Is ubiquinone diffusion rate-limiting for electron transfer?

Alternative respiratory oxidases to study the animal electron transport chain

Oxidative phosphorylation is a common process to most organisms in which the main function is to generate an electrochemical gradient across the inner mitochondrial membrane (IMM) and to make energy available to the cell. However, plants, many fungi and some animals maintain non-energy conserving oxidases which serve as a bypass to coupled respiration. Namely, the alternative NADH:ubiquinone oxidoreductase NDI1, present in the complex I (CI)-lacking Saccharomyces cerevisiae, and the alternative oxidase, ubiquinol:oxygen oxidoreductase AOX, present in many organisms across different kingdoms. In the last few years, these alternative oxidases have been used to dissect previously indivisible processes in bioenergetics and have helped to discover, understand, and corroborate important processes in mitochondria. Here, we review how the use of alternative oxidases have contributed to the knowledge in CI stability, bioenergetics, redox biology, and the implications of their use in current and future research.

Functional segmentation of CoQ and cyt c pools by respiratory complex superassembly

Electron transfer between respiratory complexes is an essential step for the efficiency of the mitochondrial oxidative phosphorylation. Until recently, it was stablished that ubiquinone and cytochrome c formed homogenous single pools in the inner mitochondrial membrane which were not influenced by the presence of respiratory supercomplexes. However, this idea was challenged by the fact that bottlenecks in electron transfer appeared after disruption of supercomplexes into their individual complexes. The postulation of the plasticity model embraced all these observations and concluded that complexes and supercomplexes co-exist and are dedicated to a spectrum of metabolic requirements. Here, we review the involvement of superassembly in complex I stability, the role of supercomplexes in ROS production and the segmentation of the CoQ and cyt c pools, together with their involvement in signaling and disease. Taking apparently conflicting literature we have built up a comprehensive model for the segmentation of CoQ and cyt c mediated by supercomplexes, discuss the current limitations and provide a prospect of the current knowledge in the field.

Induced conformational change on ferrocenyl-terminated alkyls and their application as transducers for label-free immunosensing of Alzheimer's disease biomarker

ApoE Alzheimer's disease biomarker can be sensitively detected by a label-free platform using flexible ferrocene-terminated alkyl chains. The immunorecognition triggers conformational changes, which improve the rate constants of electron-transfer.

New developments on the functions of coenzyme Q in mitochondria

The notion of a mobile pool of coenzyme Q (CoQ) in the lipid bilayer has changed with the discovery of respiratory supramolecular units, in particular the supercomplex comprising complexes I and III; in this model, the electron transfer is thought to be mediated by tunneling or microdiffusion, with a clear kinetic advantage on the transfer based on random collisions. The CoQ pool, however, has a fundamental function in establishing a dissociation equilibrium with bound quinone, besides being required for electron transfer from other dehydrogenases to complex III. The mechanism of CoQ reduction by complex I is analyzed regarding recent developments on the crystallographic structure of the enzyme, also in relation to the capacity of complex I to generate superoxide. Although the mechanism of the Q-cycle is well established for complex III, involvement of CoQ in proton translocation by complex I is still debated. Some additional roles of CoQ are also examined, such as the antioxidant effect of its reduced form and the capacity to bind the permeability transition pore and the mitochondrial uncoupling proteins. Finally, a working hypothesis is advanced on the establishment of a vicious circle of oxidative stress and supercomplex disorganization in pathological states, as in neurodegeneration and cancer.

Structural and functional organization of the mitochondrial respiratory chain: a dynamic super-assembly

The structural organization of the mitochondrial oxidative phosphorylation (OXPHOS) system has received large attention in the past and most investigations led to the conclusion that the respiratory enzymatic complexes are randomly dispersed in the lipid bilayer of the inner membrane and functionally connected by fast diffusion of smaller redox components, Coenzyme Q and cytochrome c. More recent investigations by native gel electrophoresis, however, have shown the existence of supramolecular associations of the respiratory complexes, confirmed by electron microscopy analysis and single particle image processing. Flux control analysis has demonstrated that Complexes I and III in mammalian mitochondria and Complexes I, III, and IV in plant mitochondria kinetically behave as single units with control coefficients approaching unity for each single component, suggesting the existence of substrate channelling within the supercomplexes. The reasons why the presence of substrate channelling for Coenzyme Q and cytochrome c was overlooked in the past are analytically discussed. The review also discusses the forces and the conditions responsible for the formation of the supramolecular units. The function of the supercomplexes appears not to be restricted to kinetic advantages in electron transfer: we discuss evidence on their role in the stability and assembly of the individual complexes and in preventing excess oxygen radical formation. Finally, there is increasing evidence that disruption of the supercomplex organization leads to functional derangements responsible for pathological changes.

Impedance spectroscopy of bacterial membranes: coenzyme-Q diffusion in a finite diffusion layer

The inner membrane of Escherichia coli, overexpressing an ubiquinol oxidase, cytochrome bo3 (cbo3), was "tethered" in a planar configuration to a gold electrode. Electron transfer to cbo3 was achieved via native ubiquinol-8 or added ubiquinol-10, and impedance spectroscopy was used to characterize the diffusion properties of the ubiquinol/ubiquinone in the tethered membrane system. Spectra were obtained at varying direct current (DC) potentials covering the potential window in which the voltammetric catalytic wave of cbo3 is visible. These spectra were compared to those obtained after addition of a potent inhibitor of cbo3, cyanide, and the difference in impedance was analyzed using a derived equivalent circuit, which is similar to that of open finite-length diffusion (OFLD) or the finite Warburg circuit, but with the boundary conditions modified to account for the fact that ubiquinol reoxidation is limited by enzyme activity. Analysis of the impedance spectra of the tethered membrane system gave kinetic parameters that are consistent with values obtained using cyclic voltammetry. Importantly, the diffusion rate of ubiquinone (10(-13)-10(-12) cm2/s) was found to be orders of magnitude lower than accepted values for lateral diffusion (10(-8)-10(-7) cm2/ s). It is hypothesized that this result represent perpendicular diffusion of quinone across the membrane, corresponding to a "flip" time between 0.05 and 1 s.

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.

Kinetics of integrated electron transfer in the mitochondrial respiratory chain: random collisions vs. solid state electron channeling

Recent evidence, mainly based on native electrophoresis, has suggested that the mitochondrial respiratory chain is organized in the form of supercomplexes, due to the aggregation of the main respiratory chain enzymatic complexes. This evidence strongly contrasts the previously accepted model, the Random Diffusion Model, largely based on kinetic studies, stating that the complexes are randomly distributed in the lipid bilayer of the inner membrane and functionally connected by lateral diffusion of small redox molecules, i.e., coenzyme Q and cytochrome c. This review critically examines the experimental evidence, both structural and functional, pertaining to the two models and attempts to provide an updated view of the organization of the respiratory chain and of its kinetic consequences. The conclusion that structural respiratory assemblies exist is overwhelming, whereas the expected functional consequence of substrate channeling between the assembled enzymes is controversial. Examination of the available evidence suggests that, although the supercomplexes are structurally stable, their kinetic competence in substrate channeling is more labile and may depend on the system under investigation and the assay conditions.

Kinetics of bimolecular reactions in model bilayers and biological membranes. A critical review

The quantitative study of the probability of molecular encounters giving rise to a reaction in membranes is a challenging discipline. Model systems, model in the sense that they use model bilayers and model reactants, have been widely used for this purpose, but the methodologies employed for the analysis of the results obtained in experiments, and for experimental design, are so disparate that a concerned experimentalist has difficulty in deciding about the value of each approach. This review intends to examine the several approaches that can be found in the literature showing, when feasible, the weakness, strengths and limits of application of each of them. There is not, so far, a full experimental validation of the most promising theories for the analysis of reactions in two dimensions, what leaves open a large field for new research. The major challenge resides in the time range in which the processes take place, but the possibilities of the existing techniques for these studies are far from exhausted. We review also the attempts of several authors to quantitatively analyze the kinetics of reactions in biological membranes. Especially in this field, the recently developed microspectroscopies enclose a still unexplored potential.

Mitochondrial Complex I: structural and functional aspects

This review examines two aspects of the structure and function of mitochondrial Complex I (NADH Coenzyme Q oxidoreductase) that have become matter of recent debate. The supramolecular organization of Complex I and its structural relation with the remainder of the respiratory chain are uncertain. Although the random diffusion model [C.R. Hackenbrock, B. Chazotte, S.S. Gupte, The random collision model and a critical assessment of diffusion and collision in mitochondrial electron transport, J. Bioenerg. Biomembranes 18 (1986) 331-368] has been widely accepted, recent evidence suggests the presence of supramolecular aggregates. In particular, evidence for a Complex I-Complex III supercomplex stems from both structural and kinetic studies. Electron transfer in the supercomplex may occur by electron channelling through bound Coenzyme Q in equilibrium with the pool in the membrane lipids. The amount and nature of the lipids modify the aggregation state and there is evidence that lipid peroxidation induces supercomplex disaggregation. Another important aspect in Complex I is its capacity to reduce oxygen with formation of superoxide anion. The site of escape of the single electron is debated and either FMN, iron-sulphur clusters, and ubisemiquinone have been suggested. The finding in our laboratory that two classes of hydrophobic inhibitors have opposite effects on superoxide production favours an iron-sulphur cluster (presumably N2) is the direct oxygen reductant. The implications in human pathology of better knowledge on these aspects of Complex I structure and function are briefly discussed.

Supercomplex organization of the mitochondrial respiratory chain and the role of the Coenzyme Q pool: pathophysiological implications

In this review we examine early and recent evidence for an aggregated organization of the mitochondrial respiratory chain. Blue Native Electrophoresis suggests that in several types of mitochondria Complexes I, III and IV are aggregated as fixed supramolecular units having stoichiometric proportions of each individual complex. Kinetic evidence by flux control analysis agrees with this view, however the presence of Complex IV in bovine mitochondria cannot be demonstrated, presumably due to high levels of free Complex. Since most Coenzyme Q appears to be largely free in the lipid bilayer of the inner membrane, binding of Coenzyme Q molecules to the Complex I-III aggregate is forced by its dissociation equilibrium; furthermore free Coenzyme Q is required for succinate-supported respiration and reverse electron transfer. The advantage of the supercomplex organization is in a more efficient electron transfer by channelling of the redox intermediates and in the requirement of a supramolecular structure for the correct assembly of the individual complexes. Preliminary evidence suggests that dilution of the membrane proteins with extra phospholipids and lipid peroxidation may disrupt the supercomplex organization. This finding has pathophysiological implications, in view of the role of oxidative stress in the pathogenesis of many diseases.

The mitochondrial respiratory chain is partially organized in a supercomplex assembly: kinetic evidence using flux control analysis

The model of the respiratory chain in which the enzyme complexes are independently embedded in the lipid bilayer of the inner mitochondrial membrane and connected by randomly diffusing coenzyme Q and cytochrome c is mostly favored. However, multicomplex units can be isolated from mammalian mitochondria, suggesting a model based on direct electron channeling between complexes. Kinetic testing using metabolic flux control analysis can discriminate between the two models: the former model implies that each enzyme may be rate-controlling to a different extent, whereas in the latter, the whole metabolic pathway would behave as a single supercomplex and inhibition of any one of its components would elicit the same flux control. In particular, in the absence of other components of the oxidative phosphorylation apparatus (i.e. ATP synthase, membrane potential, carriers), the existence of a supercomplex would elicit a flux control coefficient near unity for each respiratory complex, and the sum of all coefficients would be well above unity. Using bovine heart mitochondria and submitochondrial particles devoid of substrate permeability barriers, we investigated the flux control coefficients of the complexes involved in aerobic NADH oxidation (I, III, IV) and in succinate oxidation (II, III, IV). Both Complexes I and III were found to be highly rate-controlling over NADH oxidation, a strong kinetic evidence suggesting the existence of functionally relevant association between the two complexes, whereas Complex IV appears randomly distributed. Moreover, we show that Complex II is fully rate-limiting for succinate oxidation, clearly indicating the absence of substrate channeling toward Complexes III and IV.

A critical appraisal of the mitochondrial coenzyme Q pool

The function of the coenzyme Q (CoQ) pool in the inner mitochondrial membrane is reviewed in view of recent findings suggesting a supramolecular organization of the mitochondrial respiratory complexes. In spite of the structural evidence for preferential aggregations of the inner membrane components, most kinetic evidence is in favor of a dispersed organization based on random collisions of the small connecting redox components, in particular CoQ, with the individual complexes. The shape of the CoQ molecule in the pool, suggested to be a folded one, is in agreement with its very rapid lateral diffusion mobility in the membrane midplane. Since the structural evidence in favor of specific supercomplexes is rather strong, it cannot be excluded that electron transfer may follow either pool behavior or preferential channeling depending on the physiological conditions. Another function ascribed to the CoQ pool is the antioxidant action of the reduced CoQ molecules; although it cannot be excluded that protein-bound ubisemiquinones may be a source of oxygen radicals, particularly at the level of complex III, the available evidence suggests that the mitochondrial pool only behaves as an antioxidant under physiological conditions.

Localization and mobility of coenzyme Q in lipid bilayers and membranes

We have studied the mobility of coenzyme Q (CoQ) in lipid bilayers and mitochondrial membranes in relation to the control of electron transfer activities. A molecular dynamics computer simulation in the vacuum yielded a folded structure for CoQ10, with a length of only 21 A. Using this information we were able to calculate diffusion coefficients in the range of 10(-6) cm2/s in good agreement with those found experimentally by fluorescence quenching of pyrene derivatives. To investigate if CoQ diffusion may represent the rate-limiting step of electron transfer, we reconstituted complexes I and III and assayed the resulting NADH-cytochrome c reductase activity in presence of different CoQ10 levels and at different distances between complexes; the experimental turnovers were higher than the collision frequencies calculated using diffusion coefficients of 10(-9) cm2/s but compatible with values found by us by fluorescence quenching. Since the experimental turnovers are independent of the distance between complexes, we conclude that CoQ diffusion is not rate-limiting for electron transfer.

Quinone specificity of complex I

This review considers the interaction of Complex I with different redox acceptors, mainly homologs and analogs of the physiological acceptor, hydrophobic Coenzyme Q. After examining the physical properties of the different quinones and their efficacy in restoring mitochondrial respiration, a survey ensues of the advantages and drawbacks of the quinones commonly used in Complex I activity determination and of their kinetic properties. The available evidence is then displayed on structure-activity relationships of various quinone compounds in terms of electron transfer activity and proton translocation, and the present knowledge is discussed in terms of the nature of multiple quinone-binding sites in the Complex.

Steady-state kinetics of reduction of coenzyme Q analogs by glycerol-3-phosphate dehydrogenase in brown adipose tissue mitochondria

We have undertaken a study of the role of coenzyme Q (CoQ) in glycerol-3-phosphate oxidation in mitochondrial membranes from hamster brown adipose tissue, using either quinone homologs, as CoQ1 and CoQ2, or the analogs duroquinone and decylubiquinone as artificial electron acceptors. We have found that the most suitable electron acceptor for glycerol-3-phosphate:CoQ reductase activity in situ in the mitochondrial membrane is the homolog CoQ1 yielding the highest rate of enzyme activity (225 +/- 41 nmol x min(-1) x mg(-1) protein). With all acceptors tested the quinone reduction rates were completely insensitive to Complex III inhibitors, indicating that all acceptors were easily accessible to the quinone-binding site of the dehydrogenase preferentially with respect to the endogenous CoQ pool, in such a way that Complex III was kept in the oxidized state. We have also experimentally investigated the saturation kinetics of endogenous CoQ (1.35 nmol/mg protein of a mixture of 70% CoQ9 and 30% CoQ10) by stepwise pentane extraction of brown adipose tissue mitochondria and found a K(m) of the integrated activity of glycerol-3-phosphate cytochrome c reductase for endogenous CoQ of 0.22 nmol/mg protein, indicating that glycerol-3-phosphate-supported respiration is over 80% of V(max) with respect to the CoQ pool. A similar K(m) of 0.19 nmol CoQ/mg protein was found in glycerol-3-phosphate cytochrome c reductase in cockroach flight muscle mitochondria.

Major changes in complex I activity in mitochondria from aged rats may not be detected by direct assay of NADH:coenzyme Q reductase

We have investigated the respiratory activities and the concentrations of respiratory chain components of mitochondria isolated from the livers and hearts of two groups of rats aged 6 and 24 months respectively. In comparison with the adult controls (6 months), in aged rats there was a decline in total aerobic NADH oxidation in both tissues; only minor (non-significant) changes, however, were found in NADH:coenzyme Q reductase and cytochrome oxidase activities, and there was no change in ubiquinol-cytochrome c reductase activity. The coenzyme Q levels were slightly decreased in mitochondria from both organs of aged rats. The lowered NADH oxidase activity is not due to the slight decrease observed in the coenzyme Q levels, but is the result of decreased Complex I activity. Since the assay of NADH:coenzyme Q reductase requires quinone analogues, none of which can evoke its maximal turnover [Estornell, Fato, Pallotti and Lenaz (1993) FEBS Lett. 332, 127-131], its activity has been calculated indirectly by taking advantage of the relationship that exists between NADH oxidation and ubiquinol oxidation through the coenzyme Q pool. The results, expressed in this way, show a drastic loss of activity of Complex I in both the heart and the liver of aged animals in comparison with adult controls.

Saturation kinetics of coenzyme Q in NADH and succinate oxidation in beef heart mitochondria

The saturation kinetics of NADH and succinate oxidation for Coenzyme Q (CoQ) has been re-investigated in pentane-extracted lyophilized beef heart mitochondria reconstituted with exogenous CoQ10. The apparent 'Km' for CoQ10 was one order of magnitude lower in succinate cytochrome c reductase than in NADH cytochrome c reductase. The Km value in NADH oxidation approaches the natural CoQ content of beef heart mitochondria, whereas that in succinate oxidation is close to the content of respiratory chain enzymes.

Control of respiration and ATP synthesis in mammalian mitochondria and cells

We have seen that there is no simple answer to the question 'what controls respiration?' The answer varies with (a) the size of the system examined (mitochondria, cell or organ), (b) the conditions (rate of ATP use, level of hormonal stimulation), and (c) the particular organ examined. Of the various theories of control of respiration outlined in the introduction the ideas of Chance & Williams (1955, 1956) give the basic mechanism of how respiration is regulated. Increased ATP usage can cause increased respiration and ATP synthesis by mass action in all the main tissues. Superimposed on this basic mechanism is calcium control of matrix dehydrogenases (at least in heart and liver), and possibly also of the respiratory chain (at least in liver) and ATP synthase (at least in heart). In many tissues calcium also stimulates ATP usage directly; thus calcium may stimulate energy metabolism at (at least) four possible sites, the importance of each regulation varying with tissue. Regulation of multiple sites may occur (from a teleological point of view) because: (a) energy metabolism is branched and thus proportionate regulation of branches is required in order to maintain constant fluxes to branches (e.g. to proton leak or different ATP uses); and/or (b) control over fluxes is shared by a number of reactions, so that large increases in flux requires stimulation at multiple sites because each site has relatively little control. Control may be distributed throughout energy metabolism, possibly due to the necessity of minimizing cell protein levels (see Brown, 1991). The idea that energy metabolism is regulated by energy charge (as proposed by Atkinson, 1968, 1977) is misleading in mammals. Neither mitochondrial ATP synthesis nor cellular ATP usage is a unique function of energy charge as AMP is not a significant regulator (see for example Erecinska et al., 1977). The near-equilibrium hypothesis of Klingenberg (1961) and Erecinska & Wilson (1982) is partially correct in that oxidative phosphorylation is often close to equilibrium (apart from cytochrome oxidase) and as a consequence respiration and ATP synthesis are mainly regulated by (a) the phosphorylation potential, and (b) the NADH/NAD+ ratio. However, oxidative phosphorylation is not always close to equilibrium, at least in isolated mitochondria, and relative proximity to equilibrium does not prevent the respiratory chain, the proton leak, the ATP synthase and ANC having significant control over the fluxes. Thus in some conditions respiration rate correlates better with [ADP] than with phosphorylation potential, and may be relatively insensitive to mitochondrial NADH/NAD+ ratio.(ABSTRACT TRUNCATED AT 400 WORDS)

Coenzyme Q-pool function in glycerol-3-phosphate oxidation in hamster brown adipose tissue mitochondria

We have investigated the role of the Coenzyme Q pool in glycerol-3-phosphate oxidation in hamster brown adipose tissue mitochondria. Antimycin A and myxothiazol inhibit glycerol-3-phosphate cytochrome c oxidoreductase in a sigmoidal fashion, indicating that CoQ behaves as a homogeneous pool between glycerol-3-phosphate dehydrogenase and complex III. The inhibition of ubiquinol cytochrome c reductase is linear at low concentrations of both inhibitors, indicating that sigmoidicity of antimycin A and myxothiazol inhibition is not a direct property of antimycin A and myxothiazol binding. Glycerol-3-phosphate cytochrome c oxidoreductase is strongly stimulated by added CoQ3, indicating that endogenous CoQ is not saturating. Application of the pool equation for nonsaturating ubiquinone allows calculation of the Km for endogenous CoQ of glycerol-3-phosphate dehydrogenase of 3.14 mM. The results of this investigations reveal that CoQ behaves as a homogeneous pool between glycerol-3-phosphate dehydrogenase and complex III in brown adipose tissue mitochondria; moreover, its concentration is far below saturation for maximal electron transfer activity in comparison with other branches of the respiratory chain connected with the CoQ pool. HPLC analysis revealed a lower amount of CoQ in brown adipose mitochondria (0.752 nmol/mg protein) in comparison with mitochondria from other tissues and the presence of both CoQ9 and CoQ10.

The physical state of ubiquinone-10, in pure form and incorporated into phospholipid bilayers. A Fourier-transform infrared spectroscopic study

Long-chain quinones are essential components of both bacterial and eukaryotic respiratory chains, and some of the main unsolved questions on energy transduction in membranes are complicated by the lack of consistent information on the physical state of the quinones in membrane bilayers. We have recorded, at various temperatures and under different conditions, the infrared spectra of ubiquinone-10 (the main species in mitochondria) and several analogues. The C = O stretching vibration band located at 1663-1670 cm-1 has been identified as the most sensitive one to phase and environmental changes. Three distinct phases have been characterized in which pure ubiquinone-10 may exist: crystalline (LC1), isotropic liquid (IL) and liquid crystalline (Lc). The only allowed thermotropic transitions are LC1----IL, IL----Lc and Lc----LC1. Our investigations with pure quinones provide a simpler and more detailed description of their phase changes than any of the previous studies and shed light on their behaviour in membranes. When incorporated into phospholipid bilayers, ubiquinone-10 appears to be removed from the aqueous environment and is found to exist, in the 4-70 degrees C range, in an isotropic liquid phase, in the form of small aggregates.

Effects of coenzyme Q10 supplementation on exercise performance, VO2max, and lipid peroxidation in trained cyclists

The effects of dietary supplementation with Coenzyme Q10 (CoQ10), a reputed performance enhancer and antioxidant, on physiological and biochemical parameters were examined. Ten male bicycle racers performed graded cycle ergometry both before and after being given 100 mg per day CoQ10 or placebo for 8 weeks. Analysis of variance showed a significant difference between groups for postsupplementation serum CoQ10. Although both groups demonstrated training related improvements in all physiological parameters over the course of the study, there were no significant differences between the two groups (p > .05). Both groups showed a 21% increase in serum MDA (an index of lipid peroxidation) after the presupplementation exercise test. After 8 weeks this increase was only 5%, and again was identical for both groups. Supplementation with CoQ10 has no measurable effect on cycling performance, VO2max, submaximal physiological parameters, or lipid peroxidation. However, chronic intense training seems to result in marked attenuation of exercise-induced lipid peroxidation.

The mobility of a fluorescent ubiquinone in model lipid membranes. Relevance to mitochondrial electron transport

The diffusion and location of a functional, fluorescent ubiquinone molecule, NBDHA-Q, were determined as a function of temperature using microscopic observation, fluorescence recovery after photobleaching and fluorescence spectroscopy in protein-free, pure-lipid dimyristoylphosphatidylcholine and dimyristoylphosphatidylcholine/cholesterol multibilayers. The data reveal that in a liquid-crystalline membrane (1) ubiquinone is highly mobile, (2) ubiquinone uniformly diffuses laterally with the same diffusion coefficient (3.10(-8) cm2/s at 25 degrees C) as the phospholipids in which it resides, (3) the diffusion coefficients of ubiquinone and phospholipid both decrease at the exothermic phase transition of the phospholipid, (4) cholesterol affects the diffusion coefficients of ubiquinone and phospholipids to the same degree, (5) cholesterol induces a lateral phase separation progressively excluding ubiquinone from cholesterol-containing domains. These data suggest that ubiquinone does not reside at the membrane surface or in the mid-plane for any appreciable length of time. Rather, the data indicate that ubiquinone is highly mobile laterally and transversely, spending the majority of its time in the acyl chain region of the membrane, where its lateral and transverse diffusion is limited by the lateral diffusion and the transverse microviscosity gradient of the phospholipids and where its lateral location can be affected by the presence of cholesterol. In addition, based upon a comparison of the diffusion coefficients for ubiquinone, phospholipids and mitochondrial redox complexes, we hypothesize that no significant portion of the ubiquinone pool remains bound to redox complexes for any significant length of time relative to that for electron transport as resolvable by fluorescence recovery after photobleaching.

Composition of the inner mitochondrial membrane of porcine corpus luteum

An inner mitochondrial membrane fraction was prepared from porcine corpus luteum. The concentrations of the respiratory cytochromes, cytochrome P-450scc, cholesterol, ubiquinone, cardiolipin and the total phospholipids were measured. The fatty acid compositions of cardiolipin and the total phospholipid fraction were determined. Comparative data from porcine heart and liver were obtained using the same methods. Differences in both the concentration and the fatty acid composition of the phospholipids were observed between the tissues. It appeared that the phospholipid bilayer was expanded relative to haem a in luteal mitochondria. It is proposed that in the ovary this expansion may be necessary to accommodate cytochrome P-450scc and its substrate, cholesterol.

Lateral diffusion in an archipelago. Distance dependence of the diffusion coefficient

An understanding of the distance dependence of the lateral diffusion coefficient is useful in comparing the results of diffusion measurements made over different length scales, and in analyzing the kinetics of mobile redox carriers in organelles. A distance-dependent, concentration-dependent diffusion coefficient is defined, and it is evaluated by Monte Carlo calculations of a random walk by mobile point tracers in the presence of immobile obstacles on a triangular lattice, representing the diffusion of a lipid or a small protein in the presence of immobile membrane proteins. This work confirms and extends the milling crowd model of Eisinger, J., J. Flores, and W. P. Petersen (1986. Biophys J. 49:987-1001). Similar calculations for diffusion of mobile particles interacting by a hard-core repulsion yield the distance dependence of the self-diffusion coefficient. An expression for the range of short-range diffusion is obtained, and the distance scales for various diffusion measurements are summarized.

Diffusional effects in the steady state kinetics of ubiquinol cytochrome c reductase in bovine heart submitochondrial particles

The steady-state kinetics of ubiquinol cytochrome c reductase was investigated in submitochondrial particles using ubiquinol-1 as electron donor in media of increasing viscosities obtained by water-polyethylene glycol mixtures. The minimum association rate constant, kmin = kcat/km, for cytochrome c was strongly viscosity dependent, whereas kmin for ubiquinol-1 was only weakly affected by viscosity. It is concluded that the interaction of cytochrome c with the membranous reductase is largely under diffusion control, whereas the oxidation of ubiquinol by the enzyme is not significantly controlled by diffusion in either the aqueous medium or the membrane. The results are compatible with the presence of a diffusion limited step in cytochrome c but not in ubiquinone in mitochondrial electron transfer.

Quenching of the intrinsic tryptophan fluorescence of mitochondrial ubiquinol--cytochrome-c reductase by the binding of ubiquinone

1. The quenching by ubiquinone (Q) of the intrinsic fluorescence of tryptophan residues within ubiquinol--cytochrome-c reductase (complex III) has been exploited to provide direct information on the interaction between these two components of the mitochondrial respiratory chain. 2. The fluorescence quenching data have been corrected for inner filter effects and interpreted using the classical Stern-Volmer and modified Stern-Volmer plots. The latter of these plots allows computation of both the dissociation constant (Kd) of complex formation between ubiquinone and complex III, and the percentage of fluorophores accessible to quenching. 3. It is found that different Q homologues bind to complex III with different affinities depending upon the length of the isoprenoid chain: 2,3-dimethoxy-5-methyl-6-decyl-1,4-benzoquinone, an analogue of Q2, exhibits the same Kd as Q2. Furthermore, the accessibility of fluorophores to quenching was lower for Q1 than for the other quinones tested. 4. The binding affinity of Q2 to complex III depends upon the redox state of the enzyme. 5. Addition of the complex III inhibitor, antimycin, has very little effect on the binding affinity or on the accessibility of fluorophores to the quencher. 6. Addition of the inhibitor myxothiazol has a similar effect to reducing complex III with ascorbate. 7. Reconstitution of complex III into asolectin lipid vesicles gives similar qualitative results to the enzyme in solution regarding both the redox state and the addition of inhibitors.

Lipid fluidity and membrane protein dynamics

Membrane fluidity plays an important role in cellular functions. Membrane proteins are mobile in the lipid fluid environment; lateral diffusion of membrane proteins is slower than expected by theory, due to both the effect of protein crowding in the membrane and to constraints from the aqueous matrix. A major aspect of diffusion is in macromolecular associations: reduction of dimensionality for membrane diffusion facilitates collisional encounters, as those concerned with receptor-mediated signal transduction and with electron transfer chains. In mitochondrial electron transfer, diffusional control is prevented by the excess of collisional encounters between fast-diffusing ubiquinone and the respiratory complexes. Another aspect of dynamics of membrane proteins is their conformational flexibility. Lipids may induce the optimal conformation for catalytic activity. Breaks in Arrhenius plots of membrane-bound enzymes may be related to lipid fluidity: the break could occur when a limiting viscosity is reached for catalytic activity. Viscosity can affect protein conformational changes by inhibiting thermal fluctuations to the inner core of the protein molecule.

Regulation of electron transfer by the quinone pool

Strong evidence for a random collisional mechanism for ubiquinone-mediated electron transfer is provided by the characteristic kinetic properties of respiratory chains originally explored by Kröger, A., and Klingenberg, M. (1973), Eur. J. Biochem. 34, 313-323. A kinetic model which leads to this so-called "simple Q-pool behavior" has been described and we use this in reviewing evidence that electron transfer is diffusion-controlled as well as diffusion-coupled. We also consider mechanisms by which the kinetics of electron transfer might deviate from simple Q-pool behavior and how these might be implicated in the regulation of electron transport.