Partial Resolution of the Enzymes Catalyzing Oxidative Phosphorylation

Towards Synthetic Cells with Self-Producing Energy

Autonomous generation of energy, specifically adenosine triphosphate (ATP), is critical for sustaining the engineered functionalities of synthetic cells constructed from the bottom-up. In this mini-review, we categorize studies on ATP-producing synthetic cells into three different approaches: photosynthetic mechanisms, mitochondrial respiration mimicry, and utilization of non-conventional approaches such as exploiting synthetic metabolic pathways. Within this framework, we evaluate the strengths and limitations of each approach and provide directions for future research endeavors. We also introduce a concept of building ATP-generating synthetic organelle that will enable us to mimic cellular respiration in a simpler way than current strategies. This review aims to highlight the importance of energy self-production in synthetic cells, providing suggestions and ideas that may help overcome some longstanding challenges in this field.

The effect of cytochrome c oxidase on lipid polymorphism of model membranes containing cardiolipin

The effect of cytochrome c oxidase incorporation on the lipid polymorphism of the cardiolipin-Ca2+ system was investigated by 31P NMR and freeze-fracture electron microscopy. The integral membrane protein has a stabilizing effect on the bilayer organization of cardiolipin, in that it inhibits the Ca2+-induced HII phase formation of this lipid for Ca2+/cardiolipin molar ratios of 1-10. At a Ca2+/cardiolipin molar ratio of 25, about 80% of the lipid is organized in the HII phase and a structural phase separation occurs between the cardiolipin-Ca2+ complex organized in the hexagonal HII phase without protein and bilayer structures with incorporated protein.

The modes of action of long chain alkyl compounds on the respiratory chain-linked energy transducing system in submitochondrial particles

The interactions of long chain (greater than C7), alkyl compounds with tightly coupled, beef heart submitochondrial particles (SMP) have been investigated with respect to their effects upon respiratory chain-linked electron transfer and energy coupling capacity. Long chain alkyl alcohols, amines, free fatty acids, and methyl esters exhibit a general uncoupling effect, with stimulation of the succinate oxidase activity but inhibition of the NADH oxidase, in SMP. The degree of effectiveness is dependent on the nature of the functional group and the length of the alkyl chain. Submitochondrial particles depleted of F1 and the F1-inhibitor protein are similarly affected. Subsequent treatment with bovine serum albumin reverses the effects of free fatty acids and results in partial recovery of activity with alkyl amines, alcohols, and methyl esters. Differences between the effects of these alkyl compounds and those of sodium dodecyl sulfate, deoxycholate, palmitoyl carnitine, and palmitoyl CoA rule out detergent-like action as the explanation for these observations. These data suggest that specific lipophilic interactions with the membrane, modulated by the nature of the functional group, are responsible for the effects of these compounds on the energy transducing system of SMP. Analyses of the reduction kinetics of the cytochromes indicate that the sites of interaction of these compounds with the inner mitochondrial membrane are associated with the primary dehydrogenase of complex I and energy coupling site 2; alkyl amines possess an additional site of interaction in the region of complex III.

Current concepts in membrane protein reconstitution

The reconstitution of integral proteins into artificial lipid vesicles is largely prompted by the complexity of most biological membranes and the inherent difficulty of studying individual components in situ. Ideally, therefore, the reconstituted system should consist of a single protein in a lipid matrix which mimics the native membrane in all but its diversity. While such an approach allows individual components of a complex system to be studied in isolation it should also be sufficiently versatile to permit the generation of increasingly sophisticated multicomponent models. From the considerable number of reconstitution techniques which have been developed I have tried in this review to identify those characteristics of a particular system which maximise both the information it can provide and its versatility.

Mitochondrial respiratory control in the myocardium

The heart muscle has proved to be a practical model for studying respiratory control in intact tissues. It also demonstrates that control at the level of the respiratory chain is augmented by metabolic control at the substrate level as exemplified by the very narrow range of changes in the redox state of the mitochondrial NADH/NAD couple even during extensive changes in ATP and oxygen consumption. The behaviour of mitochondria when isolated can largely be duplicated in the intact myocardium. Moreover, the high intracellular concentrations of enzymes, coenzymes and adenine nucleotides create conditions of high reaction rates, enabling the formation of a near equilibrium network of certain main pathways. This equilibrium network in connection with metabolic regulation of the hydrogen pressure upon the matrix NADH/NAD pool is a prerequisite for the regulation of cellular respiration at a high efficiency of energy transfer. Experimentation on the intact myocardium also seems to be capable of resolving some of the uncertainties about prevailing mechanisms for the regulation of cellular respiration.

Reconstitution of [3H]3-cyanoimipramine ([3H]Ro 11-2465) sites solubilised from human platelets

[3H]3-Cyanoimipramine has been characterised as a pseudoirreversible ligand at imipramine binding sites making it useful for molecular characterisation studies. Using this ligand, I now report that it is possible to solubilise the binding site molecule with cholate. Gel permeation chromatography on a Sepharose 4B column equilibrated in 0.1% cholate revealed a micellar size of 4.1 nm. Subsequent removal of cholate (from an initial concentration of 2% to less than 0.02%) in the presence of Azolectin (soybean lipid extract) led to reconstituted material that could be retained on glass fibre filters. Electron microscopy revealed apparent micelles of approximately 1 micron diameter. Over 16% of the original, membrane-bound, binding sites were recovered in the reconstituted material along with a similar percentage of the protein. This reconstituted material was moderately stable for up to 3 days after its preparation.

Voltage-dependent proton fluxes in liposomes

Liposomes containing buffered KCl were prepared from bacterial lipids, were diluted into K+-free media and were treated with valinomycin to induce the formation of a diffusion potential (delta psi). Upon formation of such a potential, substantial proton influx was observed, as assayed by the quenching of 9-aminoacridine fluorescence. Complete reversal of fluorescence quenching occurred when the potential was collapsed by addition of KCl or when methylamine was added. Studies of proton influx as a function of the theoretical magnitude of the delta psi indicated that the phenomenon occurred only above a delta psi of about -60 mV. Establishment of a Na+ diffusion potential also resulted in proton influx. Treatment of K+-loaded liposomes with N,N'-dicyclohexylcarbodiimide did not reduce the delta psi-dependent proton influx. Moreover, proton influx could be demonstrated upon imposition of a diffusion potential in liposomes prepared from a synthetic lipid. The proton fluxes associated with generation of a diffusion potential in liposomes may complicate studies of reconstituted systems in which proton translocation should occur, and may affect the magnitude of the electrochemical proton gradient that is operant under some conditions.

Membrane charge as effector of cytochrome P-450LM2 catalyzed reactions in reconstituted liposomes

The phospholipid specificity of rabbit liver microsomal cytochrome P-450LM2 catalyzed hydroxylation reactions was examined in reconstituted phospholipid vesicles. An apparent linear relationship between the negative charge of the vesicles and the rate of P-450LM2-catalyzed O-dealkylation of p-nitroanisole or 7-ethoxycoumarin was obtained. The membrane charge-mediated increase in hydroxylation activities was found not to be due to (i) an altered lipid/water partition coefficient of the substrate, (ii) a change in the apparent Michaelis constant of P-450LM2 for the substrate, (iii) a different activation energy of the O-demethylation of p-nitroanisole, (iv) different spin states of P-450LM2 or (v) an altered secondary structure of this enzyme as monitored by circular dichroism. However, when the formation of the ferrous carbonyl complex of P-450LM2 was followed under aerobic or anaerobic conditions after the addition of NADPH to the vesicles, an increased negative charge of the membrane was accompanied by an increased reducibility of P-450LM2. A similar linear relationship between the reducibility of cytochrome b5 and the negative charge of the liposomes was also evident in membranes containing NADPH-cytochrome P-450 reductase and cytochrome b5. It is proposed that the interaction of the reductase with P-450LM2 is inefficient in neutral vesicles and thus rate determining for the overall hydroxylation activities.

Reconstitution of photosynthetic energy conservation. II. Photophosphorylation in liposomes containing photosystem-I reaction center and chloroplast coupling-factor complex

Photophosphorylation has been reconstituted in a liposomal system containing reaction centers of photosystem I and coupling-factor complex, both highly purified from spinach chloroplasts. This energy-converting model system was put together by diluting the preparation of the coupling-factor complex with an aqueous suspension of proteolipid vesicles, preformed from photosystem-I reaction centers and soybean phospholipids by sonication. In the presence of reduced N-methyl-phenazonium methosulfate the system catalyzed photophosphorylation with rates up to 50 mumol ATP formed x mg chlorophyll-1 x h-1, which was sensitive to uncouplers and to N,N'-dicyclohexyl-carbodiimide. The properties of the system in comparison to chloroplasts is discussed.

Structure and function of H+-ATPase

(1) Extensive studies on proton-translocating ATPase (H+-ATPase) revealed that H+-ATPase is an energy transforming device universally distributed in membranes of almost all kinds of cells. (2) Crystallization of the catalytic portion (F1) of H+-ATPase showed that F1 is a hexagonal molecule with a central hole. The diameter of F1 is about 90 A and its molecular weight is about 380,000. (3) Use of thermophilic F1 permits the complete reconstitution of F1 from its five subunits (alpha, beta, gamma, delta, epsilon) and demonstration of the gate function of the gamma delta epsilon-complex, the catalytic function of beta (supported by alpha and gamma), and the H+-translocating functions of all five subunits. (4) Studies using purified thermostable F0 showed that F0 is an H+-channel portion of H+-ATPase. The direct measurement of H+-flux through F0, sequencing of DCCD-binding protein, and isolation of F1-binding protein are described. (5) The subunit stoichiometry of F1 may be alpha 3 beta 3 gamma delta epsilon. (6) Reconstitution of stable H+-ATPase-liposomes revealed that ATP is directly synthesized by the flow of H+ driven by an electrochemical potential gradient and that H+ is translocated by ATP hydrolysis. This rules out functions for all the hypothetical components that do not belong to H+-ATPase in H+-driven ATP synthesis. The roles of conformation change and other phenomena in ATP synthesis are also discussed.

The incorporation of reaction centres into membranes from a bacteriochlorophyll-less mutant of Rhodopseudomonas sphaeroides

Reaction centres purified from a blue-green mutant R-26 of Rhodopseudomonas sphaeroides can be incorporated into bacteriochlorophyll-less membranes purified from an aerobically-grown bacteriochlorophyll-less mutant 01 of R. sphaeroides. This can be accomplished by raising the temperature of the mixture or by addition of the detergent sodium cholate and its subsequent removal by dilution or dialysis. Optimum conditions for the reconstitution are at 4 degrees C in the presence of 1% cholate and soybean phospholipid (2 : 1, w/w, with membrane protein). Isopycnic sucrose density gradient centrifugation of such preparations shows that reaction centres and light-harvesting pigment-protein complex bind to the membranes. Reconstituted membranes exhibit light-induced steady-state cytochrome absorbance changes resembling those observed in chromatophores prepared from the photosynthetically-grown mutant R-26. The effect on these absorbance changes of varying reaction centre content in the membrane has been studied, and the time course of the interaction between 01 membrane cytochrome c2 and added reaction centre examined. Cytochrome b photoreduction and cytochrome c2 photo-oxidation were observed in the reconstituted preparation; each increased following the addition of antimycin A, suggesting that a cyclic light-driven system had been reconstituted.

Partial purification and characterization of the phosphate transporter from bovine heart mitochondria

A highly active phosphate transporter was extracted with octylglucoside from bovine heart submitochondrial particles that were first partially depleted of other membrane components. It was then partially purified by ammonium sulfate fractionation. After reconstitution of the transporter into liposomes prepared with a crude mixture of soybean phospholipids, the Pi/OH exchange, but not the Pi/Pi exchange, was stimulated three- to fourfold by valinomycin and nigericin in the presence of K+. Both Pi/OH and Pi/Pi exchange activities were sensitive to mercurials and other SH reagents. The rutamycin-sensitive ATPase complex from mitochondria was reconstituted together with the phosphate transporter and adenine nucleotide transporter into liposomes. After inhibition of externally located ATPase, the hydrolysis of ATP was sensitive to atractyloside and mersalyl.

Oxidation-reduction coupled phosphorylation in the dark with isolated spinach chloroplasts

1. Spinach chloroplasts, pre-incubated with ferricyanide, acquire the ability to make ATP in the dark provided they are supplied with a reductant and a lipophilic mediator that can penetrate the membrane. The mediator must be of the type that, upon oxidation, releases protons into the surrounding medium such as 2,3,5,6-tetramethyl-p-phenylenediamine (DAD). 2. Dark phosphorylation is not affected by the electron transport inhibitor, 3(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), but is inhibited by uncouplers of photophosphorylation (e.g. NH4Cl and carbonylcyanide-m-chlorophenylhydrazone (CCCP)) and high concentrations of the energy transfer inhibitor, Dio-9. 3. Because only catalytic amounts of the mediator DAD are required to saturate dark phosphorylation, it is concluded that DAD shuttles reducing equivalents across the membrane from the reductant, ascorbate, on the outside to ferricyanide, the oxidant, trapped on the inside. 4. The results are interpreted within the framework of the chemiosmotic hypothesis for the coupling of electron transport to phosphorylation.

Energy conservation in chemotrophic anaerobic bacteria

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Perspectives and limitations of resolutions-reconstitution experiments

Reconstitutions of membranous activities can tell us how many components are required and what their functions are. The mitochondrial proton pump is used as an example. Moreover, the biological activity, such as Pi transport, can be used in reconstituted vesicles as an assay during the isolation of the transporter. Reconstitution experiments reveal the importance of membrane asymmetry and allow us to study conditions of vectorial assembly. The mechanism of action of ion pumps has been successfully analyzed in reconstituted liposomes. We can study the movement of ions and the electrogenicity of the system without interference by other unrelated processes. Based on studies with the resolved Ca2+-ATPase of sarcoplasmic reticulum, we propose a novel formulation of the mechanism of ATP-driven ion pumps in which cyclic binding of Mg2+ plays a key role.