in rat liver microsomes and brain synaptosomes and mitochondria. Ubiquinols, the reduced forms of CoQn, possess much greater antioxidant activity than the oxidized ubiquinone forms. In homogenous solution the radical scavenging activity of ubiquinol homologues does not depend on the length of their isoprenoid chain. However in membranes ubiquinols with short isoprenoid chains (Q1-Q4) are much more potent inhibitors of lipid peroxidation than the longer chain homologues (Q5-Q10). It is found that: i) the inhibitory action, that is, antioxidant efficiency of short-chain ubiquinols decreases in order Q1 greater than Q2 greater than Q3 greater than Q4; ii) the antioxidant efficiency of long-chain ubiquinols is only slightly dependent on their concentrations in the order Q5 greater than Q6 greater than Q7 greater than Q8 greater than Q9 greater than Q10 and iii) the antioxidant efficiency of Q0 is markedly less than that of other homologues. Interaction of ubiquinols with oxygen radicals was followed by their effects on luminol-activated chemiluminescence. Ubiquinols Q1-Q4 at 0.1 mM completely inhibit the luminol-activated NADPH-dependent chemiluminescent response of microsomes, while homologues Q6-Q10 exert no effect. In contrast to ubiquinol Q10 (ubiquinone Q10) ubiquinone Q1 synergistically enhances NADPH-dependent regeneration of endogenous vitamin E in microsomes thus providing for higher antioxidant protection against lipid peroxidation. The differences in the antioxidant potency of ubiquinols in membranes are suggested to result from differences in partitioning into membranes, intramembrane mobility and non-uniform distribution of ubiquinols resulting in differing efficiency of interaction with oxygen and lipid radicals as well as different efficiency of ubiquinols in regeneration of endogenous vitamin E.     

Lateral diffusion of ubiquinone during electron transfer in phospholipid- and ubiquinone-enriched mitochondrial membranes

After fusion of small unilamellar phospholipid liposomes with mitochondrial inner membranes, the rate of electron transfer between membrane dehydrogenases and cytochrome c decreases as the average distance between integral membrane proteins increases, suggesting that electron transfer is mediated through a diffusional process in the membrane plane (Schneider, H., Lemasters, J. J., H?chli, M., and Hackenbrock, C. R. (1980)., J. Biol. Chem. 255, 3748-3756). The role of ubiquinone in this process was evaluated by fusing liposomes containing ubiquinone-10 or ubiquinone-6, with inner membranes. In control membranes enriched with phospholipid only, ubiquinol-cytochrome c reductase and NADH- and succinate-cytochrome c reductase activities decreased proportionally to the increase in bilayer lipid. These decreases were restored substantially in phospholipid plus ubiquinone-supplemented membranes. The degree to which restoration occurred was dependent upon the length of the isoprenoid side chain of the ubiquinone with the shorter chain length ubiquinone-6, always giving greater restoration than ubiquinone-10. It is concluded that electron transfer between flavin-linked dehydrogenases (Complexes I and II) and cytochrome bc1 (Complex III) occurs by independent, lateral diffusion of ubiquinone as well as independent, lateral diffusion of ubiquinone as well as the protein complexes within the plane of the membrane.     

Topographical characterization of the ubiquinone reduction site of glucose dehydrogenase in Escherichia coli using depth-dependent fluorescent inhibitors.

Membrane-bound glucose dehydrogenase in Escherichia coli possesses a binding site for ubiquinone as well as glucose, metal ion and pyrroloquinoline quinone. To probe the depth of the ubiquinone binding site in the membrane environment, we synthesized two types of fluorenyl fatty acids which bear an inhibitor mimic moiety (i.e., specific inhibitor capsaicin) close to the fluorene located at different positions in the alkyl tail chain; one close to the polar carbonyl head group (alpha-(3, 4-dimethoxyphenyl)acetyloxy-7-nonyl-2-fluoreneacetic acid, alpha-DFA), and the other in the middle of the chain (theta-(3, 4-dimethoxyphenyl)acetyloxy-7-ethyl-2-fluorenenonanoic acid, theta-DFA). Mixed lipid vesicles consisting of phosphatidylcholine (PC) and alpha-DFA or theta-DFA were prepared by sonication method, and fluorescent quenching against a hydrophilic quencher, iodide anion, was examined. The vesicles containing alpha-DFA were more susceptible to quenching than those containing theta-DFA, indicating that the fluorene and consequently capsaicin mimic moiety are located at different depths in the lipid bilayer depending upon the position of attachment to the alkyl tail chain. The purified glucose dehydrogenase was reconstituted into PC vesicles which consisted of PC and alpha-DFA or theta-DFA with various molar ratios. For both types of reconstituted vesicles, the extent of inhibition of short-chain ubiquinone reduction activity increased with increases in the molar ratio of fluorenyl fatty acid to PC. The ubiquinone reduction activity was more significantly inhibited in the reconstituted vesicles containing alpha-DFA compared to those containing theta-DFA. Our findings strongly suggested that the ubiquinone reduction site in glucose dehydrogenase is located close to the membrane surface rather than in the hydrophobic membrane interior.     

The mammalian cytosolic selenoenzyme thioredoxin reductase reduces ubiquinone. A novel mechanism for defense against oxidative stress

The selenoprotein thioredoxin reductase (TrxR1) is an essential antioxidant enzyme known to reduce many compounds in addition to thioredoxin, its principle protein substrate. Here we found that TrxR1 reduced ubiquinone-10 and thereby regenerated the antioxidant ubiquinol-10 (Q10), which is important for protection against lipid and protein peroxidation. The reduction was time- and dose-dependent, with an apparent K(m) of 22 microm and a maximal rate of about 12 nmol of reduced Q10 per milligram of TrxR1 per minute. TrxR1 reduced ubiquinone maximally at a physiological pH of 7.5 at similar rates using either NADPH or NADH as cofactors. The reduction of Q10 by mammalian TrxR1 was selenium dependent as revealed by comparison with Escherichia coli TrxR or selenium-deprived mutant and truncated mammalian TrxR forms. In addition, the rate of reduction of ubiquinone was significantly higher in homogenates from human embryo kidney 293 cells stably overexpressing thioredoxin reductase and was induced along with increasing cytosolic TrxR activity after the addition of selenite to the culture medium. These data demonstrate that the selenoenzyme thioredoxin reductase is an important selenium-dependent ubiquinone reductase and can explain how selenium and ubiquinone, by a combined action, may protect the cell from oxidative damage.     

Identification of the ubiquinone-binding site of NADH:ubiquinone oxidoreductase (complex I) from Neurospora crassa.

In order to localize the ubiquinone-binding site of complex I (NADH:ubiquinone oxidoreductase), a novel photoreactive ubiquinone analogue (Q0C7ArN3) has been synthesized. It is shown that the direct chemical precursor of this analogue (Q0C7ArNO2) and the analogue itself are accepted as substrates in an enzyme assay utilizing ubiquinone-depleted mitochondrial membranes of Neurospora crassa. The activity of the enzyme applying these derivatives is inhibited by 50% at a concentration of 9 and 20 microM rotenone. Photoaffinity labeling experiments were performed with both isolated complex I and whole mitochondrial membranes of N. crassa under various conditions. In each of these experiments a protein subunit with an apparent molecular mass of about 9.5 kDa was labeled with high specificity. Radioactive labeling was totally prevented by the addition of ubiquinone-2 at concentrations higher than 500 microM but was not affected by comparable concentrations of rotenone or other hydrophobic substances. In the labeling experiments using whole membranes, the labeling signal was dramatically increased in the presence of 1.5 mM NADH. These results strongly suggest that the ubiquinone analogue interacts specifically with the enzyme.     

Relation of mevalonate synthesis to mitochondrial ubiquinone content and respiratory function in cultured neuroblastoma cells

The consequence of blocking the de novo synthesis of ubiquinone (coenzyme Q) on mitochondrial ubiquinone content and respiratory function was studied in cultured C1300 (Neuro 2A) murine neuroblastoma cells. Mevinolin, a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase, was used to suppress the synthesis of mevalonate, an essential precursor for the isoprenoid side chain of ubiquinone. At a concentration of 25 microM, mevinolin completely inhibited the incorporation of [3H]acetate into ubiquinone, isolated from cell extracts by two-dimensional thin-layer chromatography. Similar results were obtained when [14C]tyrosine was used as a precursor for the quinone ring. Through the use of reverse-phase thin-layer chromatography, it was established that the principal product of the ubiquinone pathway in murine neuroblastoma cells was ubiquinone-9. Inhibition of ubiquinone synthesis for 24h in cells cultured in the presence of 10% fetal calf serum (which contains 0.14 nmol of ubiquinone/ml of serum) resulted in a 40-57% decline in the concentration of ubiquinone in the mitochondria. However, the activities of succinate-cytochrome c reductase and succinate dehydrogenase in whole-cell homogenates or mitochondria were not inhibited. The state 3 and uncoupled rates of respiration, determined by polarographic measurements of oxygen consumption in homogenates and mitochondria, were elevated slightly in the mevinolin-treated cells. The data demonstrate that, although mevalonate synthesis is important for the maintenance of the intramitochondrial ubiquinone pool in cultured cells, major changes in the ubiquinone content of the mitochondria can occur in intact cells without perturbation of respiratory function. However, the coincidence of decreased mitochondrial ubiquinone concentration and the inhibition of cell cycling previously observed in mevinolin-treated cells (Maltese, W.A. (1984) Biochem. Biophys. Res. Commun. 120, 454-460) suggests that the availability of ubiquinone may play a role in the regulation of mitochondrial and cellular proliferation.     

Diffusion measurements of water, ubiquinone and lipid bilayer inside a cylindrical nanoporous support: A stimulated echo pulsed-field gradient MAS-NMR investigation

Stimulated echo pulsed-field gradient ¹H magic angle spinning NMR has been used to investigate the mobility of water, ubiquinone and tethered phospholipids, components of a biomimetic model membrane. The diffusion constant of water corresponds to an isotropic motion in a cylinder. When the lipid bilayer is obtained after the fusion of small unilamellar vesicles, the extracted value of lipid diffusion indicates unrestricted motion. The cylindrical arrangement of the lipids permits a simplification of data analysis since the normal bilayer is perpendicular to the gradient axis. This feature leads to a linear relation between the logarithm of the attenuation of the signal intensity and a factor depending on the gradient strength, for lipids covering the inner wall of aluminium oxide nanopores as well as for lipids adsorbed on a polymer sheet rolled into a cylinder. The effect of the bilayer formation on water diffusion has also been observed. The lateral diffusion coefficient of ubiquinone is in the same order of magnitude as the lipid lateral diffusion coefficient, in agreement with its localization within the bilayer.     

²H solid-state nuclear magnetic resonance investigation of whole Escherichia coli interacting with antimicrobial peptide MSI-78

A key aspect of the activity of antimicrobial peptides (AMPs) is their interaction with membranes. Efforts to elucidate their detailed mechanisms have focused on applying biophysical methods, including nuclear magnetic resonance (NMR), to AMPs in model lipid systems. However, these highly simplified systems fail to capture many of the features of the much more complex cell envelopes with which AMPs interact in vivo. To address this issue, we have designed a procedure to incorporate high levels of (2)H NMR labels specifically into the cell membrane of Escherichia coli and used this approach to study the interactions between the AMP MSI-78 and the membranes of intact bacteria. The (2)H NMR spectra of these membrane-deuterated bacteria can be reproduced in the absence and presence of MSI-78. Because the (2)H NMR data provide a quantitative measure of lipid disorder, they directly report on the lipid bilayer disruption central to the function of AMPs, in the context of intact bacteria. Addition of MSI-78 to the bacteria leads to decreases in the order of the lipid acyl chains. The molar peptide:lipid ratios required to observe the effects of MSI-78 on acyl chain order are approximately 30 times greater than the ratios needed to observe effects in model lipid systems and approximately 100 times less than the ratios required to observe inhibition of cell growth in biological assays. The observations thus suggest that MSI-78 disrupts the bilayer even at sublethal AMP levels and that a large fraction of the peptide does not actually reach the inner membrane.     

Diffusion measurements of water, ubiquinone and lipid bilayer inside a cylindrical nanoporous support: a stimulated echo pulsed-field gradient MAS-NMR investigation

Stimulated echo pulsed-field gradient 1H magic angle spinning NMR has been used to investigate the mobility of water, ubiquinone and tethered phospholipids, components of a biomimetic model membrane. The diffusion constant of water corresponds to an isotropic motion in a cylinder. When the lipid bilayer is obtained after the fusion of small unilamellar vesicles, the extracted value of lipid diffusion indicates unrestricted motion. The cylindrical arrangement of the lipids permits a simplification of data analysis since the normal bilayer is perpendicular to the gradient axis. This feature leads to a linear relation between the logarithm of the attenuation of the signal intensity and a factor depending on the gradient strength, for lipids covering the inner wall of aluminium oxide nanopores as well as for lipids adsorbed on a polymer sheet rolled into a cylinder. The effect of the bilayer formation on water diffusion has also been observed. The lateral diffusion coefficient of ubiquinone is in the same order of magnitude as the lipid lateral diffusion coefficient, in agreement with its localization within the bilayer.     

The effects of inducers of the endoplasmic reticulum, peroxisomes and mitochondria on the amounts and synthesis of ubiquinone in rat liver subcellular membranes

Rats were treated with inducers of peroxisomes, mitochondria and the endoplasmic reticulum, as well as receiving diets and drug known to influence the mevalonate pathway. Treatment with clofibrate and 2-diethylhexylphthalate (DEHP) increased microsomal and mitochondrial ubiquinone contents, but a decrease was observed in lysosomes. In vivo labeling of this lipid with [3H]mevalonate was also elevated. The amount of cholesterol did not change upon exposure to these inducers of peroxisomes and mitochondria, but its rate of labeling was decreased. The concentration of dolichol increased only after treatment with DEHP and only in lysosomes. The inducers of the endoplasmic reticulum phenobarbital, 3-methylcholanthrene and N-nitrosodiethylamine enhanced the rate of ubiquinone synthesis and exposure to the latter two substances also elevated the amount of this lipid in microsomes. A cholesterol-rich diet increased the labeling of ubiquinone and decreased cholesterol labeling, while cholestyramine treatment had opposite effects on lipid labeling in both microsomes and mitochondria. The results demonstrate that the ubiquinone contents of the various membranes of hepatocytes change in a characteristic manner under the influence of inducers and dietary factors. Clearly, the level of ubiquinone and its biosynthesis are regulated separately from those of the other products of the mevalonate pathway, cholesterol and dolichol.     

The role of Coenzyme Q in mitochondrial electron transport

In mitochondria, most Coenzyme Q is free in the lipid bilayer; the question as to whether tightly bound, non-exchangeable Coenzyme Q molecules exist in mitochondrial complexes is still an open question.We review the mechanism of inter-complex electron transfer mediated by ubiquinone and discuss the kinetic consequences of the supramolecular organization of the respiratory complexes (randomly dispersed vs. super-complexes) in terms of Coenzyme Q pool behavior vs. metabolic channeling, respectively, both in physiological and in some pathological conditions. As an example of intra-complex electron transfer, we discuss in particular Complex I, a topic that is still under active investigation.     

The function of ubiquinone in Escherichia coli

1. The function of ubiquinone in Escherichia coli was studied by using whole cells and membrane preparations of normal E. coli and of a mutant lacking ubiquinone. 2. The mutant lacking ubiquinone, strain AN59 (Ubi(-)), when grown under aerobic conditions, gave an anaerobic type of growth yield and produced large quantities of lactic acid, indicating that ubiquinone plays a vital role in electron transport. 3. NADH and lactate oxidase activities in membranes from strain AN59 (Ubi(-)) were greatly impaired and activity was restored by the addition of ubiquinone (Q-1). 4. Comparison of the percentage reduction of flavin, cytochrome b(1) and cytochrome a(2) in the aerobic steady state in membranes from the normal strain (AN62) and strain AN59 (Ubi(-)) and the effect of respiratory inhibitors on these percentages in membranes from strain AN62 suggest that ubiquinone functions at more than one site in the electron-transport chain. 5. Membranes from strain AN62, in the absence of substrate, showed an electron-spin-resonance signal attributed to ubisemiquinone. The amount of reduced ubiquinone (50%) found after rapid solvent extraction is consistent with the existence of ubiquinone in membranes as a stabilized ubisemiquinone. 6. The effects of piericidin A on membranes from strain AN62 suggest that this inhibitor acts at the ubiquinone sites: thus inhibition of electron transport is reversed by ubiquinone (Q-1); the aerobic steady-state oxidation-reduction levels of flavins and cytochrome b(1) in the presence of the inhibitor are raised to values approximating those found in the membranes of strain AN59 (Ubi(-)); the inhibitor rapidly eliminates the electron-spin-resonance signal attributed to ubisemiquinone and allows slow oxidation of endogenous ubiquinol in the absence of substrate and prevents reduction of ubiquinone in the presence of substrate. It is concluded that piericidin A separates ubiquinone from the remainder of the electron-transport chain. 7. A scheme is proposed in which ubisemiquinone, complexed to an electron carrier, functions in at least two positions in the electron-transport sequence.     

Rate constants in two dimensions of electron transfer between pyruvate oxidase, a membrane enzyme, and ubiquinone (coenzyme Q8), its water-insoluble electron carrier

The functionality of the membrane-bound, ubiquinone-dependent pyruvate oxidase from the respiratory chain of Escherichia coli was reconstituted with a supported lipidic structure. The artificial structure was especially designed to allow the electrochemical control of the quinone pool through the lateral mobility of the ubiquinone (Q(8)) molecules. The kinetic coupling of the enzyme bound to the lipid structure with the quinone pool was ensured by the regeneration of the oxidized form of ubiquinone at the electrochemical interface. Such an experimental approach enabled us to carry out an unprecedented determination of the kinetic parameters controlling the reaction between the enzyme bound and the electron carrier under conditions taking rigorously into account the fact that the freedom of motion is restricted to two dimensions. The kinetic constants we found show that the activated enzyme can be efficiently regulated by the oxidation level of the quinone pool in natural membranes.     

On the sidedness of the ubiquinone redox cycle. Kinetic studies in mitochondrial membranes

The inhibition of NADH oxidation but not of succinate oxidation by the low ubiquinone homologs UQ-2 and UQ-3 is not due to a lower rate of reduction of ubiquinone by NADH dehydrogenase: experiments in submitochondrial particles and in pentane-extracted mitochondria show that UQ-3 is reduced at similar rates using either NADH or succinate as substrates. The fact that reduced UQ-3 cannot be reoxidized when reduced by NADH but can be reoxidized when reduced by succinate may be explained by a compartmentation of ubiquinone.Using reduced ubiquinones as substrates of ubiquinol oxidase activity in intact mitochondria and in submitochondrial particles we found that ubiquinol-3 is oxidized at higher rates in submitochondrial particles than in mitochondria. The initial rates of ubiquinol oxidation increased with increasing lengths of isoprenoid side chains in mitochondria, but decreased in submitochondrial particles. These findings suggest that the site of oxidation of reduced ubiquinone is on the matrix side of the membrane; reduced ubiquinones may reach their oxidation site in mitochondria only crossing the lipid bilayer: the rate of diffusion of ubiquinol-3 is presumably lower than that of ubiquinol-7 due to the differences in hydrophobicity of the two quinones.     

Induction of non-bilayer lipid structures by functional signal peptides.

Using 31P NMR and freeze-fracture electron microscopy we investigated the effect of several synthetic signal peptides on lipid structure in model membranes mimicking the lipid composition of the Escherichia coli inner membrane. It is demonstrated that the signal peptide of the E. coli outer membrane protein PhoE, as well as that of the M13 phage coat protein, strongly promote the formation of non-bilayer lipid structures. This effect appears to be correlated to in vivo translocation efficiency, since a less functional analogue of the PhoE signal peptide was found to be less active in destabilizing the bilayer. It is proposed that signal sequences can induce local changes in lipid structure that are involved in protein translocation across the membrane.