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

Observation of anomalous carotenoid and blind chlorophyll activations in photosystem I under synthetic membrane confinements

The role of natural thylakoid membrane confinements in architecting the robust structural and electrochemical properties of PSI is not fully understood. Most PSI studies till date extract the proteins from their natural confinements that can lead to non-native conformations. Recently our group had successfully reconstituted PSI in synthetic lipid membranes using detergent-mediated liposome solubilizations. In this study, we investigate the alterations in chlorophylls and carotenoids interactions and reorganization in PSI based on spectral property changes induced by its confinement in anionic DPhPG and zwitterionic DPhPC phospholipid membranes. To this end, we employ a combination of absorption, fluorescence, and circular dichroism (CD) spectroscopic measurements. Our results indicate unique activation and alteration of photoresponses from the PSI carotenoid (Car) bands in PSI-DPhPG proteoliposomes that can tune the Excitation Energy Transfer (EET), otherwise absent in PSI at non-native environments. Specifically, we observe broadband light harvesting via enhanced absorption in the otherwise non-absorptive green region (500-580 nm) of the Chlorophylls (Chl) along with ~64% increase in the full-width half maximum of the Qy band (650-720 nm). The CD results indicate enhanced Chl-Chl and Chl-Car interactions along with conformational changes in protein secondary structures. Such distinct changes in the Car and Chl bands are not observed in PSI confined in DPhPC. The fundamental insights into membrane microenvironments tailoring PSI subunits reorganization and interactions provide novel strategies for tuning photoexcitation processes and rational designing of biotic-abiotic interfaces in PSI-based photoelectrochemical energy conversion systems.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Cyanobacterial photosystem I: Morphology and aggregation behavior

A simple procedure is described for the preparation of cyanobacterial photosystem I particles with a full complement of antenna chlorophyll a from Triton X-100-solubilized thylakoid membranes. In the presence of >/=0.1% Triton X-100, photosystem I particles from Synechococcus 6301 were largely monodisperse. These particles, when negatively stained, appeared to approximate prolate ellipsoids 18 x 8 nm. A value of 4-5 x 10(-19) cm(3) was estimated for the volume of the stain-exclusion envelopes of the particles. At low concentrations of Triton X-100, photosystem I particles formed linear aggregates or sheets one-layer thick. The manner of aggregation was strongly dependent on protein concentration. Shadowed preparations of the sheets indicated a thickness of 8.0-8.5 nm.

Analysis of light-induced transmembrane ion gradients and membrane potential in Photosystem I proteoliposomes

Photosystem I (PSI) complexes can support a light-driven electrochemical gradient for protons, which is the driving force for energy-conserving reactions across biological membranes. In this work, a computational model that enables a quantitative description of the light-induced proton gradients across the membrane of PSI proteoliposomes is presented. Using a set of electrodiffusion equations, a compartmental model of a vesicle suspended in aqueous medium was studied. The light-mediated proton movement was modeled as a single proton pumping step with backpressure of the electric potential. The model fits determinations of pH obtained from PSI proteoliposomes illuminated in the presence of mediators of cyclic electron transport. The model also allows analysis of the proton gradients in relation to the transmembrane ion fluxes and electric potential. Sensitivity analysis enabled a determination of the parameters that have greater influence on steady-state levels and onset/decay rates of transmembrane pH and electric potential. This model could be used as a tool for optimizing PSI proteoliposomes for photo-electrochemical applications.

Incorporation of transmembrane hydroxide transport into the chemiosmotic theory

A cornerstone of textbook bioenergetics is that oxidative ATP synthesis in mitochondria requires, in normal conditions of internal and external pH, a potential difference (delta psi) of well over 100 mV between the aqueous compartments that the energy-transducing membrane separates. Measurements of delta psi inferred from diffusion of membrane-permeant ions confirm this, but those using microelectrodes consistently find no such delta psi--a result ostensibly irreconcilable with the chemiosmotic theory. Transmembrane hydroxide transport necessarily accompanies mitochondrial ATP synthesis, due to the action of several carrier proteins; this nullifies some of the proton transport by the respiratory chain. Here, it is proposed that these carriers' structure causes the path of this "lost" proton flow to include a component perpendicular to the membrane but within the aqueous phases, so maintaining a steady-state proton-motive force between the water at each membrane surface and in the adjacent bulk medium. The conflicting measurements of delta psi are shown to be consistent with the response of this system to its chemical environment.

Functional reconstitution of photosystem I reaction center from cyanobacterium Synechocystis sp PCC6803 into liposomes using a new reconstitution procedure

Photosystem I reaction center from the cyanobacterium Synechocystis sp PCC6803 was reconstituted into phosphatidylcholine/phosphatidic acid liposomes. Liposomes prepared by reversephase evaporation were treated with various amounts of different detergents and protein incorporation was analyzed at each step of the solubilization process. After detergent removal the activities of the resulting proteoliposomes were measured. The most efficient reconstitution was obtained by insertion of the protein complex into preformed liposomes destabilized by saturating amounts of octylglucoside. In the presence of N-methylphenazonium methosulfate and ascorbic acid, liposomes containing the reaction center catalyzed a light-dependent net H+ uptake as measured by the 9-aminoacridine fluorescence quenching and the pH meter. An important benefit of the new reconstitution procedure is that it produces a homogeneous population of large-size proteoliposomes with a low ionic permeability and with a majority inwardly directed H+ transport activity. In optimal conditions, a light-induced delta pH of about 1.8 units could be sustained at 20 degrees C in the presence of valinomycin. In the absence of valinomycin, a "back-pressure" effect of an electrical transmembrane potential decreased both the rate and the extent of the H+ transport. The reaction center was also co-reconstituted with F0F1 H(+)-ATPases from chloroplasts and from the thermophilic bacterium, PS3. The co-reconstituted system was shown to catalyze a light-dependent phosphorylation which could only be measured in the presence of a high concentration of PSI (low lipid/PSI ratios) while no delta pH could be detected.

ATP-synthesis by proteoliposomes incorporating Rhodospirillum rubrum F0F1 as measured with firefly luciferase: dependence on delta psi and delta pH

ATP-synthesis catalyzed by proteoliposomes incorporating Rhodospirillum rubrum F0F1 was driven by artificially applied electrochemical proton gradients. The time-course of ATP-synthesis was followed continuously by means of firefly luciferase. Correction methods were developed which allow one to calculate the initial rate of ATP-synthesis from the observed luminescence kinetics. The following results were obtained: (1) ATP-synthesis occurred above a threshold delta mu H+ of 90 mV; this threshold is not imposed by the activation requirement of the enzyme; (2) delta psi and delta pH appear to be equivalent as driving forces for ATP-synthesis if allowance is made for the effect of the electrical capacitance of the liposome membrane on the distribution of K+ at equilibrium; and (3) the highest rate observed so far is 200 mol ATP per mol F0F1 per s.

Reaction centers from Rhodopseudomonas sphaeroides in reconstituted phospholipid vesicles. II. Light-induced proton translocation

Unidirectional light-dependent proton translocation was demonstrated in a suspension of reconstituted reaction center (RC) vesicles supplemented with cytochrome c and 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UQ0), a lipid- and water-soluble quinone. Proton translocation was detected only at alkaline pH. The pH dependence can be accounted for by the slow redox reaction between the reduced quinone (UQ0H2) and oxidized cytochrome c. This conclusion is based on (i) the pH dependence of partial reactions of the reconstituted proton translocation cycle, measured either optically or electrometrically and (ii) titration studies with cytochrome c and UQ0. At 250 and 25 microM UQ0 and cytochrome c, respectively, maximal proton translocation was observed at pH 9.6. This pH optimum can be extended to a more acidic pH by increasing the concentration of the soluble redox mediators in the reconstituted cyclic electron transfer chain. At the alkaline side of the pH optimum, proton translocation appears to be limited by electron transfer from the endogenous primary to the secondary quinone within the RCs. The light intensity limits the reconstituted proton pump at the optimal pH. The results are discussed in the context of a reaction scheme for the cyclic redox reactions and the associated proton translocation events.

Coreconstitution of bacterial ATP synthase with monomeric bacteriorhodopsin into liposomes. A comparison between the efficiency of monomeric bacteriorhodopsin and purple membrane patches in coreconstitution experiments

The conditions for coreconstitution of a bacterial ATP synthase and bacteriorhodopsin into lecithin liposomes and for light driven ATP synthesis have been optimized. A rate of maximally 280 nmol ATP min-1 mg ATP synthase-1 was achieved with monomerized bacteriorhodopsin compared with a rate of up to 45 nmol ATP min-1 mg-1 found for proteoliposomes containing bacteriorhodopsin in the form of purple membrane patches. The different rates are explained by the finding that monomeric bacteriorhodopsin is more homogeneously distributed among the liposomes than the purple membrane patches. The final activities depended on both the purification method for the two proteins and the coreconstitution procedure. Furthermore, the ratio (lipid to bacteriorhodopsin to ATP synthase) could be optimized. Light-driven ATP synthesis depends also on the type of detergent used. The best result was obtained by deoxycholate. Also the relationship between proton translocation (by bacteriorhodopsin) and ATP synthesis activity was measured. A constant H+/ATP ratio was found at higher light intensities. This ratio increased strongly at lower light intensities.

Steady-state ATP synthesis by bacteriorhodopsin and chloroplast coupling factor co-reconstituted into asolectin vesicles

A method was developed for the co-reconstitution of bacteriorhodopsin and chloroplast coupling factor in asolectin vesicles. First, bacteriorhodopsin was reconstituted from a mixture of octyl glucoside, asolectin, and protein in the presence of ethylenediaminetetraacetic acid by passage through a Sephadex G-50 centrifuge column. Then, the purified coupling factor was reconstituted from a mixture of sodium cholate, bacteriorhodopsin vesicles, and coupling factor in the presence of Mg2+ by passage through the centrifuge column. Sucrose density-gradient centrifugation indicated a band of vesicles with slightly different positions in the gradient for maximum vesicle concentration, bacteriorhodopsin vesicle concentration, ATP synthesis, and ATP hydrolysis. The rate of light-driven ATP synthesis reaches a limiting value as the concentration of bacteriorhodopsin and the light intensity are increased. A steady-state rate of ATP synthesis of 1 mumol per mg of coupling factor X min-1 has been achieved. Apparently this rate is limited by the heterogeneity within the vesicle population and by the ability of bacteriorhodopsin to form a sufficiently large pH gradient.

A time lag in the onset of ATP-Pi exchange catalyzed by purified ATP-synthase (CF0-CF1) proteoliposomes and by chloroplasts

The time course of ATP-Pi exchange which is catalyzed by the isolated chloroplast ATP synthase in phospholipid vesicles was studied. The following observations were made. (i) The onset of 32Pi incorporation into ATP lags behind ATP hydrolysis. The lag lasts for about 2 min at 37 degrees C and is followed by a steady-state rate which is constant for more than 30 min. Under the same experimental conditions, ATP hydrolysis shows an initial burst followed by a constant, slower rate. (ii) The initial lag is independent of Mg-ATP concentration in the range 0.2-5 mM and of the presence of ADP. In contrast, the steady-state rate of ATP-Pi exchange has an apparent Km of 0.3 mM for Mg-ATP and is stimulated by ADP. (iii) Increasing the temperature from 30 to 45 degrees C decreases the lag from 6 min to zero. The steady-state rate of ATP-Pi exchange is affected to a much smaller extent by the temperature in this range. (iv) The lag is insensitive to valinomycin or tetraphenylboron, while the steady-state rate is partially inhibited. Nigericin and protonophores affect both the lag and steady-state rate. (v) ATP-induced membrane potential formation, as followed by oxonol VI, does not correlate with the lag in its kinetics and temperature dependence. ATP-induced pH gradient formation could not be detected in the proteoliposome system. (vi) Light-triggered ATP-Pi exchange in chloroplasts shows essentially the same time course as the proteoliposome system, but the lag lasts for only about 20 s at room temperature and is unaffected by a preexisting proton gradient. These results suggest that the initial lag in ATP-Pi exchange does not reflect the time required for the buildup of a protomotive force (delta - mu H+) nor the time required to produce ADP. It is suggested, therefore, that the lag reflects an internal autocatalytic conformational change in the ATP-synthase complex which is initiated by ATP hydrolysis and which converts the enzyme from an "exclusive ATPase state" to a "reversible ATP-synthase state". This slow transition is not directly coupled to a trans-membrane pH or potential gradient.

Inorganic pyrophosphate-driven ATP-synthesis in liposomes containing membrane-bound inorganic pyrophosphatase and F0-F1 complex from Rhodospirillum rubrum

PPi driven ATP synthesis has been reconstituted in a liposomal system containing the membrane-bound energy-linked PPiase and coupling factor complex, both highly purified from Rhodospirillum rubrum. This energy converting model system was made by mixing both enzyme preparations with an aqueous suspension of sonicated soybean phospholipids and subjecting to a freeze-thaw procedure. In the presence of ADP, Mg2+, Pi and PPi the system catalyzed phosphorylation by up to 25 nmol ATP formed X mg protein-1 X min-1, at 20 degrees C, which was sensitive to uncouplers and inhibitors of phosphorylation such as oligomycin, efrapeptin and N,N'-dicyclohexylcarbodiimide.

Genes and transcripts for the P700 chlorophylla apoprotein and subunit 2 of the photosystem I reaction center complex from spinach thylakoid membranes

A photosystem I reaction center complex has been purified to homogeneity by a procedure involving partial solubilization of spinach thylakoid membranes, ion exchange chromatography and centrifugation in sucrose gradients. The complex contains 7 polypeptides: the P700 chlorophylla apoprotein with an apparent molecular weight of 67 kd, which at high resolution splits into two bands, and smaller polypeptides of 22 (subunit 2), 18.5, 18, 16, 12 and 10 kd.Stable transcripts for the P700 chlorophylla apojprotein and subunit 2 were found in plastid and cytosolic RNA, respectively. The apoprotein product obtained by translation in a mRNA-dependent cell-free rabbit reticulocyte lysate and also by DNA-programmed transcription-translation of cloned plastid DNA fragments inE. coli lysates was indistinguishable immunologically and electrophoretically from the authentic protein. However, the product immunologically related to subunit 2 was 4 kd larger than the mature compound indicating that this protein is encoded in the nucleus and synthesized as a precursor.The gene for the P700 chlorophylla apoprotein has been physically mapped on the spinach plastid chromosome by hybrid selection mapping and DNA-programmed cell-free transcription-translation using cloned restriction fragments of plastid DNA. There is one gene copy per chromosome and it is located centrally in the large single-copy region of the circular DNA molecule. This gene is uninterrupted and is transcribed in the same direction as that of the large subunit of ribulose bisphosphate carboxylase/oxygenase. Its transcript is approximately 4 kb longer than the 2 kbp structural gene.

Photosystem I reaction centers from Chlamydomonas and higher plant chloroplasts

A photosystem I reaction center has been isolated from Chlamydomonas chloroplasts and compared with the photosystem I reaction center from higher plants. While the higher plant reaction center is active in cytochrome 552 photooxidation, the Chlamydomonas preparation was not active unless salts were included in the assay medium or the pH was lowered to 5. Subunit III-depleted photosystem I reaction center from higher plants is also inactive in cytochrome 552 photooxidation in the absence of salts. As with the Chlamydomonas reaction center, salts induced its activity. Subunit I of the photosystem I reaction center has tentatively been identified as the binding site of cytochrome 552.

pH dependence of adenosine 5'-triphosphate synthesis and hydrolysis catalyzed by reconstituted chloroplast coupling factor

The purified ATP-synthesizing complex from chloroplasts has been reconstituted into phospholipid vesicles with bacteriorhodopsin by use of octyl glucoside. Phosphorylation rates up to 90 mmol of ATP (mg of protein)-1 min-1 have been achieved. The dependence of the steady-state kinetic parameters on external and internal pH for both synthesis and hydrolysis was determined. The Michaelis constants are independent of the magnitude of the pH gradient at external pH values of 6.6 and 8.0. The dependence of the maximum velocity for ATP synthesis on the external pH is bell shaped at a constant pH gradient with a maximum at about pH 6.7. The variation of the maximum velocity with external pH is not dependent on the magnitude of the pH gradient. At external pH values of 6.6 and 8.0, the maximum velocity for ATP synthesis varies with approximately the 2.3 power of the internal hydrogen ion concentration. The maximum velocity for ATP hydrolysis also is dependent on the external pH, with a maximum at about pH 8.4; however, most of the ATPase activity is not coupled to the proton flux. Both Mg2+ and Mn2+ are good cofactors for ATP synthesis and hydrolysis whereas Ca2+ is completely ineffective for synthesis and only about 10% as effective as Mg2+ and Mn2+ for hydrolysis. The results obtained suggest that ATP synthesis or hydrolysis may be coupled to proton pumping indirectly, as, for example, by conformational changes.

31P-Nuclear magnetic resonance and freeze-fracture electron microscopic studies on reconstituted bacteriorhodopsin vesicles

Bacteriorhodopsin has been reconstituted into egg-phosphatidylcholine vesicles by various methods. The resulting preparations have been analyzed on density gradients and by freeze-fracture electron microscopy. The homogeneity of the vesicle preparations and the light-induced intravesicular pH changes have been studied by 31P-NMR, using glucose 6-phosphate as pH probe. It is concluded that bacteriorhodopsin is incorporated in the inside-out mode in vesicles up to about 100 nm. Above this diameter, more or less random insertion takes place.

A cytochrome f/b6 complex of five polypeptides with plastoquinol-plastocyanin-oxidoreductase activity from spinach chloroplasts

The isolation of a cytochrome f/b6 complex from spinach chloroplasts, with high yield and purity is reported. The complex consists of five polypeptides with a molecular mass of 34, 33, 23.5, 20 and 17.5 kDa, and contains one cytochrome f, two cytochromes b6 and the Rieske Fe-S center with two non-heme irons. It does not contain plastocyanin and is almost completely devoid of chlorophyll and carotenoids, but lipid and detergent are present. It is lacking cytochrome b-559, although three of the five polypeptides seem to carry heme groups. The preparation has plastoquinol-plastocyanin oxidoreductase activity with plastoquinol-1 and plastoquinol-9, which is sensitive to 2,5-dibromomethylisopropyl-p-benzoquinone, to 2-iodo-6-isopropyl-3-methyl-2',4',4'-trinitrodiphenyl ether, to 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole, and to bathophenanthroline. Characteristics of this activity with respect to substrate concentrations, pH, detergent effect and other parameters are described.

Stepwise molecular evolution of bacterial photosynthetic energy conversion

The concept of continuity in molecular evolution implies a stepwise formation of metabolic systems and processes. In this manner, chemical and biological evolution have given rise, step by step, to such complicated systems as the photosynthetic apparatus and thus, such elaborate processes as photosynthesis in the living cell. Among currently living organisms, the bacteria contain a much less complex photosynthetic system than the algae and higher plants, which uniquely are capable fo splitting H2O. But also the bacterial system is a very highly evolved and sophisticated, membrane-bound apparatus for the transformation of light energy to other biologically useful energy forms. The study of its molecular evolution is here undertaken by the method of attempting to break down the system into its main components and functions in order to elucidate how they had originated and evolved, and how, by divergent and convergent evolutionary steps, the stage was set for the arrival of bacterial photophosphorylation.

Reconstitution of photosynthetic energy conservation. I. Proton movements in liposomes containing reaction center of photosystem I from spinach chloroplasts

A preparation of reaction centers of photosystem I from spinach chloroplasts was incorporated into lipid vesicles by sonication. Incorporation was tested by chromatography on Sepharose-4B, by comparison of the elution profiles of photosystem-I reaction center liposomes and of free reaction centers. In the presence of reduced N-methylphenazonium methosulfate the reaction center liposomes catalyzed net proton extrusion in the light, but at the same time showed light-induced quenching of 9-aminoacridine fluorescence, with similar extent and kinetics as known for chloroplasts. We conclude that we are dealing with two vesicle populations, one right side-out and one inside-out with respect to the orientation of the incorporated reaction center complex. Net proton movements are influenced by the nature of the cations present in the suspending medium, and the possibilities for effects of surface charge on these movements are discussed.