K+-dependence of electrogenic transport by the NaK-ATPase

Distinctive differences in the granulation of saline and non-saline enriched anaerobic ammonia oxidizing (AMX) bacteria

The growing interest in the anaerobic ammonium oxidizing (AMX) process in treating high nitrogen containing wastewaters and a comprehensive study into the granulation mechanism of these bacteria under diverse environmental conditions over the years have been unequal. To this effect, the distinctive differences in saline adapted AMX (S_AMX) and non-saline adapted AMX (NS_AMX) granules are presented in this study. It was observed that substrate utilisation profiles, granule formation mechanism, and pace towards granulation differed marginally for the two adaptation conditions. The different microbial dominant aggregation types aided in splitting the 471 days operated lab-scale SBRs into three distinct phases. In both reactors, phase III (granules dominant phase) showed the highest average nitrogen removal efficiency of 87.9% ± 4.8% and 85.6% ± 3.6% for the S_AMX and NS_AMX processes, respectively. The extracellular polymeric substances (EPS) quantity and major composition determined its role either as a binding agent in granulation or a survival mechanism in saline adaptation. It was also observed that granules of the S_AMX reactor were mostly loosely and less condensed aggregates of smaller sub-units and flocs while those of the NS_AMX reactor were compact agglomerates. The ionic gradient in saline enrichment led to an increased activity of the Na⁺/K⁺ - ATPase, hence enriched granules produced higher cellular adenosine triphosphate molecules which finally improved the granules active biomass ratio by 32.96%. Microbial community showed that about three to four major known AMX species made up the granules consortia in both reactors. Proteins and expression of functional genes differed for these different species.

A quantitative flux assay for the study of reconstituted Cl- channels and transporters

The recent deluge of high-resolution structural information on membrane proteins has not been accompanied by a comparable increase in our ability to functionally interrogate these proteins. Current functional assays often are not quantitative or are performed in conditions that significantly differ from those used in structural experiments, thus limiting the mechanistic correspondence between structural and functional experiments. A flux assay to determine quantitatively the functional properties of purified and reconstituted Cl^- channels and transporters in membranes of defined lipid compositions is described. An ion-sensitive electrode is used to measure the rate of Cl^- efflux from proteoliposomes reconstituted with the desired protein and the fraction of vesicles containing at least one active protein. These measurements enable the quantitative determination of key molecular parameters such as the unitary transport rate, the fraction of proteins that are active, and the molecular mass of the transport protein complex. The approach is illustrated using CLC-ec1, a CLC-type H⁺/Cl^- exchanger as an example. The assay enables the quantitative study of a wide range of Cl^- transporting molecules and proteins whose activity is modulated by ligands, voltage, and membrane composition as well as the investigation of the effects of compounds that directly inhibit or activate the reconstituted transport systems. The present assay is readily adapted to the study of transport systems with diverse substrate specificities and molecular characteristics, and the necessary modifications needed are discussed.

A Loose Relationship: Incomplete H+/Sugar Coupling in the MFS Sugar Transporter GlcP

The glucose transporter from Staphylococcus epidermidis, GlcP_Se, is a homolog of the human GLUT sugar transporters of the major facilitator superfamily. Together with the xylose transporter from Escherichia coli, XylE_Ec, the other prominent prokaryotic GLUT homolog, GlcP_Se, is equipped with a conserved proton-binding site arguing for an electrogenic transport mode. However, the electrophysiological analysis of GlcP_Se presented here reveals important differences between the two GLUT homologs. GlcP_Se, unlike XylE_Ec, does not perform steady-state electrogenic transport at symmetrical pH conditions. Furthermore, when a pH gradient is applied, partially uncoupled transport modes can be generated. In contrast to other bacterial sugar transporters analyzed so far, in GlcP_Se sugar binding, translocation and release are also accomplished by the deprotonated transporter. Based on these experimental results, we conclude that coupling of sugar and H⁺ transport is incomplete in GlcP_Se. To verify the viability of the observed partially coupled GlcP_Se transport modes, we propose a universal eight-state kinetic model in which any degree of coupling is realized and H⁺/sugar symport represents only a specific instance. Furthermore, using sequence comparison with strictly coupled XylE_Ec and similar sugar transporters, we identify an additional charged residue that may be essential for effective H⁺/sugar symport.

SSM-Based Electrophysiology for Transporter Research

Functional characterization of transport proteins using conventional electrophysiology can be challenging, especially for low turnover transporters or transporters from bacteria and intracellular compartments. Solid-supported membrane (SSM)-based electrophysiology is a sensitive and cell-free assay technique for the characterization of electrogenic membrane proteins. Purified proteins reconstituted into proteoliposomes or membrane vesicles from cell culture or native tissues are adsorbed to the sensor holding an SSM. A substrate or a ligand is applied via rapid solution exchange. The electrogenic transporter activity charges the sensor, which is recorded as a transient current. The high stability of the SSM allows cumulative measurements on the same sensor using different experimental conditions. This allows the determination of kinetic properties including EC₅₀, IC₅₀, K_m, K_D, and rate constants of electrogenic reactions. About 100 different transporters have been measured so far using this technique, among them symporters, exchangers, uniporters, ATP-, redox-, and light-driven ion pumps, as well as receptors and ion channels. Different instruments apply this technique: the laboratory setups use a closed flow-through arrangement, while the commercially available SURFE²R N1 resembles a pipetting robot. For drug screening purposes high-throughput systems, such as the SURFE²R 96SE enable the simultaneous measurement of up to 96 sensors.

Kinetics of K(+) occlusion by the phosphoenzyme of the Na(+),K(+)-ATPase

Investigations of K(+)-occlusion by the phosphoenzyme of Na(+),K(+)-ATPase from shark rectal gland and pig kidney by stopped-flow fluorimetry reveal major differences in the kinetics of the two enzymes. In the case of the pig enzyme, a single K(+)-occlusion step could be resolved with a rate constant of 342 (± 26) s⁻¹. However, in the case of the shark enzyme, two consecutive K(+)-occlusions were detected with rate constants of 391 (± 19) s⁻¹ and 48 (± 2) s⁻¹ at 24°C and pH 7.4. A conformational change of the phosphoenzyme associated with K(+)-occlusion is, thus, the major rate-determining step of the shark enzyme under saturating concentrations of all substrates, whereas for the pig enzyme the major rate-determining step under the same conditions is the E2 → E1 transition and its associated K(+) deocclusion and release to the cytoplasm. The differences in rate constants of the K(+) occlusion reactions of the two enzymes are paralleled by compensating changes to the rate constant for the E2 → E1 transition, which explains why the differences in the enzymes' kinetic behaviors have not previously been identified.

Versatile (1)H-(31)P-(31)P COSY 2D NMR techniques for the characterization of polyphosphorylated small molecules

Di- and triphosphorylated small molecules represent key intermediates in a wide range of biological and chemical processes. The importance of polyphosphorylated species in biology and medicine underscores the need to develop methods for the detection and characterization of this compound class. We have reported two-dimensional HPP-COSY spectroscopy techniques to identify diphosphate-containing metabolic intermediates at submillimolar concentrations in the methylerythritol phosphate (MEP) isoprenoid biosynthetic pathway. (1) In this work, we explore the scope of HPP-COSY-based techniques to characterize a diverse group of small organic molecules bearing di- and triphosphorylated moieties. These include molecules containing P-O-P and P-C-P connectivities, multivalent P(III)-O-P(V) phosphorus nuclei with widely separated chemical shifts, as well as virtually overlapping (31)P resonances exhibiting strong coupling effects. We also demonstrate the utility of these experiments to rapidly distinguish between mono- and diphosphates. A detailed protocol for optimizing these experiments to achieve best performance is presented.

Delineating electrogenic reactions during lactose/H+ symport

Electrogenic reactions accompanying downhill lactose/H(+) symport catalyzed by the lactose permease of Escherichia coli (LacY) have been assessed using solid-supported membrane-based electrophysiology with improved time resolution. Rates of charge translocation generated by purified LacY reconstituted into proteoliposomes were analyzed over a pH range from 5.2 to 8.5, which allows characterization of two electrogenic steps in the transport mechanism: (i) a weak electrogenic reaction triggered by sugar binding and observed under conditions where H(+) translocation is abolished either by acidic pH or by a Glu325 --> Ala mutation in the H(+) binding site (this step with a rate constant of approximately 200 s(-1) for wild-type LacY leads to an intermediate proposed to represent an "occluded" state) and (ii) a major electrogenic reaction corresponding to 94% of the total charge translocated at pH 8, which is pH-dependent with a maximum rate of approximately 30 s(-1) and a pK of 7.5. This partial reaction is assigned to rate-limiting H(+) release on the cytoplasmic side of LacY during turnover. These findings together with previous electrophysiological results and biochemical-biophysical studies are included in an overall kinetic mechanism that allows delineation of the electrogenic steps in the reaction pathway.

Electrophysiological characterization of ATPases in native synaptic vesicles and synaptic plasma membranes

Vesicular V-ATPase (V-type H+-ATPase) and the plasma membrane-bound Na+/K+-ATPase are essential for the cycling of neurotransmitters at the synapse, but direct functional studies on their action in native surroundings are limited due to the poor accessibility via standard electrophysiological equipment. We performed SSM (solid supported membrane)-based electrophysiological analyses of synaptic vesicles and plasma membranes prepared from rat brains by sucrose-gradient fractionation. Acidification experiments revealed V-ATPase activity in fractions containing the vesicles but not in the plasma membrane fractions. For the SSM-based electrical measurements, the ATPases were activated by ATP concentration jumps. In vesicles, ATP-induced currents were inhibited by the V-ATPase-specific inhibitor BafA1 (bafilomycin A1) and by DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulfonate). In plasma membranes, the currents were inhibited by the Na+/K+-ATPase inhibitor digitoxigenin. The distribution of the V-ATPase- and Na+/K+-ATPase-specific currents correlated with the distribution of vesicles and plasma membranes in the sucrose gradient. V-ATPase-specific currents depended on ATP with a K0.5 of 51+/-7 microM and were inhibited by ADP in a negatively co-operative manner with an IC50 of 1.2+/-0.6 microM. Activation of V-ATPase had stimulating effects on the chloride conductance in the vesicles. Low micromolar concentrations of DIDS fully inhibited the V-ATPase activity, whereas the chloride conductance was only partially affected. In contrast, NPPB [5-nitro-2-(3-phenylpropylamino)-benzoic acid] inhibited the chloride conductance but not the V-ATPase. The results presented describe electrical characteristics of synaptic V-ATPase and Na+/K+-ATPase in their native surroundings, and demonstrate the feasibility of the method for electrophysiological studies of transport proteins in native intracellular compartments and plasma membranes.

Inhibitory effect of Pb2+ on the transport cycle of the Na+,K+-ATPase

The effect of Pb(2+) on the transport cycle of the Na(+),K(+)-ATPase was characterized in detail at a molecular level by combining electrical and biochemical measurements. Electrical measurements were performed by adsorbing purified membrane fragments containing Na(+),K(+)-ATPase on a solid-supported membrane. Upon adsorption, the Na(+),K(+)-ATPase was activated by carrying out concentration jumps of different activating substrates, for example, Na(+) and ATP. Charge movements following Na(+),K(+)-ATPase activation were measured in the presence of various Pb(2+) concentrations to investigate the effect of Pb(2+) on different ion translocating steps of the pump cycle. These charge measurements were then compared to biochemical measurements of ATPase activity in the presence of increasing Pb(2+) concentration. Our results indicate that Pb(2+) inhibits cycling of the enzyme, but it does not affect cytoplasmic Na(+) binding and release of Na(+) ions at the extracellular side at concentrations below 10 muM. To explain the inhibitory effect of Pb(2+) on the Na(+),K(+)-ATPase, we propose that Pb(2+) may interfere with the hydrolytic cleavage of the phosphorylated intermediate E(2)P, which occurs in the K(+)-related branch of the pump cycle.

Electrogenic ion pumps investigated on a solid supported membrane: comparison of current and voltage measurements

Current and voltage measurements were performed on Na,K-ATPase and sarcoplasmic reticulum (SR) Ca-ATPase. Measurements of current transients under short-circuit conditions and of voltage transients under open-circuit conditions were carried out by employing a solid supported membrane (SSM). Purified membrane fragments containing Na,K-ATPase or native SR vesicles were adsorbed on a SSM and were activated by performing substrate concentration jumps. Current and voltage transients were recorded in the external circuit. They are related to pump activity and can be attributed to electrogenic events in the reaction cycles of the two enzymes. While current transients of very small amplitude are difficult to detect, the corresponding voltage transients can be measured with higher accuracy because of a much more favorable signal-to-noise ratio. Therefore, voltage measurements are preferable for the investigation of slow processes generating low current signals, e.g., for the analysis of low turnover transporters.

Caged protein prenyltransferase substrates: tools for understanding protein prenylation

Originally designed to block the prenylation of oncogenic Ras, inhibitors of protein farnesyltransferase currently in preclinical and clinical trials are showing efficacy in cancers with normal Ras. Blocking protein prenylation has also shown promise in the treatment of malaria, Chagas disease and progeria syndrome. A better understanding of the mechanism, targets and in vivo consequences of protein prenylation are needed to elucidate the mode of action of current PFTase (Protein Farnesyltransferase) inhibitors and to create more potent and selective compounds. Caged enzyme substrates are useful tools for understanding enzyme mechanism and biological function. Reported here is the synthesis and characterization of caged substrates of PFTase. The caged isoprenoid diphosphates are poor substrates prior to photolysis. The caged CAAX peptide is a true catalytically caged substrate of PFTase in that it is to not a substrate, yet is able to bind to the enzyme as established by inhibition studies and X-ray crystallography. Irradiation of the caged molecules with 350 nm light readily releases their cognate substrate and their photolysis products are benign. These properties highlight the utility of those analogs towards a variety of in vitro and in vivo applications.

An automatic electrophysiological assay for the neuronal glutamate transporter mEAAC1

A rapid and robust electrophysiological assay based on solid supported membranes (SSM) for the murine neuronal glutamate transporter mEAAC1 is presented. Measurements at different concentrations revealed the EAAC1 specific affinities for l-glutamate (K(m)=24microM), l-aspartate (K(m)=5microM) and Na(+) (K(m)=33mM) and an inhibition constant K(i) for dl-threo-beta-benzyloxyaspartic acid (TBOA) of 1microM. Inhibition by 3-hydroxy-4,5,6,6a-tetrahydro-3aH-pyrrolo[3,4-d]isoxazole-6-carboxylic acid (HIP-B) was not purely competitive with an IC(50) of 13microM. Experiments using SCN(-) concentration jumps yielded large transient currents in the presence of l-glutamate showing the characteristics of the glutamate-gated anion conductance of EAAC1. Thus, SSM-based electrophysiology allows the analysis of all relevant transport modes of the glutamate transporter on the same sample. K(+) and Na(+) gradients could be applied to the transporter. Experiments in the presence and absence of Na(+) and K(+) gradients demonstrated that the protein is still able to produce a charge translocation when no internal K(+) is present. In this case, the signal amplitude is smaller and a lower apparent affinity for l-glutamate of 144microM is found. Finally the assay was adapted to a commercial fully automatic system for SSM-based electrophysiology and was validated by determining the substrate affinities and inhibition constants as for the laboratory setup. The combination of automatic function and its ability to monitor all transport modes of EAAC1 make this system an universal tool for industrial drug discovery.

SSM-based electrophysiology

An assay technique for the electrical characterization of electrogenic transport proteins on solid supported membranes is presented. Membrane vesicles, proteoliposomes or membrane fragments containing the transporter are adsorbed to the solid supported membrane and are activated by providing a substrate or a ligand via a rapid solution exchange. This technique opens up new possibilities where conventional electrophysiology fails like transporters or ion channels from bacteria and from intracellular compartments. Its rugged design and potential for automation make it suitable for drug screening.

Identification and function of a cytoplasmic K+ site of the Na+, K+ -ATPase

A cytoplasmic nontransport K(+)/Rb(+) site in the P-domain of the Na(+), K(+)-ATPase has been identified by anomalous difference Fourier map analysis of crystals of the [Rb(2)].E(2).MgF(4)(2-) form of the enzyme. The functional roles of this third K(+)/Rb(+) binding site were studied by site-directed mutagenesis, replacing the side chain of Asp(742) donating oxygen ligand(s) to the site with alanine, glutamate, and lysine. Unlike the wild-type Na(+), K(+)-ATPase, the mutants display a biphasic K(+) concentration dependence of E(2)P dephosphorylation, indicating that the cytoplasmic K(+) site is involved in activation of dephosphorylation. The affinity of the site is lowered significantly (30-200-fold) by the mutations, the lysine mutation being most disruptive. Moreover, the mutations accelerate the E(2) to E(1) conformational transition, again with the lysine substitution resulting in the largest effect. Hence, occupation of the cytoplasmic K(+)/Rb(+) site not only enhances E(2)P dephosphorylation but also stabilizes the E(2) dephosphoenzyme. These characteristics of the previously unrecognized nontransport site make it possible to account for the hitherto poorly understood trans-effects of cytoplasmic K(+) by the consecutive transport model, without implicating a simultaneous exposure of the transport sites toward the cytoplasmic and extracellular sides of the membrane. The cytoplasmic K(+)/Rb(+) site appears to be conserved among Na(+), K(+)-ATPases and P-type ATPases in general, and its mode of operation may be associated with stabilizing the loop structure at the C-terminal end of the P6 helix of the P-domain, thereby affecting the function of highly conserved catalytic residues and promoting helix-helix interactions between the P- and A-domains in the E(2) state.

Charge transfer in P-type ATPases investigated on planar membranes

Planar lipid bilayers, e.g., black lipid membranes (BLM) and solid supported membranes (SSM), have been employed to investigate charge movements during the reaction cycle of P-type ATPases. The BLM/SSM method allows a direct measurement of the electrical currents generated by the cation transporter following chemical activation by a substrate concentration jump. The electrical current transients provides information about the reaction mechanism of the enzyme. In particular, the BLM/SSM technique allows identification of electrogenic steps which in turn may be used to localize ion translocation during the reaction cycle of the pump. In addition, using the high time resolution of the technique, especially when rapid activation via caged ATP is employed, rate constants of electrogenic and electroneutral steps can be determined. In the present review, we will discuss the main results obtained by the BLM and SSM methods and how they have contributed to unravel the transport mechanism of P-type ATPases.

Importance of Leu99 in Transmembrane Segment M1 of the Na+,K+-ATPase in the Binding and Occlusion of K+

Twenty-six point mutations were introduced into the N-terminal and middle parts of transmembrane segment M1 of the Na+, K+ -ATPase and its cytosolic extension. None of the alterations to charged and polar residues in the N-terminal part of M1 and its cytosolic extension had any major effect on the cation binding properties, thus rejecting the hypothesis that these residues are involved in cation selectivity. By contrast, specific residues in the middle part of M1, particularly Leu(99), were found critical to K+ interaction of the enzyme. Hence, mutation L99A reduced the affinity for K+ activation of E2P dephosphorylation 17-fold, and L99F reduced the equilibrium level of the K+-occluded intermediate [K2]E2 and increased the rate of K+ deocclusion 39-fold, i.e. more than seen for mutation E329Q of the cation-binding glutamate in M4. L99Q affected K+ interaction in yet another way, the equilibrium level of [K2]E2 being slightly increased despite an increased rate of K+ deocclusion, suggesting that the K+ ions leave and enter the occlusion pocket more frequently than in the wild type. L99Q furthermore affected the ability to discriminate between Na+ and K+ on the extracellular side. Our findings can be explained by a structural model in which Leu(99) and Glu(329) interact and cooperate in K+ binding and gating of the K+ sites. The disturbance of K+ interaction in mutants with alteration to Leu(91), Phe(95), Ser(96), or Leu(98) could be a consequence of the roles of these residues in positioning the M1 helix optimally for the interaction between Leu(99) and Glu(329). Phe(95) may serve to stabilize the pivot for movement of M1 through interaction with Ile(287) in M3.

Mutation of Gly-94 in transmembrane segment M1 of Na+,K+-ATPase interferes with Na+ and K+ binding in E2P conformation

The importance of Gly-93 and Gly-94 in transmembrane segment M1 of the Na+,K+-ATPase for interaction with Na+ and K+ was demonstrated by functional analysis of mutants Gly-93-Ala and Gly-94-Ala. In the crystal structures of the Ca2+-ATPase, the corresponding residues, Asp-59 and Leu-60, are located exactly where M1 bends. Rapid kinetic measurements of K+-induced dephosphorylation allowed determination of the affinity of the E2P phosphoenzyme intermediate for K+. In Gly-94-Ala, the K+ affinity was reduced 9-fold, i.e., to the same extent as seen for mutation of the cation-binding residue Glu-329. Furthermore, Gly-94-Ala showed strongly reduced sensitivity of the E1P-E2P equilibrium to Na+, with accumulation of E2P even at 600 mM Na+, indicating that interaction of E2P with extracellular Na+ is impaired. On the contrary, in Gly-93-Ala, the affinity for K+ was slightly increased, and the E1P-E2P equilibrium was displaced in favor of E1P. In both mutants, the affinity of the cytoplasmically facing sites of E1 for Na+ was reduced, but this effect was relatively small compared with the effects seen for E2P in Gly-94-Ala. Comparison with Ca2+-ATPase mutagenesis data suggests that the role of M1 in binding of the transported ions is universal among P-type ATPases, despite the low sequence homology in this region. Structural modeling of Na+,K+-ATPase mutant Gly-94-Ala on the basis of the Ca2+-ATPase crystal structures indicates that the alanine side chain comes close to Ile-287 of M3, particularly in E2P, thus resulting in a steric clash that may explain the present observations.

Effect of ADP on Na(+)-Na(+) exchange reaction kinetics of Na,K-ATPase

The whole-cell voltage-clamp technique was used in rat cardiac myocytes to investigate the kinetics of ADP binding to phosphorylated states of Na,K-ATPase and its effects on presteady-state Na(+)-dependent charge movements by this enzyme. Ouabain-sensitive transient currents generated by Na,K-ATPase functioning in electroneutral Na(+)-Na(+) exchange mode were measured at 23 degrees C with pipette ADP concentrations ([ADP]) of up to 4.3 mM and extracellular Na(+) concentrations ([Na](o)) between 36 and 145 mM at membrane potentials (V(M)) from -160 to +80 mV. Analysis of charge-V(M) curves showed that the midpoint potential of charge distribution was shifted toward more positive V(M) both by increasing [ADP] at constant Na(+)(o) and by increasing [Na](o) at constant ADP. The total quantity of mobile charge, on the other hand, was found to be independent of changes in [ADP] or [Na](o). The presence of ADP increased the apparent rate constant for current relaxation at hyperpolarizing V(M) but decreased it at depolarizing V(M) as compared to control (no added ADP), an indication that ADP binding facilitates backward reaction steps during Na(+)-Na(+) exchange while slowing forward reactions. Data analysis using a pseudo three-state model yielded an apparent K(d) of approximately 6 mM for ADP binding to and release from the Na,K-ATPase phosphoenzyme; a value of 130 s(-1) for k(2), a rate constant that groups Na(+) deocclusion/release and the enzyme conformational transition E(1) approximately P --> E(2)-P; a value of 162 s(-1)M(-1) for k(-2), a lumped second-order V(M)-independent rate constant describing the reverse reactions; and a Hill coefficient of approximately 1 for Na(+)(o) binding to E(2)-P. The results are consistent with electroneutral release of ADP before Na(+) is deoccluded and released through an ion well. The same approach can be used to study additional charge-moving reactions and associated electrically silent steps of the Na,K-pump and other transporters.

Channelrhodopsin-2, a directly light-gated cation-selective membrane channel

Microbial-type rhodopsins are found in archaea, prokaryotes, and eukaryotes. Some of them represent membrane ion transport proteins such as bacteriorhodopsin, a light-driven proton pump, or channelrhodopsin-1 (ChR1), a recently identified light-gated proton channel from the green alga Chlamydomonas reinhardtii. ChR1 and ChR2, a related microbial-type rhodopsin from C. reinhardtii, were shown to be involved in generation of photocurrents of this green alga. We demonstrate by functional expression, both in oocytes of Xenopus laevis and mammalian cells, that ChR2 is a directly light-switched cation-selective ion channel. This channel opens rapidly after absorption of a photon to generate a large permeability for monovalent and divalent cations. ChR2 desensitizes in continuous light to a smaller steady-state conductance. Recovery from desensitization is accelerated by extracellular H+ and negative membrane potential, whereas closing of the ChR2 ion channel is decelerated by intracellular H+. ChR2 is expressed mainly in C. reinhardtii under low-light conditions, suggesting involvement in photoreception in dark-adapted cells. The predicted seven-transmembrane alpha helices of ChR2 are characteristic for G protein-coupled receptors but reflect a different motif for a cation-selective ion channel. Finally, we demonstrate that ChR2 may be used to depolarize small or large cells, simply by illumination.

Charge displacements during ATP-hydrolysis and synthesis of the Na+-transporting FoF1-ATPase of Ilyobacter tartaricus

Transient electrical currents generated by the Na(+)-transporting F(o)F(1)-ATPase of Ilyobacter tartaricus were observed in the hydrolytic and synthetic mode of the enzyme. Two techniques were applied: a photochemical ATP concentration jump on a planar lipid membrane and a rapid solution exchange on a solid supported membrane. We have identified an electrogenic reaction in the reaction cycle of the F(o)F(1)-ATPase that is related to the translocation of the cation through the membrane bound F(o) subcomplex of the ATPase. In addition, we have determined rate constants for the process: For ATP hydrolysis this reaction has a rate constant of 15-30 s(-1) if H(+) is transported and 30-60 s(-1) if Na(+) is transported. For ATP synthesis the rate constant is 50-70 s(-1).

Electrogenic properties of the Na+,K+-ATPase probed by presteady state and relaxation studies

Electrical measurements on planar lipid bilayers, patch/voltage clamp experiments, and spectroscopic investigations involving a potential sensitive dye are reviewed. These experiments were performed to analyze the kinetics of charge translocation of the Na+,K+-ATPase. High time resolution was achieved by applying caged ATP, voltage-jump, and stopped-flow techniques, respectively. Kinetic parameters and the electrogenicity of the relevant transitions in the Na+,K+-ATPase reaction cycle are discussed.

Zn(2+) binding to the cytoplasmic side of Paracoccus denitrificans cytochrome c oxidase selectively uncouples electron transfer and proton translocation

Using a combination of stopped-flow spectrophotometric proton pumping measurements and time-resolved potential measurements on black lipid membranes, we have investigated the effect of Zn(2+) ions on the proton transfer properties of Paracoccus denitrificans cytochrome c oxidase. When zinc was enclosed in the interior of cytochrome c oxidase containing liposomes, the H/e stoichiometry was found to gradually decrease with increasing Zn(2+) concentration. Half-inhibition of proton pumping was observed at [Zn(2+)](i)=75 microM corresponding to about 5-6 Zn(2+) ions per oxidase molecule. In addition, there was a significant increase in the respiratory control ratio of the proteoliposomes upon incorporation of Zn(2+). Time-resolved potential measurements on a black lipid membrane showed that the electrogenic phases slowed down in the presence of Zn(2+) correspond to phases that have been attributed to proton uptake from the cytoplasmic side and to proton pumping. We conclude that Zn(2+) ions bind close to or within the two proton transfer pathways of the bacterial cytochrome c oxidase.

Single-electron reduction of the oxidized state is coupled to proton uptake via the K pathway in Paracoccus denitrificans cytochrome c oxidase

The reductive part of the catalytic cycle of cytochrome c oxidase from Paracoccus denitrificans was examined by using time-resolved potential measurements on black lipid membranes. Proteoliposomes were adsorbed to the black lipid membranes and Ru(II)(2, 2'-bipyridyl)(3)(2+) was used as photoreductant to measure flash-induced membrane potential generation. Single-electron reduction of the oxidized wild-type cytochrome c oxidase reveals two phases of membrane potential generation (tau(1) approximately 20 micros and tau(2) approximately 175 micros) at pH 7.4. The fast phase is not sensitive to cyanide and is assigned to electron transfer from Cu(A) to heme a. The slower phase is inhibited completely by cyanide and shows a kinetic deuterium isotope effect by a factor of 2-3. Although two enzyme variants mutated in the so-called D pathway of proton transfer (D124N and E278Q) show the same time constants and relative amplitudes as the wild-type enzyme, in the K pathway variant K354M, tau(2) is increased to 900 micros. This result suggests uptake of a proton through the K pathway during the transition from the oxidized to the one-electron reduced state. After the second laser flash under anaerobic conditions, a third electrogenic phase with a time constant of approximately 1 ms appears. The amplitude of this phase grows with increasing flash number. We explain this growth by injection of a second electron into the single-electron reduced enzyme. On multiple flashes, both D pathway mutants behave differently compared with the wild type and two additional slow phases of tau(3) approximately 2 ms and tau(4) approximately 15 ms are observed. These results suggest that the D pathway is involved in proton transfer coupled to the uptake of the second electron.

Mutation of Arg-54 strongly influences heme composition and rate and directionality of electron transfer in Paracoccus denitrificans cytochrome c oxidase

The effect of a single site mutation of Arg-54 to methionine in Paracoccus denitrificans cytochrome c oxidase was studied using a combination of optical spectroscopy, electrochemical and rapid kinetics techniques, and time-resolved measurements of electrical membrane potential. The mutation resulted in a blue-shift of the heme a alpha-band by 15 nm and partial occupation of the low-spin heme site by heme O. Additionally, there was a marked decrease in the midpoint potential of the low-spin heme, resulting in slow reduction of this heme species. A stopped-flow investigation of the reaction with ferrocytochrome c yielded a kinetic difference spectrum resembling that of heme a(3). This observation, and the absence of transient absorbance changes at the corresponding wavelength of the low-spin heme, suggests that, in the mutant enzyme, electron transfer from Cu(A) to the binuclear center may not occur via heme a but that instead direct electron transfer to the high-spin heme is the dominating process. This was supported by charge translocation measurements where Deltapsi generation was completely inhibited in the presence of KCN. Our results thus provide an example for how the interplay between protein and cofactors can modulate the functional properties of the enzyme complex.

Charge translocation by the Na+/K+-ATPase investigated on solid supported membranes: cytoplasmic cation binding and release

In the preceding publication (. Biophys. J. 76:000-000) a new technique was described that was able to produce concentration jumps of arbitrary ion species at the surface of a solid supported membrane (SSM). This technique can be used to investigate the kinetics of ion translocating proteins adsorbed to the SSM. Charge translocation of the Na+/K+-ATPase in the presence of ATP was investigated. Here we describe experiments carried out with membrane fragments containing Na+/K+-ATPase from pig kidney and in the absence of ATP. Electrical currents are measured after rapid addition of Na+. We demonstrate that these currents can be explained only by a cation binding process on the cytoplasmic side, most probably to the cytoplasmic cation binding site of the Na+/K+-ATPase. An electrogenic reaction of the protein was observed only with Na+, but not with other monovalent cations (K+, Li+, Rb+, Cs+). Using Na+ activation of the enzyme after preincubation with K+ we also investigated the K+-dependent half-cycle of the Na+/K+-ATPase. A rate constant for K+ translocation in the absence of ATP of 0.2-0.3 s-1 was determined. In addition, these experiments show that K+ deocclusion, and cytoplasmic K+ release are electroneutral.