Ubiquinone Molecule Research Articles

Biochemical and spectroscopic properties of the four-subunit quinol oxidase (cytochrome ba 3) from Paracoccus denitrificans

The ba 3 quinol oxidase from Paracoccus denitrificans has been purified by a new protocol leading to significantly higher yields than previously reported (Richter et al. (1994) J. Biol. Chem. 269, 23079–23086). In an SDS PAG an additional protein band compared with the previous preparation appears, which can be identified as the major form of subunit II. All protein bands can be assigned to genes of the qox operon by N-terminal sequencing, indicating that the oxidase consists of four subunits. In addition to one heme A, one heme B, and one copper atom, the preparation contains two ubiquinone molecules per enzyme. The oxidase is further characterized by electron paramagnetic resonance (EPR), circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopy.

Energy conservation by bifurcated electron-transfer in the cytochrome- bc1 complex

The overall electron- and proton-pathways within the cytochrome- bc 1 complex are described by a widely accepted mechanism known as the protonmotive Q-cycle. Within this reaction scheme, the unique bifurcation of electron flow into a high potential and a low potential pathway occurring at the ubihydroquinone-oxidation center is the energy conserving reaction. It is this reaction, which results in vectorial proton translocation, as it allows the ‘recycling’ of every second electron across the membrane onto the ubiquinone-reduction center. However, the Q-cycle reaction scheme does not address the detailed chemistry of this central step. Based on a structural model of the ubihydroquinone-oxidation pocket and the assumption that the reaction involves two ubiquinone molecules in a stacked configuration, here I propose a detailed chemical model for the reactions occurring during steady-state catalysis. In this proton-gated charge-transfer mechanism the reaction is controlled by the deprotonation of the substrate ubihydroquinone and not, as proposed earlier, by the formation of a highly unstable semiquinone species.

Reaction of the Escherichia coli quinol oxidase cytochrome bo3 with dioxygen: the role of a bound ubiquinone molecule.

We have studied the kinetics of the oxygen reaction of the fully reduced quinol oxidase, cytochrome bo3, using flow-flash and stopped flow techniques. This enzyme belongs to the heme-copper oxidase family but lacks the CuA center of the cytochrome c oxidases. Depending on the isolation procedure, the kinetics are found to be either nearly monophasic and very different from those of cytochrome c oxidase or multiphasic and quite similar to cytochrome c oxidase. The multiphasic kinetics in cytochrome c oxidase can largely be attributed to the presence Of CuA as the donor of a fourth electron, which rereduces the originally oxidized low-spin heme and completes the reduction of O2 to water. Monophasic kinetics would thus be expected, a priori, for cytochrome bo3 since it lacks the CuA center, and in this case we show that the oxygen reaction is incomplete and ends with the ferryl intermediate. Multiphasic kinetics thus suggest the presence of an extra electron donor (analogous to CuA). We observe such kinetics exclusively with cytochrome bo3 that contains a single equivalent of bound ubiquinone-8, whereas we find no bound ubiquinone in an enzyme exhibiting monophasic kinetics. Reconstitution with ubiquinone-8 converts the reaction kinetics from monophasic to multiphasic. We conclude that a single bound ubiquinone molecule in cytochrome bo3 is capable of fast rereduction of heme b and that the reaction with O2 is quite similar in quinol and cytochrome c oxidases.

Potential Dependence of Transmembrane Electron Transfer across Phospholipid Bilayers Mediated by Ubiquinone 10

Transmembrane electron transfer across phospholipid bilayer membranes (BLMs) mediated by ubiquinone have been investigated. The membranes were prepared by the brush method and patch-clamp micropipet techniques and were used to separate identical aqueous Fe(II)/Fe(III) citrate solution. Electron transfer across the BLMs was observed on applying a potential difference across the system. The current, and thus the transmembrane electron transfer rate, were found to be dependent on the redox potential of the bathing solutions. The overall transfer coefficient was 0.5, and it was concluded that the rate-determining step is the transfer of the semiubiquinone radical anion across the hydrophobic interior of the membrane, instead of charge transfer reactions at the membrane/electrolyte solution boundaries. The results obtained could not be understood in terms of tunneling to the neutral ubiquinone molecule residing in the bilayer.

FTIR Study of Redox Switching of .pi.-Interactions and Reorientation of Adsorbed Ubiquinone-10 on Gold

The reduction of ubiquinone-10 adsorbed onto a gold electrode in borax and sodium hydroxide solutions has been studied by subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS). A reorientation of the adsorbed ubiquinone molecule on reduction could be observed. The SNIFTIRS spectra show that the molecule is anchored to the electrode surface through the isoprenoid groups, with the quinone ring away from the surface. Upon reduction, the aromatic ring formed is adsorbed by π interactions with the metal surface and is aligned flat to the surface slightly lifting the isoprenoid chains

Monomolecular Langmuir-Blodgett Films at Electrodes. Electrochemistry at Single Molecule "Gate Sites"

The Langmuir-Blodgett (L-B) technique is used to coat electrode surfaces with bifunctional monolayer films that are designed to carry out two functions : passivation and gating. We demonstrate that incorporation of ubiquinone (Q 50 ) in otherwise passivating L-B monolayer films allows us to control access to the electrode surface via a controlled number of single molecule gate sites. The electrode passivation is accomplished with mixed octadecanethiol/octadecanol(C 18 SH/C IB OH) monolayers, as described previously (Bilewicz, R. ; Majda, M. Langmuir 1991, 7, 2794). A detailed characterization ofthe passivating properties of these L-B monolayers is provided in view of the Amatore-Saveant-Tessier (A-S-T) theory (Amatore, C. ; Saveant, J.-M. ; Tessier, D. J. Electroanal. Chem. 1983, 147, 39). The incorporation of low levels (10 -12 -10 -17 mol/cm 2 ) of ubiquinone allows electroreduction of probe species such as Ru(NH 3 ) 6 3+ . At Q 50 concentrations below 10 -15 mol/cm 2 , these monolayers behave as a random array of individual, 5-A-radius disk microelectrodes each created by a single ubiquinone molecule. We postulate that a loose conformation of the ubiquinone's isoprenoid chain creates a channel that allows for a direct approach ofRu(NH 3 ) 6 3+ ions to the electrode surface. Electrochemical results are evaluated on the basis ofA-S-T theory using direct comparison of the experimental and calculated cyclic voltammetric responses. Negative deviations in the observed voltammetric response compared with the theory at the Q 50 concentrations higher than 10 -15 mol/cm 2 are explained by postulating aggregation and loss of Q 50 during L-B transfer.

Reconstitution of a functional electron transfer in a biomimetic structure, including an electrode interface, phospholipid and ubiquinone molecules, and a membrane enzyme.

Annals of the New York Academy of SciencesVolume 750, Issue 1 p. 112-115 Reconstitution of a Functional Electron Transfer in a Biomimetic Structure, Including an Electrode Interface, Phospholipid and Ubiquinone Molecules, and a Membrane Enzyme ELISABETH TORCHUT, ELISABETH TORCHUT Laboratoire de Technologie Enzymatique URA 1442 du CNRS Université de Compiègne BP 649, 60206 Compiègne, FranceSearch for more papers by this authorCHRISTIAN BOURDILLON, CHRISTIAN BOURDILLON Laboratoire de Technologie Enzymatique URA 1442 du CNRS Université de Compiègne BP 649, 60206 Compiègne, FranceSearch for more papers by this authorJEAN-MARC LAVAL, JEAN-MARC LAVAL Laboratoire de Technologie Enzymatique URA 1442 du CNRS Université de Compiègne BP 649, 60206 Compiègne, FranceSearch for more papers by this author ELISABETH TORCHUT, ELISABETH TORCHUT Laboratoire de Technologie Enzymatique URA 1442 du CNRS Université de Compiègne BP 649, 60206 Compiègne, FranceSearch for more papers by this authorCHRISTIAN BOURDILLON, CHRISTIAN BOURDILLON Laboratoire de Technologie Enzymatique URA 1442 du CNRS Université de Compiègne BP 649, 60206 Compiègne, FranceSearch for more papers by this authorJEAN-MARC LAVAL, JEAN-MARC LAVAL Laboratoire de Technologie Enzymatique URA 1442 du CNRS Université de Compiègne BP 649, 60206 Compiègne, FranceSearch for more papers by this author First published: March 1995 https://doi.org/10.1111/j.1749-6632.1995.tb19937.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Volume750, Issue1Enzyme Engineering XIIMarch 1995Pages 112-115 RelatedInformation

Evidence that serine L223 is involved in the proton transfer pathway to QB in the photosynthetic reaction center of Rhodopseudomonas viridis

In the reaction center of purple photosynthetic bacteria, the reducing equivalents produced by primary charge separation are exported via an ubiquinone molecule working as a two-electron shuttle. This loosely-bound quinone, called QB, accepts in successive flashes two electrons from the tightly bound primary quinone acceptor QA, along with two protons from the external medium. The surrounding protein plays an important role in stabilizing the semiquinone anion and in providing a pathway for protons from the cytoplasmic phase to QB. Herbicides of the triazine type compete with QB for the binding pocket and their binding is controlled by nearby amino acid residues. We have studied the kinetics of the first and second electron transfer from QA to QB in two herbicide-resistant mutants from Rhodopseudomonas viridis, T1 (ArgL217-->His,Ser L223-->Ala) and MAV5 (Arg L217-->His, Val L220-->Leu), in order to determine whether these residues are involved in proton transfer to the reduced QB. The main effect of the mutant T1 was a drastic (600-fold at pH 7) decrease in the rate of the second electron transfer to QB compared to the wild type. In contrast, the rate of the second electron transfer in the mutant MAV5 was decreased only slightly (10-fold) in the pH range from 7 to 11. We attribute the inhibition of the second electron transfer in the Ser L223-->Ala mutation to an essential role of Ser L223 in the donation of the first proton to the reduced QB.(ABSTRACT TRUNCATED AT 250 WORDS)

On coenzyme Q orientation in membranes: A linear dichroism study of ubiquinones in a model bilayer

A general approach is developed to interpret linear dichroism (LD) spectra of ubiquinones (Qn) in host bilayers. Information is reported in terms of guest-host mutual orientation and localization. The overall orientational anisotropy of guest ubiquinone molecules is described by a basic set of limiting orientation/localization modes. Assignments of the UV transitions of the ubiquinone chromophore were obtained by the liquid crystal-linear dichroism technique and molecular orbital (CNDO/S) calculations. The LD spectra of Qn in the bilayers provided by the lyotropic nematic mesophase exhibited by water solutions of potassium laurate and decanol were interpreted on the basis of the above assignments. The resulting experimental evidence showed a multisite distribution in the host bilayer for the aromatic heads of all the investigated Qn derivatives except Q0. The orientational distribution suggested by the LD spectra fits the solubilization model recently proposed by G. Lenaz [J. Membrane Biol. (1988) 104:193-209] for ubiquinone in lipid membranes. Within this model Qn molecules are located in the midplane and their headgroups oscillate transversally across the membrane. Q0 instead has a single site location, close to the polar bilayer interface. Experimental evidence that the headgroup carbonyls tend to grasp the polar interface of the host bilayer was also obtained. Orientation and location distributions of Qn guest molecules are therefore likely to result from the tendency of their aromatic heads to grasp the polar heads of the host bilayer and from the concurrent tendency of their chains to settle into the hydrocarbon host interior.

Voltammetric trace determination of ubiquinones at mercury electrodes

Voltammetric methods were developed for the determination of ubiquinones on the basis of their adsorption and redox behaviour at mercury electrodes. Linear calibration plots were obtained in the concentration range between 10−4 mol/l and 2.3·10−5 mol/l from differential pulse voltammetric measurements under experimental conditions avoiding significant adsorption of the analyte. A remarkable decrease of the detection limit down to 10−7 mol/l was achieved with the help of adsorptive accumulation of ubiquinone molecules at the mercury electrode followed by differential pulse detection. The resulting non-linear calibration plot was explained with a model, which takes into account both adsorption and diffusion of the analyte.

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.

The effect of the size of the quinone pool on the electrogenic reactions in the ubiquinol-cytochrome c2 oxidoreductase of Rhodobacter capsulatus. Pool behaviour at the quinone reductase site

(1) Electron transfer within the ubiquinol-cytochrome c 2 oxidoreductase ( bc 1) complex produces a charge separation across the membrane which, in some bacterial chromatophores, can be monitored by the electrochromic band shift of carotenoids. The electrogenic steps are believed to coincide with the electron transfer through the cytochrome b chain to the quinone reductase site (Q c) of the complex. (2) The kinetics of these electrogenic reactions have been studied in chromatophores from Rhodobacter capsulatus, activated by a single turnover flash, as a function of the ambient redox potential, and of the size of the ubiquinone pool in isooctane-extracted membranes. (3) In chromatophores containing the native ubiquinone (UQ) pool the rate of charge transfer, monitored by the electrochromic red shift of caretenoids, is progressively increased by lowering the E h below 160 mV. At 100 mV the rate, when normalized to the number of reaction centers turning over, reaches a maximum and declines significantly when the E h is lowered further (down to 40 mV). This rate remains constant for E h values between 40 mV and −40 mV. (4) When the size of the UQ pool is decreased by isoctane extraction, the stimulation in the rate of charge separation takes place only at more negative E h values and the value of the maximum rate is decreased. Also at negative E h values, however, when no pre-oxidized quinone from the pool is present in the membrane, the rate does not decline significantly from its maximum value. (5) The size of the UQ pool affects also the number of charges translocated per flash by the bc 1 complex, since it affects the probability of rapid multiple turnover of the complex, which occurs at E h < 160 mV in unextracted membranes. (6) These data are interpreted as evidence that oxidized UQ from the pool interacts collisionally with the Q c site. At E h < 40 mV, when the pool is totally reduced before the flash, the oxidized UQ molecule produced at every turnover at the Q z site of the bc 1 complex is sufficient for sustaining a rather high reaction rate at the Q c site. The apparent K m estimated for quinone reduction at the Q c site is approx. 1 mM, less than one order of magnitude smaller than that estimated for QH 2 at the Q z site. The rate of transfer of ubiquinone from Q z to Q c required by such a Q cycle mechanism does not appear incompatible with the experimental data.

Solvation properties of ubiquinone-10 in solvents of different polarity.

The solvation properties of ubiquinone-10 and ubiquinol-10 in a wide variety of solvents of polarity varying from alkanes to water are reported. Greatest solubility is observed in solvents of intermediate polarity and particularly where low polarity is combined with a pronounced tendency to interact with the benzoquinone substituent of the ubiquinone molecule. This includes solvents like chloroform and benzene. Ubiquinone-10 is somewhat less polar than ubiquinol-10 as judged by comparative solubilities of the two molecules. Proton-NMR chemical shift measurements and aggregation studies in selected solvents indicate that in ubiquinone-10 in the liquid phase and in solution in hydrocarbons like dodecane the molecules have a preferred association possibly involving stacking of the benzoquinone rings. Surface balance studies indicated that the surface-active character of ubiquinone-10 is relatively weak and only in a comparatively polar and highly structured solvent, formamide, was there evidence of an effect on surface tension of the solvent. The critical micelle concentration in this solvent was estimated to be about 5 microM on the basis of surface tension measurements. Ubiquinone-10 is well known to form virtually insoluble monolayers at the air/water interface. Studies of the partition of ubiquinone-10 in binary mixtures of solvents suggest that the interaction of the benzoquinone ring substituent with structured polar solvents is considerably weaker than the internal cohesion between molecules of the solvent. No evidence on the basis of wide-angle X-ray diffraction measurements was obtained to indicate that solvent molecules were a component of the crystal lattice of ubiquinone-10 that had precipitated from solvent mixtures.

Resonance Raman scattering from Rhodopseudomonas sphaeroides reaction centers absorbed on a silver electrode

The reaction center complex from Rhodopseudomonas sphaeroides carotenoidless mutant R-26 has been well characterized as to its protein, pigment (4 bacteriochlorophylls (BChl) and 2 bacteriopheophytins (BPh)) and quinone content [ 11. The function of 3 or 4 of the pigment molecules as well as the quinones in the reaction center primary photochemistry is also becoming better understood [2,3]. Two of the BChls, the special pair, are believed to function as the primary electron donor on photoexcitation and transfer an electron to a ubiquinone molecule via one of the BPhs, which has been termed the intermediate electron acceptor. In addition, there is evidence that one of the remaining BChls is involved in the initial electron transfer from the special pair to BPh [3]. New spectroscopic methods are needed which can provide additional support to the picosecond optical absorption data, as well as structural information about the bonding interactions of BChl and BPh in the reaction center. finitive method for identifying the proposed transient intermediates in reaction center photochemistry. The vibrational spectrum of the BChl cation radical differs substantially from the neutral species [7,8]. We have undertaken a RR study of isolated Rps. sphaeroides reaction centers spontaneously adsorbed onto a Ag electrode: The advantages of studying the adsorbed complex include increased RR scattering intensity as a result of the surface enhanced effect on Ag [9], quenching of background fluorescence [lo], and the possibility of changing the redox state of the reaction center by varying the electrode potential.

Differential extraction and structural specificity of specialized ubiquinone molecules in secondary electron transfer in chromatophores from Rhodopseudomonas sphaeroides, Ga

In chromatophores from the facultative photosynthetic bacterium, Rhodopseudomonas sphaeroides, Ga, the function of ubiquinone-10 (UQ-10) at two specialized binding sites (Q B and Q Z) has been determined by kinetic criteria. These were the rate of rereduction of flash-oxidized [BChl] 2 + through the back reaction, or the binary pattern of cytochrome b 561 (for the Q b site), and the rapid rate of rereduction of flash-oxidized cytochrome c, or the relative amplitude of the antimycin-sensitive Phase III ( t 1 2 ~ 1.5 ms ) of the carotenoid spectral shift induced by a single turnover flash at E h ~ 100 mV (for the Q Z site). The phenomenon associated with the two binding sites behaved differently on extraction of UQ from lyophilized chromatophores using isooctane. By this selective extraction procedure it has been possible to show that UQ-10 molecules are required at different concentrations in the membrane for specific redox events in secondary electron transfer. The reduction of cytochrome b occurs in particles which no longer show the phenomena associated with Q Z, but still possess a large proportion of Q b, while rapid rereduction of flash-oxidized cytochrome c requires an additional complement of UQ-10 (Q Z). Extracted particles lacking Q Z and a large amount of Q B have been reconstituted with different UQ homologs (UQ-1, UQ-3, and UQ-10). Specific redox events have been studied in reconstituted particles. All UQ homologs act as secondary acceptors from the reaction center; UQ-3 and UQ-10, but not UQ-1, are also able to reconstitute the function of Q Z as electron donor to cytochrome c. Only UQ-10, however, is able to restore normal rates of the overall cyclic electron transfer induced by a train of flashes, and maximal rates of the light-induced ATP synthesis. The results are interpreted in terms of Q-cycle mechanisms in which quinone and quinol at both the Q Z and Q b sites are in rapid equilibrium with the quinone pool.

The polyisoprenoid chain length influences the interaction of ubiquinones with phospholipid bilayers

The interaction between ubiquinone homologues with polyisoprenoid chain lengths varying from 3 to 10 units and dipalmitoylphosphatidylcholine bilayers has been examined by differential scanning calorimetry and wide angle X-ray diffraction analysis. Decreasing the polyisoprenoid chain lengths of ubiquinone in mixed dispersions with phospholipid in mol ratios of about 10 mol% caused a decrease in the gel-liquid crystalline phase transition temperature of the phospholipid and a broadening of the transition. Enthalpy measurements showed that most of the phospholipid (>92%) was involved in the transition endotherm and the formation of a gel phase was also confirmed by the presence of a sharp X-ray reflection at 0.42 nm. These results are consistent with a model in which all of the ubiquinone homologous ultimately undergo a phase separation from phospholipid molecules entering a gel phase on cooling below the phase transition temperature. Reducing the length of the polyisoprenoid chain alters the amphipathic balance of the ubiquinone molecules and is reflected in the tendency of shorter chain ubiquinones to intercalate between the phospholipid molecules upon reheating through the main phase transition.

ATP synthesis coupled to light-dependent non-cyclic electron flow in chromatophores of Rhodopseudomonas capsulata

Light-dependent non-cyclic electron transport in membranes from aerobic dark-grown cells of Rhodopseudomonas capsulata, i.e., light-induced oxygen uptake, is clearly linked to energy transduction. It is proposed that the open chain performing the electron transfer from exogenous donors such as 2,6-dichlorophenolindophenol (DCIP) or N, N, N′, N′-tetramethyl- p-phenylenediamine (TMPD) to oxygen includes cytochrome c 2, reaction centre bacteriochlorophyll, ubiquinone molecules and the alternate oxidase (cytochrome b-260). In contrast, cytochrome b-60 previously suggested to act at the branching point of the dual electron transport system of Rps. capsulata (Zannoni, D., Melandri, B.A. and Baccarini-Melandri, A. (1976) Biochim. Biophys. Acta 449, 386–400) does not appear to be essential for aerobic photophosphorylation. This conclusion is derived mainly from experiments with Rps. capsulata R126 chromatophores deficient in cytochrome b-60 photoreduction and electron transfer through the cytochrome b- c segment of the chain. Indeed, these mutant membranes present normal rates of light-induced oxygen consumption. Moreover, this activity is coupled to Δμ H + formation which in turn drives ATP synthesis.

Photoelectric currents across planar bilayer membranes containing bacterial reaction centers. Response under conditions of single electron turnover

Light-induced electric current and potential responses have been measured across planar phospholipid membranes containing reaction centers from the photosynthetic bacterium Rhodopseudomonas sphaeroides. Under conditions in which the reaction centers are restricted to a single electron turnover, the responses can be correlated with the light-induced electron transfer reactions associated with the reaction center. The results indicate that electron transfer from the bacteriochlorophyll dimer to the primary ubiquinone molecule, and from ferrocytochrome c to the oxidized dimer occur in series across the planar membrane. Electron transfer from the primary to secondary ubiquinone molecule is not electrogenic.

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.

Structural requirements of quinone coenzymes for endogenous and dye-mediated coupled electron transport in bacterial photosynthesis.

Electron transport in continuous light has been investigated in chromatophores of Rhodopseudomonas capsulata. Ala pho+, depleted in ubiquinone-10 and subsequently reconstituted with various ubiquinone homologs and analogs. In addition the restoration of electron transport in depleted chromatophores by the artificial redox compounds N-methylphenazonium methosulfate and N,N,N',N'-tetramethyl-p-phenylenediamine was studied. The following pattern of activities was obtained: (1) Reconstitution of cyclic photophosphorylation with ubiquinone-10 was saturated at about 40 ubiquinone molecules per reaction center. (2) Reconstitution by ubiquinone homologs was dependent on the length of the isoprenoid side chain and the amount of residual ubiquinone in the extracted chromatophores. If two or more molecules of ubiquinone-10 per reaction center were retained, all homologs with a side chain longer than two isoprene units were as active as ubiquinone-10 in reconstitution, and the double bonds in the side chain were not required. If less than two molecules per reaction center remained, an unsaturated side chain longer than five units was necessary for full activity. Plastoquinone, alpha-tocopherol, and naphthoquinones of the vitamin K series were relatively inactive in both cases. (3) All ubiquinone homologs, also ubiquinone-1 and -2, could be reduced equally well by the photosynthetic reaction center, as measured by light-induced proton binding in the presence of antimycin A and uncoupler. Plastoquinone was found to be a poor electron acceptor. (4) Photophosphorylation could be reconstituted by N-methylphenazonium methosulfate as well as by N,N,N',N'-tetramethyl-p-phenylenediamine in an antimycin-insensitive way, if more than two ubiquinones per reaction center remained. These compounds were active also in more extensively extracted particles reconstituted with ubiquinone-1, which itself was inactive.