Lipid-impregnated filters as a tool for studying the electric current-generating proteins

Electrometric and Electron Paramagnetic Resonance Measurements of a Difference in the Transmembrane Electrochemical Potential: Photosynthetic Subcellular Structures and Isolated Pigment-Protein Complexes

A transmembrane difference in the electrochemical potentials of protons (ΔμH+) serves as a free energy intermediate in energy-transducing organelles of the living cell. The contributions of two components of the ΔμH+ (electrical, Δψ, and concentrational, ΔpH) to the overall ΔμH+ value depend on the nature and lipid composition of the energy-coupling membrane. In this review, we briefly consider several of the most common instrumental (electrometric and EPR) methods for numerical estimations of Δψ and ΔpH. In particular, the kinetics of the flash-induced electrometrical measurements of Δψ in bacterial chromatophores, isolated bacterial reaction centers, and Photosystems I and II of the oxygenic photosynthesis, as well as the use of pH-sensitive molecular indicators and kinetic data regarding pH-dependent electron transport in chloroplasts, have been reviewed. Further perspectives on the application of these methods to solve some fundamental and practical problems of membrane bioenergetics are discussed.

Features of the Mechanism of Proton Transport in ESR, Retinal Protein from Exiguobacterium sibiricum

Retinal-containing light-sensitive proteins - rhodopsins - are found in many microorganisms. Interest in them is largely explained by their role in light energy storage and photoregulation in microorganisms, as well as the prospects for their use in optogenetics to control neuronal activity, including treatment of various diseases. One of the representatives of microbial rhodopsins is ESR, the retinal protein of Exiguobacterium sibiricum. What distinguishes ESR from homologous proteins is the presence of a lysine residue (Lys96) as a proton donor for the Schiff base. This feature, along with the hydrogen bond of the proton acceptor Asp85 with the His57 residue, determines functional characteristics of ESR as a proton pump. This review examines the results of ESR studies conducted using various methods, including direct electrometry. Comparison of the obtained data with the results of structural studies and with other retinal proteins allows us to draw conclusions about the mechanisms of transport of hydrogen ions in ESR and similar retinal proteins.

Generation of Electric Potential Difference by Chromatophores from Photosynthetic Bacteria in the Presence of Trehalose under Continuous Illumination

Measurement of electrical potential difference (Δψ) in membrane vesicles (chromatophores) from the purple bacterium Rhodobacter sphaeroides associated with the surface of a nitrocellulose membrane filter (MF) impregnated with a phospholipid solution in decane or immersed into it in the presence of exogenous mediators and disaccharide trehalose demonstrated an increase in the amplitude and stabilization of the signal under continuous illumination. The mediators were the ascorbate/N,N,N'N'-tetramethyl-p-phenylenediamine pair and ubiquinone-0 (electron donor and acceptor, respectively). Although stabilization of photoelectric responses upon long-term continuous illumination was observed for both variants of chromatophore immobilization, only the samples immersed into the MF retained the functional activity of reaction centers (RCs) for a month when stored in the dark at room temperature, which might be due to the preservation of integrity of chromatophore proteins inside the MF pores. The stabilizing effect of the bioprotector trehalose could be related to its effect on both the RC proteins and the phospholipid bilayer membrane. The results obtained will expand current ideas on the use of semi-synthetic structures based on various intact photosynthetic systems capable of converting solar energy into its electrochemical form.

Lel A. Drachev and the Direct Electrometric Method

In the bioenergetics studies, the direct electrometric method played an important role. This method is based on measuring the electrical potential difference (Δψ) between two compartments of the experimental cell generated by some membrane proteins. These proteins are incorporated into closed lipid-protein membrane vesicles associated with an artificial lipid membrane that separates the compartments. The very existence of such proteins able to generate Δψ was one of the consequences of Peter Mitchell's chemiosmotic concept. The discovery and investigation of their functioning contributed to the recognition of this concept and, eventually the well-deserved awarding of the Nobel Prize to P. Mitchell. Lel A. Drachev (1926-2022) was one of the main authors of the direct electrometrical method. With his participation, key studies were carried out on the electrogenesis of photosynthetic and respiratory membrane proteins, including bacteriorhodopsin, visual rhodopsin, photosynthetic bacterial reaction centers, cytochrome oxidase and others.

Application of direct electrometry in studies of microbial rhodopsins reconstituted in proteoliposomes

Microbial rhodopsins are the family of retinal-containing proteins that perform primarily the light-driven transmembrane ion transport and sensory functions. They are widely distributed in nature and can be used for optogenetic control of the cellular activities by light. Functioning of microbial rhodopsins results in generation of the transmembrane electric potential in response to a flash that can be measured by direct time-resolved electrometry. This method was developed by L. Drachev and his colleagues at the Belozersky Institute and successfully applied in the functional studies of microbial rhodopsins. First measurements were performed using bacteriorhodopsin from Halobacterium salinarum-the prototype member of the microbial retinal protein family. Later, direct electrometric studies were conducted with proteorhodopsin from Exiguobacterium sibiricum (ESR), the sodium pump from Dokdonia, and other proteins. They allowed detailed characterization of the charge transfer steps during the photocycle of microbial rhodopsins and provided new insights for profound understanding of their mechanism of action.

Photovoltage generation by photosystem II core complexes immobilized onto a Millipore filter on an indium tin oxide electrode

The light-induced functioning of photosynthetic pigment-protein complex of photosystem II (PSII) is linked to the vectorial translocation of charges across the membrane, which results in the formation of voltage. Direct measurement of the light-induced voltage (∆V) generated by spinach oxygen-evolving PSII core complexes adsorbed onto a Millipore membrane filter (MF) on an indium tin oxide (ITO) electrode under continuous illumination has been performed. PSII was shown to participate in electron transfer from water to the ITO electrode, resulting in ∆V generation. No photovoltage was detected in PSII deprived of the water-oxidizing complex. The maximal and stable photoelectric signal was observed in the presence of disaccharide trehalose and 2,6-dichloro-1,4-benzoquinone, acting as a redox mediator between the primary quinone acceptor Q_A of PSII and electrode surface. Long time preservation of the steady-state photoactivity at room temperature in a simple in design ITO|PSII-MF|ITO system may be related to the retention of water molecules attached to the PSII surface in the presence of trehalose.

His57 controls the efficiency of ESR, a light-driven proton pump from Exiguobacterium sibiricum at low and high pH

ESR, a light-driven proton pump from Exiguobacterium sibiricum, contains a lysine residue (Lys96) in the proton donor site. Substitution of Lys96 with a nonionizable residue greatly slows reprotonation of the retinal Schiff base. The recent study of electrogenicity of the K96A mutant revealed that overall efficiency of proton transport is decreased in the mutant due to back reactions (Siletsky et al., BBA, 2019). Similar to members of the proteorhodopsin and xanthorhodopsin families, in ESR the primary proton acceptor from the Schiff base, Asp85, closely interacts with His57. To examine the role of His57 in the efficiency of proton translocation by ESR, we studied the effects of H57N and H57N/K96A mutations on the pH dependence of light-induced pH changes in suspensions of Escherichia coli cells, kinetics of absorption changes and electrogenic proton transfer reactions during the photocycle. We found that at low pH (<5) the proton pumping efficiency of the H57N mutant in E. coli cells and its electrogenic efficiency in proteoliposomes is substantially higher than in the WT, suggesting that interaction of His57 with Asp85 sets the low pH limit for H⁺ pumping in ESR. The electrogenic components that correspond to proton uptake were strongly accelerated at low pH in the mutant indicating that Lys96 functions as a very efficient proton donor at low pH. In the H57N/K96A mutant, a higher H⁺ pumping efficiency compared with K96A was observed especially at high pH, apparently from eliminating back reactions between Asp85 and the Schiff base by the H57N mutation.

Electron Transfer on the Donor Side of Manganese-Depleted Photosystem 2

After removal of manganese ions responsible for light-driven water oxidation, redox-active tyrosine Y_Z (tyrosine 161 of the D1 subunit) still remains the dominant electron donor to the photooxidized chlorophyll P₆₈₀ (P₆₈₀⁺) in the reaction center of photosystem 2 (PS2). Here, we investigated P₆₈₀⁺ reduction by Y_Z under single-turnover flashes in Mn-depleted PS2 core complexes in the presence of weak acids and NH₄Cl. Analysis of changes in the light-induced absorption at 830 nm (reflecting P₆₈₀ redox transitions) at pH 6.0 showed that P₆₈₀⁺ reduction is well approximated by two kinetic components with the characteristic times (τ) of ~7 and ~31 μs and relative contributions of ~54 and ~37%, respectively. In contrast to the very small effect of sodium formate (200 mM), addition of sodium acetate and NH₄Cl increased the rate of electron transfer between Y_Z and P₆₈₀⁺ approx. by a factor of 5. The suggestion that direct electron transfer from Y_Z to P₆₈₀⁺ has a biphasic kinetics and reflects the presence of two different populations of PS2 centers was confirmed by the data obtained using direct electrometrical technique. It was demonstrated that the submillisecond two-phase kinetics of the additional electrogenic phase in the kinetics of photoelectric response due to the electron transfer between Y_Z and P₆₈₀⁺ is significantly accelerated in the presence of acetate or ammonia. These results contribute to the understanding of the mechanism of interaction between the oxidized tyrosine Y_Z and exogenous substances (including synthetic manganese-containing compounds) capable of photooxidation of water molecule in the manganese-depleted PS2 complexes.

Biomimetic membranes with aqueous nano channels but without proteins: impedance of impregnated cellulose ester filters

Earlier we have shown that many important properties of ionic aqueous channels in biological membranes can be imitated using simple biomimetic membranes. These membranes are composed of mixed cellulose ester-based filters, impregnated with isopropyl myristate or other esters of fatty acids, and can be used for high-throughput drug screening. If the membrane separates two aqueous solutions, combination of relatively hydrophilic polymer support with immobilized carboxylic groups results in the formation of thin aqueous layers covering inner surface of the pores, while the pore volume is filled by lipid-like substances. Because of these aqueous layers biomimetic membranes even without proteins have a cation/anion ion selectivity and specific (per unit of thickness) electrical properties, which are similar to typical properties of biological membranes. Here we describe frequency-dependent impedance of the isopropyl myristate-impregnated biomimetic membranes in the 4-electrode arrangement and present the results as Bode and Nyquist diagrams. When the membranes are placed in deionized water, it is possible to observe three different dispersion processes in the frequency range 0.1 Hz to 30 kHz. Only one dispersion is observed in 5 mM KH(2)PO(4) solution. It is suggested that these three dispersion features are determined by (a) conductivity in aqueous structures/channels, formed near the internal walls of the filter pores at high frequencies, (b) dielectric properties of the whole membrane at medium frequencies, determined by polymer support, aqueous layers and impregnating oil, and, finally, (c) by the processes in hydrated liquid crystal structures formed in pores by impregnating oil in contact with water at low frequencies.

Elevated proton leak of the intermediate OH in cytochrome c oxidase

The kinetics of the formation and relaxation of transmembrane electric potential (Deltapsi) during the complete single turnover of CcO was studied in the bovine heart mitochondrial and the aa(3)-type Paracoccus denitrificans enzymes incorporated into proteoliposome membrane. The real-time Deltapsi kinetics was followed by the direct electrometry technique. The prompt oxidation of CcO and formation of the activated, oxidized (O(H)) state of the enzyme leaves the enzyme trapped in the open state that provides an internal leak for protons and thus facilitates dissipation of Deltapsi (tau(app) < or = 0.5-0.8 s). By contrast, when the enzyme in the O(H) state is rapidly re-reduced by sequential electron delivery, Deltapsi dissipates much slower (tau(app) > 3 s). In P. denitrificans CcO proteoliposomes the accelerated Deltapsi dissipation is slowed down by a mutational block of the proton conductance through the D-, but not K-channel. We concluded that in contrast to the other intermediates the O(H) state of CcO is vulnerable to the elevated internal proton leak that proceeds via the D-channel.

Molecular mechanism of proton translocation by cytochrome c oxidase

Cytochrome c oxidase (CcO) is a terminal protein of the respiratory chain in eukaryotes and some bacteria. It catalyzes most of the biologic oxygen consumption on earth done by aerobic organisms. During the catalytic reaction, CcO reduces dioxygen to water and uses the energy released in this process to maintain the electrochemical proton gradient by functioning as a redox-linked proton pump. Even though the structures of several terminal oxidases are known, they are not sufficient in themselves to explain the molecular mechanism of proton pumping. Thus, additional extensive studies of CcO by varieties of biophysical and biochemical approaches are involved to shed light on the mechanism of proton translocation. In this review, we summarize the current level of knowledge about CcO, including the latest model developed to explain the CcO proton-pumping mechanism.

Plasticity of Proton Pathway Structure and Water Coordination in Cytochrome c Oxidase

Cytochrome c oxidase (CytcO) is a redox-driven, membrane-bound proton pump. One of the proton transfer pathways of the enzyme, the D pathway, used for the transfer of both substrate and pumped protons, accommodates a network of hydrogen-bonded water molecules that span the distance between an aspartate (Asp(132)), near the protein surface, and glutamate Glu(286), which is an internal proton donor to the catalytic site. To investigate how changes in the environment around Glu(286) affect the mechanism of proton transfer through the pathway, we introduced a non-hydrogen-bonding (Ala) or an acidic residue (Asp) at position Ser(197) (S197A or S197D), located approximately 7 A from Glu(286). Although Ser(197) is hydrogen-bonded to a water molecule that is part of the D pathway "proton wire," replacement of the Ser by an Ala did not affect the proton transfer rate. In contrast, the S197D mutant CytcO displayed a turnover activity of approximately 35% of that of the wild-type CytcO, and the O(2) reduction reaction was not linked to proton pumping. Instead, a fraction of the substrate protons was taken from the positive ("incorrect") side of the membrane. Furthermore, the pH dependence of the proton transfer rate was altered in the mutant CytcO. The results indicate that there is plasticity in the water coordination of the proton pathway, but alteration of the electrostatic potential within the pathway results in uncoupling of the proton translocation machinery.

Probing biological interfaces by tracing proton passage across them

The properties of water at the surface, especially at an electrically charged one, differ essentially from those in the bulk phase. Here we survey the traits of surface water as inferred from proton pulse experiments with membrane enzymes. In such experiments, protons that are ejected (or captured) by light-triggered enzymes are traced on their way between the membrane surface and the bulk aqueous phase. In several laboratories it has been shown that proton exchange between the membrane surface and the bulk aqueous phase takes as much as about 1 ms, but could be accelerated by added mobile pH-buffers. Since the accelerating capacity of the latter decreased with increase in their electric charge, it was suggested that the membrane surface is separated from the bulk aqueous phase by a barrier of electrostatic nature. In terms of ordinary electrostatics, the barrier could be ascribed to dielectric saturation of water at a charged surface. In terms of nonlocal electrostatics, the barrier could result from the dielectric overscreening in the surface water layers. It is discussed how the interfacial potential barrier can affect the reactions at interface, especially those coupled with biological energy conversion and membrane transport.

Transport across artificial membranes–an analytical perspective

Biosensors that make use of transport processes across lipid membranes are very rare even though a stimulus, the binding of a single analyte molecule, can enhance the sensor response manifold if the analyte leads to the transport of more than one ion or molecule across the membrane. Prerequisite for a proper function of such membrane based biosensors is the formation of lipid bilayers attached to a support that allow for the insertion of membrane peptides and proteins in a functional manner. In this review, the current state of the art technologies to obtain lipid membranes on various supports are described. Solid supported membranes on transparent and electrically conducting surfaces, lipid bilayers on micromachined apertures and on porous materials are discussed. The focus lies on the applicability of such membranes for the investigation of transport phenomena across lipid bilayers facilitated by membrane embedded peptides, channel proteins and transporters. Carriers and channel forming peptides, which are easy to handle and rather robust, are used frequently to build up membrane based biosensors. However, channel forming proteins and transporters are more difficult to insert functionally and thus, there are yet only few examples that demonstrate the applicability of such systems as biosensor devices.

The catalytic cycle of cytochrome c oxidase is not the sum of its two halves

Membrane-bound cytochrome c oxidase catalyzes cell respiration in aerobic organisms and is a primary energy transducer in biology. The two halves of the catalytic cycle may be studied separately: in an oxidative phase, the enzyme is oxidized by O(2), and in a reductive phase, the oxidized enzyme is reduced before binding the next O(2) molecule. Here we show by time-resolved membrane potential and pH measurements with cytochrome oxidase liposomes that, with both phases in succession, two protons are translocated during each phase, one during each individual electron transfer step. However, when the reductive phase is not immediately preceded by oxidation, it follows a different reaction pathway no longer coupled to proton pumping. Metastable states with altered redox properties of the metal centers are accessed during turnover and relax when external electron donors are exhausted but recover after enzyme reduction and reoxidation by O(2). The efficiency of ATP synthesis might be regulated by switching between the two catalytic pathways.

Photoelectric studies of the transmembrane charge transfer reactions in photosystem I pigment-protein complexes

The results of studies of charge transfer in cyanobacterial photosystem I (PS I) using the photoelectric method are reviewed. The electrogenicity in the PS I complex and its interaction with natural donors (plastocyanin, cytochrome c(6)), natural acceptors (ferredoxin, flavodoxin), or artificial acceptors and donors (methyl viologen and other redox dyes) were studied. The operating dielectric constant values in the vicinity of the charge transfer carriers in situ were calculated. The profile of distribution of the dielectric constant along the PS I pigment-protein complex (from plastocyanin or cytochrome c(6) through the chlorophyll dimer P700 to the acceptor complex) was estimated, and possible mechanisms of correlation between the local dielectric constant and electron transfer rate constant were discussed.

Proton translocation by cytochrome c oxidase in different phases of the catalytic cycle

Since mitochondrial cytochrome c oxidase was found to be a redox-linked proton pump, most enzymes of the haem-copper oxidase family have been shown to share this function. Here, the most recent knowledge of how the individual reactions of the enzyme's catalytic cycle are coupled to proton translocation is reviewed. Two protons each are pumped during the oxidative and reductive halves of the cycle, respectively. An apparent controversy that concerns proton translocation during the reductive half is resolved. If the oxidised enzyme is allowed to relax in the absence of reductant, the binuclear haem-copper centre attains a state that lies outside the main catalytic cycle. Reduction of this form of the enzyme is not linked to proton translocation, but is necessary for a return to the main cycle. This phenomenon might be related to the previously described "pulsed" vs. "resting" and "fast" vs."slow" forms of haem-copper oxidases.

On the kinetics of voltage formation in purple membranes of Halobacterium salinarium

The kinetics of the bacteriorhodopsin photocycle, measured by voltage changes in a closed membrane system using the direct electrometrical method (DEM) of Drachev, L.A., Jasaitus, A.A., Kaulen, A.D., Kondrashin, A.A., Liberman, E.A., Nemecek, I.B., Ostroumov, S.A., Semenov, Yu, A. & Skulachev, V.P. (1974) Nature 249, 321-324 are sixfold slower than the kinetics obtained in optical studies with suspensions of purple membrane patches. In this study, we have investigated the reasons for this discrepancy. In the presence of the uncouplers carbonyl cyanide m-chlorophenylhydrazone or valinomycin, the rates in the DEM system are similar to the rates in suspensions of purple membrane. Two alternative explanations for the effects of uncouplers were evaluated: (a) the 'back-pressure' of the Deltamicro;H+ slows the kinetic steps leading to its formation, and (b) the apparent difference between the two systems is due to slow major electrogenic events that produce little or no change in optical absorbance. In the latter case, the uncouplers would decrease the RC time constant for membrane capacitance leading to a quicker discharge of voltage and concomitant decrease in photocycle turnover time. The experimental results show that the primary cause for the slower kinetics of voltage changes in the DEM system is thermodynamic back-pressure as described by Westerhoff, H.V. & Dancshazy, Z. (1984) Trends Biochem. Sci. 9, 112-117.

Conformationally controlled pK-switching in membrane proteins: one more mechanism specific to the enzyme catalysis?

Internal proton displacements in several membrane photosynthetic enzymes are analyzed in relation to general mechanisms of enzymatic catalysis. In the bacterial photosynthetic reaction center (RC) and in bacteriorhodopsin (BR), carboxy residues (Glu-212 in the RC L-subunit and Asp-96 in BR) serve as indispensable intrinsic proton donors. Both carboxyls are protonated prior to the proton-donation step, because their pK values are shifted to >/=12.0 by the interaction with the protein and/or substrate. In both cases, the proton transfer reactions are preceded by conformational changes that, supposedly, let water interact with the carboxyls. These changes switch over the pK values of the carboxyls to </=6.0 and 7.1 in the RC and BR, respectively. The sharp increase in the proton-donating ability of the carboxyls drives the reaction cycles. This kind of catalytic mechanism, where a strong general acid or base emerges, when needed, as a result of a conformational change can be denoted as a conformationally controlled pK-switching. Generally, the ability of enzymes to go between isoenergetic conformations that differ widely in the reactivity of the catalytic group(s) may be of crucial importance to the understanding of enzymatic catalysis. Particularly, the pK-switching concept could help to reconcile the contradictory views on the functional protonation state of the redox-active tyrosine Y(Z) in the oxygen-evolving photosystem II. It is conceivable that Y(Z) switches its pK from approximately 4.5 to >/=10.0 upon the last, rate-limiting step of water oxidation. By turning into a strong base, tyrosine assists then in abstracting a proton from the bound substrate water and helps to drive the dioxygen formation.

A new system for the measurement of electrogenicity produced by ion pumps using a thin polymer film: examination of wild type bacteriorhodopsin and the D96N mutant over a wide pH range

We developed a new assay system for the measurement of capacitive electric currents generated by ion pumps using the thin polymer film 'Lumirror' (Toray Co., Japan). This system enables us to examine the electrogenicity of ion pumps over a wide range of experimental conditions with high reproducibility due to the mechanical and chemical stability, the high electric resistance and the high electric capacitance of the thin polymer film. Using this method, we examined the photoelectric response of wild type bacteriorhodopsin and its D96N mutant over a wide pH range (2.8-10.0). The results were explained in terms of the affinities of the proton binding sites for translocated protons. A possibility that the direction of the proton transfer from the Schiff base was influenced by the protonation/deprotonation state of the surrounding proton binding sites was suggested. We also found that this film can be used as a substrate for atomic force microscopy (AFM) samples and hence the active purple membrane was observed with AFM.

The roles of the two proton input channels in cytochrome c oxidase from Rhodobacter sphaeroides probed by the effects of site-directed mutations on time-resolved electrogenic intraprotein proton transfer

The crystal structures of cytochrome c oxidase from both bovine and Paracoccus denitrificans reveal two putative proton input channels that connect the heme-copper center, where dioxygen is reduced, to the internal aqueous phase. In this work we have examined the role of these two channels, looking at the effects of site-directed mutations of residues observed in each of the channels of the cytochrome c oxidase from Rhodobacter sphaeroides. A photoelectric technique was used to monitor the time-resolved electrogenic proton transfer steps associated with the photo-induced reduction of the ferryl-oxo form of heme a3 (Fe4+ = O2-) to the oxidized form (Fe3+OH-). This redox step requires the delivery of a "chemical" H+ to protonate the reduced oxygen atom and is also coupled to proton pumping. It is found that mutations in the K channel (K362M and T359A) have virtually no effect on the ferryl-oxo-to-oxidized (F-to-Ox) transition, although steady-state turnover is severely limited. In contrast, electrogenic proton transfer at this step is strongly suppressed by mutations in the D channel. The results strongly suggest that the functional roles of the two channels are not the separate delivery of chemical or pumped protons, as proposed recently [Iwata, S., Ostermeier, C., Ludwig, B. & Michel, H. (1995) Nature (London) 376, 660-669]. The D channel is likely to be involved in the uptake of both "chemical" and "pumped" protons in the F-to-Ox transition, whereas the K channel is probably idle at this partial reaction and is likely to be used for loading the enzyme with protons at some earlier steps of the catalytic cycle. This conclusion agrees with different redox states of heme a3 in the K362M and E286Q mutants under aerobic steady-state turnover conditions.

Temperature dependence of the electrogenic reaction in the QB site of the Rhodobacter sphaeroides photosynthetic reaction center: the QA-QB --> QAQB- transition

The temperature dependencies for the kinetics and relative amplitudes of electrogenic reaction(s) coupled with the first reduction of the secondary quinone acceptor QB were measured with dark-adapted chromatophores of Rhodobacter sphaeroides. The kinetics, while acceptably fitted by a single exponent at room temperature, clearly split into two components below 15 degrees C (rise times, 25 micros and 300 micros at pH 7.0 and 10 degrees C) with the slow phase ousting the fast one at pH > 9.0. The activation energies of the fast and slow phases were estimated at pH 7.0 as < 10 kJ/mol and 60-70 kJ/mol, respectively. To explain the kinetic heterogeneity of the QB --> QB- transition, we suggest two possible conformations for the neutral oxidized ubiquinone at the QB site: one with a hydrogen bond between the side chain carboxyl of Glu-L212 and the methoxy oxygen at C3 of the QB ring (QB-H-Glu centers) and the other one, without this bond (QB:Glu- centers). The fast phase is attributed to QA- QB-H-Glu --> QA QB-H-Glu transition, whereas the slow one to the QA- QB:Glu- --> QA- QB-H-Glu --> QA QB(-)-H-Glu transition.

Near-IR absorbance changes and electrogenic reactions in the microsecond-to-second time domain in Photosystem I

The back-reaction kinetics in Photosystem I (PS I) were studied on the microsecond-to-s time scale in cyanobacterial preparations, which differed in the number of iron-sulfur clusters to assess the contributions of particular components to the reduction of P700+. In membrane fragments and in trimeric P700-FA/FB complexes, the major contribution to the absorbance change at 820 nm (delta A820) was the back-reaction of FA- and/or FB- with lifetimes of approximately 10 and 80 ms (approximately 10% and 40% relative amplitude). The decay of photoinduced electric potential (delta psi) across a membrane with directionally incorporated P700-FA/FB complexes had similar kinetics. HgCl2-treated PS I complexes, which contain FA but no FB, retain both of these kinetic components, indicating that neither can be assigned uniquely to a specific acceptor. These results suggest that FA- reduces P700+ directly and argue for a rapid electron equilibration between FA and FB, which would eliminate their kinetic distinction in a back-reaction. In PsaC-depleted P700-Fx cores, as well as in P700-FA/FB complexes with chemically reduced FA and FB, the major contribution to the delta A820 and the delta psi decay is a biphasic back-reaction of F-X (approximately 400 microseconds and 1.5 ms) with some contribution from A-1 (approximately 10 microseconds and 100 microseconds), the latter of which is variable depending on experimental conditions. The delta A820 decay in a P700-A1 core devoid of all iron-sulfur clusters comprises two phases with lifetimes of 10 microseconds and 130 microseconds (2.7:1 ratio). The biexponential back-reaction kinetics found for each of the electron acceptors may be related to existence of different conformational states of the PS I complex. In all preparations studied, excitation at 532 nm with flash energies exceeding 10 mJ gives rise to formation of antenna 3Chl, which also contributes to delta A820 decay on the tens-of-microsecond time scale. A distinction between delta A820 components related to back-reactions and to 3Chl decay can be made by analysis of flash saturation dependencies and by measurements of kinetics with preoxidized P700.

Flash-induced membrane potential generation by cytochrome c oxidase

Flash-induced single-electron reduction of cytochrome c oxidase. Compound F (oxoferryl state) by RuII(2,2'-bipyridyl)3(2+) [Nilsson (1992) Proc. Natl. Acad. Sci. USA 89, 6497-6501] gives rise to three phases of membrane potential generation in proteoliposomes with tau values and contributions of ca. 45 microsecond (20%), 1 ms (20%) and 5 ms (60%). The rapid phase is not sensitive to the binuclear centre ligands, such as cyanide or peroxide, and is assigned to vectorial electron transfer from CuA to heme a. The two slow phases kinetically match reoxidation of heme a, require added H2O2 or methyl peroxide for full development, and are completely inhibited by cyanide; evidently, they are associated with the reduction of Compound F to the Ox state by heme a. The charge transfer steps associated with the F to Ox conversion are likely to comprise (i) electrogenic uptake of a 'chemical' proton from the N phase required for protonation of the reduced oxygen atom and (ii) electrogenic H+ pumping across the membrane linked to the F to Ox transition. Assuming heme a 'electrical location' in the middle of the dielectric barrier, the ratio of the rapid to slow electrogenic phase amplitudes indicates that the F to Ox transition is linked to transmembrane translocation of 1.5 charges (protons) in addition to an electrogenic uptake of one 'chemical' proton required to form Fe(3+)-OH- from Fe4+ = O2-. The shortfall in the number of pumped protons and the biphasic kinetics of the millisecond part of the electric response matching biphasic reoxidation of heme a may indicate the presence of 2 forms of Compound F, reduction of only one of which being linked to full proton pumping.

Electrogenic steps in the redox reactions catalyzed by photosynthetic reaction-centre complex from Rhodopseudomonas viridis

Electrogenic and redox events in the reaction-centre complexes from Rhodopseudomonas viridis have been studied. In contrast to the previous points of view it is shown that all the four hemes of the tightly bound cytochrome c have different Em values (-60, +20, +310 and +380 mV). The first three hemes reveal alpha absorption maxima at 554 nm, 552 nm and 556 nm respectively. The 380-mV heme displays a split alpha band with a maximum at 559 nm and a shoulder at 552 nm. Such a splitting is due to non-degenerated Qx and Qy transitions in the iron-porphyrin ring as demonstrated by magnetic circular dichroism spectra. Fast kinetic measurements show that, at redox potentials when only high-potential hemes c-559 and c-556 are reduced, heme c-559 appears to be the electron donor to P-960+ (tau = 0.32 microsecond) whereas heme c-556 serves to rereduce c-559 (tau = 2.5 microsecond). Upon reduction of the third heme (c-552), the P-960+ reduction rate increases twofold (tau = 0.17 microsecond) and all photoinduced redox events within the cytochrome appear to be complete in less than 1 microsecond after the flash. The following sequence of the redox centers is tentatively suggested: c-554, c-556, c-552, c-559, P-960. To study electrogenesis, the reaction-centre complexes from Rps. viridis were incorporated into asolectin liposomes, and fast kinetics of laser flash-induced electric potential difference has been measured in proteoliposomes adsorbed on a phospholipid-impregnated film. The electrical difference induced by a single 15-ns flash was found to be as high as 100 mV. The photoelectric response has been found to involve four electrogenic stages associated with (I) QA reduction by P-960; (II) reduction of P-960+ by heme c-559; (III) reduction of c-559 by c-556 and (IV) protonation of Q2-B. The relative contributions of stages I, II, III and IV are found to be equal to 70%, 15%, 5% and 10%, respectively, of the overall electrogenic process. At the same time, the first three respective distances along the axis normal to the membrane plane covered by electrons, calculated from X-ray data of Deisenhofer et al. [J. Mol. Biol. 180, 385-398 (1984)], are 22%, 18.5% and 26%. This indicates that the efficiency of electrogenic phases depends first of all upon the value of the dielectric constant of the respective membrane regions rather than upon the distance between the redox groups involved.(ABSTRACT TRUNCATED AT 400 WORDS)

The sodium cycle. I. Na+-dependent motility and modes of membrane energization in the marine alkalotolerant vibrio Alginolyticus

Respiration, membrane potential generation and motility of the marine alkalotolerant Vibrio alginolyticus were studied. Subbacterial vesicles competent in NADH oxidation and delta psi generation were obtained. The rate of NADH oxidation by the vesicles was stimulated by Na+ in a fashion specifically sensitive to submicromolar HQNO (2-heptyl-4-hydroxyquinoline N-oxide) concentrations. The same amounts of HQNO completely suppressed the delta psi generation. Delta psi was also inhibited by cyanide, gramicidin D and by CCCP + monensin. CCCP (carbonyl cyanide m-chlorophenylhydrazone) added without monensin exerted a much weaker effect on delta psi. Na+ was required to couple NADH oxidation with delta psi generation. These findings are in agreement with the data of Tokuda and Unemoto on Na+-motive NADH oxidase in V. alginolyticus. Motility of V. alginolyticus cells was shown to be (i) Na+-dependent, (ii) sensitive to CCCP + monensin combination, whereas CCCP and monensin, added separately, failed to paralyze the cells, (iii) sensitive to combined treatment by HQNO, cyanide or anaerobiosis and arsenate, whereas inhibition of respiration without arsenate resulted only in a partial suppression of motility. Artificially imposed delta pNa, i.e., addition of NaCl to the K+ -loaded cells paralyzed by HQNO + arsenate, was shown to initiate motility which persisted for several minutes. Monensin completely abolished the NaCl effect. Under the same conditions, respiration-supported motility was only slightly lowered by monensin. The artificially-imposed delta pH, i.e., acidification of the medium from pH 8.6 to 6.5 failed to activate motility. It is concluded that delta mu Na+ produced by (i) the respiratory chain and (ii) an arsenate-sensitive anaerobic mechanism (presumably by glycolysis + Na+ ATPase) can be consumed by an Na+ -motor responsible for motility of V. alginolyticus.

Proteinase-treated photoreceptor discs. Photoelectric activity of the partially-digested rhodopsin and membrane orientation

Photoreceptor discs from rod outer segments of cattle retina were treated with (a) papain, (b) thermolysin or (c) trypsin, the procedures resulting in the cleavage of the rhodopsin polypeptide chain between (a) 323 and 324, 236 and 237, 241 and 242, (b) 327 and 328, 240 and 241, or (c) 339 and 340 amino acid residues, respectively. In all the cases, partially digested rhodopsins proved to be competent in generating photoelectric potential and increasing membrane conductance of the discs adsorbed onto phospholipid-impregnated collodion film. The kinetics of generation and dissipation of photopotential as well as of formation of metarhodopsin II and of the light-induced rhodopsin protonation were found to be the same in the partially digested preparations and in the intact one. Incubation of papain-treated or thermolysin-treated discs at pH 6.0 induced formation of inside-out vesicles which, when incorporated into the collodion film, generated an oppositely directed photopotential. Treatment of such vesicles with papain gave rise to further cleavages of the polypeptide localized between 30 and 31, 186 and 187 amino acid residues. One more proteinase-sensitive site, localized between 104 and 105 residues, has been discovered in the inside-out vesicles treated with thermolysin. This fact consistent with the scheme of the 'seven column' arrangement of the visual rhodopsin [Ovchinnikov, Yu. A. (1982) FEBS Lett. 148, 179-191]. Rhodopsin, when treated with papain on both sides, was deprived of sixty amino acid residues being split in two sites in the middle part of the polypeptide, but was still active as a photoelectric energy transducer. The main specific feature inherent in the photoelectric response of the papain-treated or thermolysin-treated rhodopsin and absent from the native protein is that the former survives addition of long trains of saturating flashes when the response of the intact preparation becomes negligible. This effect was shown to be due to conversion of partially digested rhodopsin to a photolytic product that at room temperature lived for minutes even in the presence of NH2OH. A 532-nm laser flash effectively converted this product back to rhodopsin.

The action of lanthanum ions and formaldehyde on the proton-pumping function of bacteriorhodopsin

The photochemical cycle and the proton-pumping function of bacteriorhodopsin modified with lanthanum and formaldehyde has been studied. In both preparations, the M412 leads to BR570 transition time has been found to increase considerably. The deceleration of the photochemical cycle has been shown to be accompanied by inhibition of the millisecond phase of the photoelectrical response of bacteriorhodopsin membranes associated with phospholipid-impregnated collodion film. Photoelectrogenic activity measured with permeable ion probe in proteoliposomes was also inhibited. Effects of lanthanum were reversed by EDTA. Formation of M412 was slightly accelerated and the microsecond electrogenic phase was not affected by lanthanum and by formaldehyde. It is concluded that lanthanum, but not formaldehyde, can be used as a specific reversible inhibitor of the second half of the bacteriorhodopsin photocycle and of the associated H+ uptake on the cytoplasmic side of the halobacterial membrane. Possible mechanisms of these effects are discussed.

Fast stages of photoelectric processes in biological membranes. I. Bacteriorhodopsin

Bacteriorhodopsin-containing fragments of Halobacterium halobium membrane (bacteriorhodopsin sheets) were incorporated into a lecithin-impregnated collodion film, and fast stages of flash-induced electrogenesis were measured by two electrodes separated by this film. It is found that a single turnover of bacteriorhodopsin results in an electrogenic response composed of three main stages of the following tau: the first less than 200 ns, the second 15 - 70 microseconds and the third 10 ms. The second and third phases are of the same direction as an electric response to continuous illumination, whereas the first one is oppositely directed. The microseconds and ms stages were shown to correlate, in the first approximation, with formation and decomposition of the bacteriorhodopsin intermediate absorbing with 412 nm, respectively. Both the second and third phases of the photoelectric response are sums of at least two exponents. The third stage is specifically inhibition by La3+ ions which are also shown to decrease the rate of regeneration of the original bacteriorhodopsin absorbing at 570 nm from the intermediate absorbing at 412 nm. Acidification of the medium induces parallel inhibition of the second and third phases and of formation of the intermediate absorbing at 412 nm as if protonation of a group with pK = 3.6 were responsible for this inhibition. The first (opposite) phase survives acidification. It even increases at pH lower than 1.5. At such a low pH, one can show a good correlation of decays of photopotential and of a bacteriorhodopsin bathointermediate. The decays are biphasic (tau 1 = 200 microseconds and tau 2 = 2 ms), formation of both the photopotential and the bathointermediate being faster than 200 ns. At higher pH, when a three-phase photoelectric response is revealed, decay of the formed electric potential difference gives the average tau value of about 1 s. It can be accelerated by compounds that increase ionic conductance of biomembranes. At pH below 4, fluoride is found to completely inhibit the second and third phases, so that only the first phase is observed. The results are discussed in terms of a scheme postulating that the first electrogenic phase is a result of translocation of the protonated Schiff base inside the membrane due to a light-induced conformation change in retinal or protein. The second and third phases are explained by H+ transfer from the Schiff base to the outer membrane surface and from inner (cytoplasmic) surface of membrane to the Schiff base, respectively.

Fast stages of photoelectric processes in biological membranes. II. Visual rhodopsin

The functioning of visual rhodopsin as a photoelectric generator has been demonstrated with a direct method. Photoreceptor discs were incorporated into a phospholipid-impregnated collodion film. Illumination of the resulting system with continuous light was found to induce formation of an electric potential (the disc-free side positive) that was measured with two electrodes separated by the film. A photopotential exceeding 40 mV was shown. It dissipated before the light source was switched off. A 15 ns 530-nm laser flash induced the formation of a photopotential of up to 35 mV whose appearance was preceded with a small oppositely directed electrogenic phase. This "negative" photoresponse took less than 200 ns. The "positive" photoresponse was composed of at least two phases (t 1/2 about 500 microseconds and several milliseconds). The latter was shown to correlate with formation of metarhodopsin II. A 347-nm laser flash added after a 530-nm flash resulted in a photoelectric effect similar to that initiated by 530-nm flash but of opposite direction. The 347-nm response was completely abolished by hydroxylamine preventing the accumulation of metarhodopsin II. The response at 530 nm proved to be hydroxylamine-resistant. Both the amplitude and the decay time of the flash-induced potential were maximal in the response to the first flash, each subsequent flash being less effective than the preceding one. Flashes were found to cause acceleration of the photopotential decay. The latter effect proved to be due to a increase of membrane conductance that developed faster than in 50 ms. Addition of 11-cis retinal after illumination improved the amplitude of the photoresponse but not the conductance. The light-induced increase in conductance was insensitive to hydroxylamine. It is suggested that a function of visual rhodopsin consists of generating a potential difference across the photoreceptor disc membrane which responds with a increase in membrane permeability to a rise of the membrane potential. A possible role of an electric break-down of the membrane, induced by the rhodopsin-generated local or partially delocalized electric field has been discussed.

Reconstitution of biological molecular generators of electric current. Inorganic pyrophosphatase

Proteoliposomes have been reconstituted from soy-bean phospholipids (asolectin) and inorganic pyrophosphatase isolated from Rhodospirillum rubrum chromatophores. In the presence of Mg2+ ions, pyrophosphatase proteoliposomes were incorporated into a phospholipid-impregnated Teflon filter separating two solutions of an identical electrolyte content. Addition of inorganic pyrophosphate to the same compartment as proteoliposomes was found to induce generation of an electric potential difference between the two filter-separated compartments, the proteoliposomes-containing compartment being negatively charged. An electric potential difference of 15 mV and a current of 20 pA were observed. The electrogenic effect required Mg2+ and proved to be sensitive to fluoride, an inorganic pyrophosphatase inhibitor. Treatment with 10 microM N,N'-dicyclohexylcarbodiimide for several minutes was without influence upon pyrophosphate-induced membrane potential generation. Similar results were obtained in experiments with a proteoliposome suspension and a penetrating anion, tetraphenyl borate, which is a probe for membrane potential. The obtained data are discussed in connection with the results of studies on other enzymes as molecular generators of electric current.

Reconstitution of biological molecular generators of electric current. Transhydrogenase

Direct measurement of the electrogenic activity of purified mitochondrial transhydrogenase has been carried out. To this end, beef-heart transhydrogenase was isolated and reconstituted with phospholipids to form proteoliposomes. The transhydrogenase proteoliposomes were incorporated into a membrane filter impregnated with a decane solution of phospholipids. It is shown that addition of substrates of either the forward (NADPH and NAD+) or the reverse (NADH and NADP+) transhydrogenase reaction gives rise to an electric potential difference across the proteoliposome-treated membrane filter. The electric vector depends upon the direction of the reaction. The proteoliposome-supplemented compartment charges negatively in the case of the forward reaction and positively in the case of the reverse one. Addition of the reaction products after substrates equalizes the potentials. The transhydrogenase-treated membrane filter retains the ability to perform transhydrogenase-linked electrogenesis after removal of excess non-incorporated proteoliposomes. The electric potential difference reaching 20 mV immediately after the transhydrogenase substrate addition, slowly decreases due to accumulation of the reaction products. Such decay is prevented when the mixture is supplemented with the substrate-regenerating and product-utilizing enzymic systems. Under these conditions, a steady continuous electric current of about 10 pA can be observed.