E2P phosphoforms of Na,K-ATPase. II. Interaction of substrate and cation-binding sites in Pi phosphorylation of Na,K-ATPase

The Na,K-ATPase in complex with beryllium fluoride mimics an ATPase phosphorylated state

The Na⁺,K⁺-ATPase generates electrochemical gradients of Na⁺ and K⁺ across the plasma membrane via a functional cycle that includes various phosphoenzyme intermediates. However, the structure and function of these intermediates and how metal fluorides mimick them require further investigation. Here, we describe a 4.0 Å resolution crystal structure and functional properties of the pig kidney Na⁺,K⁺-ATPase stabilized by the inhibitor beryllium fluoride (denoted E2-BeF_x). E2-BeF_x is expected to mimic properties of the E2P phosphoenzyme, yet with unknown characteristics of ion and ligand binding. The structure resembles the E2P form obtained by phosphorylation from inorganic phosphate (P_i) and stabilized by cardiotonic steroids, including a low-affinity Mg²⁺ site near ion binding site II. Our anomalous Fourier analysis of the crystals soaked in Rb⁺ (a K⁺ congener) followed by a low-resolution rigid-body refinement (6.9-7.5 Å) revealed pre-occlusion transitions leading to activation of the dephosphorylation reaction. We show that the Mg²⁺ location indicates a site of initial K⁺ recognition and acceptance upon binding to the outward-open E2P state after Na⁺ release. Furthermore, using binding and activity studies, we find that the BeF_x-inhibited enzyme is also able to bind ADP/ATP and Na⁺. These results relate the E2-BeF_x complex to a transient K⁺- and ADP-sensitive E*P intermediate of the functional cycle of the Na⁺,K⁺-ATPase, prior to E2P.

Cryoelectron microscopy of Na+,K+-ATPase in the two E2P states with and without cardiotonic steroids

The E2P state of Na⁺,K⁺-ATPase, in which the ATPase is phosphorylated and of low affinity to Na⁺ with the extracellular gate opened, shows different biochemical properties depending on whether the phosphate is transferred from ATP in the forward reaction or from inorganic phosphate (P_i) in the backward reaction. We present here cryoelectron microscopy structures of Na⁺,K⁺-ATPase in the two E2P states, explaining their different biochemical properties established a half century ago. The new electron microscopy maps show previously unseen structural features, including unexpected binding modes of cardiotonic steroids, specific and medically important inhibitors of the ATPase, and stabilization by ATP of the E2P state.

Cryoelectron microscopy (cryo-EM) was applied to Na⁺,K⁺-ATPase (NKA) to determine the structures of two E2P states, one (E2P^ATP) formed by ATP and Mg²⁺ in the forward reaction, and the other (E2P^Pi) formed by inorganic phosphate (P_i) and Mg²⁺ in the backward reaction, with and without ouabain or istaroxime, representatives of classical and new-generation cardiotonic steroids (CTSs). These two E2P states exhibit different biochemical properties. In particular, K⁺-sensitive acceleration of the dephosphorylation reaction is not observed with E2P^Pi, attributed to the presence of a Mg²⁺ ion in the transmembrane cation binding sites. The cryo-EM structures of NKA demonstrate that the two E2P structures are nearly identical but Mg²⁺ in the transmembrane binding cavity is identified only in E2P^Pi, corroborating the idea that it should be denoted as E2P^Pi·Mg²⁺. We can now explain why the absence of transmembrane Mg²⁺ in E2P^ATP confers the K⁺ sensitivity in dephosphorylation. In addition, we show that ATP bridges the actuator (A) and nucleotide binding (N) domains, stabilizing the E2P^ATP state; CTS binding causes hardly any changes in the structure of NKA, both in E2P^ATP and E2P^Pi·Mg²⁺, indicating that the binding mechanism is conformational selection; and istaroxime binds to NKA, extending its aminoalkyloxime group deep into the cation binding site. This orientation is upside down compared to that of classical CTSs with respect to the steroid ring. Notably, mobile parts of NKA are resolved substantially better in the electron microscopy (EM) maps than in previous X-ray structures, including sugars sticking out from the β-subunit and many phospholipid molecules.

Structure and Function of Na,K-ATPase-The Sodium-Potassium Pump

Na,K-ATPase is an ubiquitous enzyme actively transporting Na-ions out of the cell in exchange for K-ions, thereby maintaining their concentration gradients across the cell membrane. Since its discovery more than six decades ago the Na-pump has been studied extensively and its vital physiological role in essentially every cell has been established. This article aims at providing an overview of well-established biochemical properties with a focus on Na,K-ATPase isoforms, its transport mechanism and principle conformations, inhibitors, and insights gained from crystal structures. © 2021 American Physiological Society. Compr Physiol 11:1-21, 2021.

Binding of cardiotonic steroids to Na+,K+-ATPase in the E2P state

The sodium pump (Na⁺, K⁺-ATPase, NKA) is vital for animal cells, as it actively maintains Na⁺ and K⁺ electrochemical gradients across the cell membrane. It is a target of cardiotonic steroids (CTSs) such as ouabain and digoxin. As CTSs are almost unique strong inhibitors specific to NKA, a wide range of derivatives has been developed for potential therapeutic use. Several crystal structures have been published for NKA-CTS complexes, but they fail to explain the largely different inhibitory properties of the various CTSs. For instance, although CTSs are thought to inhibit ATPase activity by binding to NKA in the E2P state, we do not know if large conformational changes accompany binding, as no crystal structure is available for the E2P state free of CTS. Here, we describe crystal structures of the BeF₃ ^- complex of NKA representing the E2P ground state and then eight crystal structures of seven CTSs, including rostafuroxin and istaroxime, two new members under clinical trials, in complex with NKA in the E2P state. The conformations of NKA are virtually identical in all complexes with and without CTSs, showing that CTSs bind to a preformed cavity in NKA. By comparing the inhibitory potency of the CTSs measured under four different conditions, we elucidate how different structural features of the CTSs result in different inhibitory properties. The crystal structures also explain K⁺-antagonism and suggest a route to isoform specific CTSs.

The interaction of Na+, K+, and phosphate with the gastric H,K-ATPase. Kinetics of E1-E2 conformational changes assessed by eosin fluorescence measurements

H,K-ATPase and Na,K-ATPase show the highest degree of sequence similarity among all other members of the P-type ATPases family. To explore their common features in terms of ligand binding, we evaluated conformational transitions due to the binding of Na⁺, K⁺ and Pi in the H,K-ATPase, and compared the results with those obtained for the Na,K-ATPase. This work shows that eosin fluorescence time courses provide a reasonably precise method to study the kinetics of the E1-E2 conformational changes in the H,K-ATPase. We found that, although Na⁺ shifts the equilibrium toward the E1 conformation and seems to compete with H⁺ in ATPase activity assays, it was neither possible to isolate a Na⁺-occluded state, nor to reveal an influx of Na⁺ related to H,K-ATPase activity. The high rate of the E2K → E1 transition found for the H,K-ATPase, which is not compatible with the presence of a K⁺-occluded form, agrees with the negligible level of occluded Rb⁺ (used as a K⁺ congener) found in the absence of added ligands. The use of vanadate and fluorinated metals to induce E2P-like states increased the level of occluded Rb⁺ and suggests that-during dephosphorylation-the probability of K⁺ to remain occluded increases from the E2P-ground to the E2P-product state. From kinetic experiments we found an unexpected increase in the values of k_obs for E2P formation with [Pi]; consequently, to obey the Albers-Post model, the binding of Pi to the E2 state cannot be a rapid-equilibrium reaction.

Cholesterol depletion inhibits Na+,K+-ATPase activity in a near-native membrane environment

Cholesterol's effects on Na+,K+-ATPase reconstituted in phospholipid vesicles have been extensively studied. However, previous studies have reported both cholesterol-mediated stimulation and inhibition of Na+,K+-ATPase activity. Here, using partial reaction kinetics determined via stopped-flow experiments, we studied cholesterol's effect on Na+,K+-ATPase in a near-native environment in which purified membrane fragments were depleted of cholesterol with methyl-β-cyclodextrin (mβCD). The mβCD-treated Na+,K+-ATPase had significantly reduced overall activity and exhibited decreased observed rate constants for ATP phosphorylation (ENa3+ → E2P, i.e. phosphorylation by ATP and Na+ occlusion from the cytoplasm) and K+ deocclusion with subsequent intracellular Na+ binding (E2K2+ → E1Na3+). However, cholesterol depletion did not affect the observed rate constant for K+ occlusion by phosphorylated Na+,K+-ATPase on the extracellular face and subsequent dephosphorylation (E2P → E2K2+). Thus, partial reactions involving cation binding and release at the protein's intracellular side were most dependent on cholesterol. Fluorescence measurements with the probe eosin indicated that cholesterol depletion stabilizes the unphosphorylated E2 state relative to E1, and the cholesterol depletion-induced slowing of ATP phosphorylation kinetics was consistent with partial conversion of Na+,K+-ATPase into the E2 state, requiring a slow E2 → E1 transition before the phosphorylation. Molecular dynamics simulations of Na+,K+-ATPase in membranes with 40 mol % cholesterol revealed cholesterol interaction sites that differ markedly among protein conformations. They further indicated state-dependent effects on membrane shape, with the E2 state being likely disfavored in cholesterol-rich bilayers relative to the E1P state because of a greater hydrophobic mismatch. In summary, cholesterol extraction from membranes significantly decreases Na+,K+-ATPase steady-state activity.

Intrinsic reaction-cycle time scale of Na+,K+-ATPase manifests itself in the lipid-protein interactions of nonequilibrium membranes

Interaction between integral membrane proteins and the lipid-bilayer component of biological membranes is expected to mutually influence the proteins and the membrane. We present quantitative evidence of a manifestation of the lipid-protein interactions in liposomal membranes, reconstituted with actively pumping Na(+),K(+)-ATPase, in terms of nonequilibrium shape fluctuations that contain a relaxation time, τ, which is robust and independent of the specific fluctuation modes of the membrane. In the case of pumping Na(+)-ions, analysis of the flicker-noise temporal correlation spectrum of the liposomes leads to τ ~/= 0.5 s, comparing favorably with an intrinsic reaction-cycle time of about 0.4 s from enzymology.

Oleic and linoleic acids are active principles in Nigella sativa and stabilize an E(2)P conformation of the Na,K-ATPase. Fatty acids differentially regulate cardiac glycoside interaction with the pump

Nigella sativa seed oil was found to contain a modulator of Na,K-ATPase. Separation analyses combined with (1)H NMR and GCMS identified the inhibitory fraction as a mixture of oleic and linoleic acids. These two fatty acids are specifically concentrated in several medicinal plant oils, and have particularly been implicated in decreasing high blood pressure. The ouabain binding site on Na,K-ATPase has also been implicated in blood pressure regulation. Thus, we aimed to determine how these two molecules modify pig kidney Na,K-ATPase. Oleic and linoleic acids did not modify reactions involving the E(1) (Na(+)) conformations of the Na,K-ATPase. In contrast, K(+) dependent reactions were strongly modified after treatment. Oleic and linoleic acids were found to stabilize a pump conformation that binds ouabain with high affinity, i.e., an ion free E(2)P form. Time-resolved binding assays using anthroylouabain, a fluorescent ouabain analog, revealed that the increased ouabain affinity is unique to oleic and linoleic acids, as compared with γ-linolenic acid, which decreased pump-mediated ATP hydrolysis but did not equally increase ouabain interaction with the pump. Thus, the dynamic changes in plasma levels of oleic and linoleic acids are important in the modulation of the sensitivity of the sodium pump to cardiac glycosides. Given the possible involvement of the cardiac glycoside binding site on Na,K-ATPase in the regulation of hypertension, we suggest oleic acid to be a specific chaperon that modulates interaction of cardiac glycosides with the sodium pump.

Interaction of ATP with the phosphoenzyme of the Na+,K+-ATPase

The interaction of ATP with the phosphoenzyme of Na(+),K(+)-ATPase from pig kidney, rabbit kidney, and shark rectal gland was investigated using the voltage-sensitive fluorescent probe RH421. In each case, ATP concentrations >or=100 microM caused a drop in fluorescence intensity, which, because RH421 is sensitive to the formation of enzyme in the E2P state, can be attributed to ATP binding to the E2P phosphoenzyme. Simulations of the experimental behavior using kinetic models based on either a monomeric or a dimeric enzyme mechanism yielded a K(d) for ATP binding in the range 140-500 muM. Steady-state activity measurements and independent measurements of the phosphoenzyme level via a radioactive assay indicated that ATP binding to E2P causes a deceleration in its dephosphorylation when acting in the Na(+)-ATPase mode, i.e., in the absence of K(+) ions. Both the ATP-induced drop in RH421 fluorescence and the effect on the dephosphorylation reaction could be attributed to an inhibition of dissociation from the E2P(Na(+))(3) state of the one Na(+) ion necessary to allow dephosphorylation. Stopped-flow studies on the shark enzyme indicated that the ATP-induced inhibition of dephosphorylation is abolished in the presence of 1 mM KCl. A possible physiological role of allosteric binding of ATP to the phosphoenzyme could be to stabilize the E2P state and stop the enzyme running backward, which would cause dissipation of the Na(+) electrochemical potential gradient and the resynthesis of ATP from ADP. ATP binding to E2P could also fix ATP within the enzyme ready to phosphorylate it in the subsequent turnover.

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.

Structural changes in the catalytic cycle of the Na+,K+-ATPase studied by infrared spectroscopy

Pig kidney Na(+),K(+)-ATPase was studied by means of reaction-induced infrared difference spectroscopy. The reaction from E1Na(3)(+) to an E2P state was initiated by photolysis of P(3)-1-(2-nitrophenyl)ethyl ATP (NPE caged ATP) in samples that contained 3 mM free Mg(2+) and 130 mM NaCl at pH 7.5. Release of ATP from caged ATP produced highly detailed infrared difference spectra indicating structural changes of the Na(+),K(+)-ATPase. The observed transient state of the enzyme accumulated within seconds after ATP release and decayed on a timescale of minutes at 15 degrees C. Several controls ensured that the observed difference signals were due to structural changes of the Na(+),K(+)-ATPase. Samples that additionally contained 20 mM KCl showed similar spectra but less intense difference bands. The absorbance changes observed in the amide I region, reflecting conformational changes of the protein backbone, corresponded to only 0.3% of the maximum absorbance. Thus the net change of secondary structure was concluded to be very small, which is in line with movement of rigid protein segments during the catalytic cycle. Despite their small amplitude, the amide I signals unambiguously reveal the involvement of several secondary structure elements in the conformational change. Similarities and dissimilarities to corresponding spectra of the Ca(2+)-ATPase and H(+),K(+)-ATPase are discussed, and suggest characteristic bands for the E1 and E2 conformations at 1641 and 1661 cm(-1), respectively, for alphabeta heterodimeric ATPases. The spectra further indicate the participation of protonated carboxyl groups or lipid carbonyl groups in the reaction from E1Na(3)(+) to an E2P state. A negative band at 1730 cm(-1) is in line with the presence of a protonated Asp or Glu residue that coordinates Na(+) in E1Na(3)(+). Infrared signals were also detected in the absorption regions of ionized carboxyl groups.

Mechanism of allosteric effects of ATP on the kinetics of P-type ATPases

The roles of allosteric effects of ATP and protein oligomerisation in the mechanisms of P-type ATPases belong to the most controversial and least well understood topics in the field. Recent crystal structural and kinetic data, however, now allow certain hypotheses to be definitely excluded and consistent hypotheses to be developed. The aim of this review is to critically discuss recent results and, in the light of them, to present a set of conclusions which could form the basis of future research. The major conclusions are: (1) at saturating ATP concentrations P-type ATPases function as monomeric enzymes, (2) the catalytic units of P-type ATPases only possess a single ATP binding site, (3) at non-saturating ATP concentrations P-type ATPases exist as diprotomeric (or higher oligomeric) complexes, (4) protein-protein interactions within a diprotomeric complex enhances the enzymes' ATP binding affinity, (5) ATP binding to both protomers within a diprotomeric complex causes it to dissociate into two separate monomers. The physiological role of protein-protein interactions within a diprotomer may be to enhance ATP binding affinity so as to scavenge ATP and maximize the ion pumping rate under hypoxic or anoxic conditions. For the first time a structural basis for the well-known ATP allosteric acceleration of the E2 --> E1 transition is presented. This is considered to be due to a minimization of steric hindrance between neighbouring protomers because of the ability of ATP to induce a compact conformation of the enzymes' cytoplasmic domains.

Photochemical behavior and Na+,K+-ATPase sensitivity of voltage-sensitive styrylpyridinium fluorescent membrane probes

RH421 is a widely used voltage-sensitive fluorescent membrane probe. Its exposure to continuous illumination with 577 nm light from an Hg lamp leads, however, to an increase in its steady-state fluorescence level when bound to lipid membranes. The increase occurs on the second time scale at typical light intensities and was found to be due to a single-photon excited-state isomerization. Modifications to the dye structure are, therefore, necessary to increase photochemical stability and allow wider application of such dyes in kinetic studies of ion-transporting membrane proteins. The related probe ANNINE 5, which has a rigid polycyclic structure, shows no observable photochemical reaction when bound to DMPC vesicles on irradiation with 436 nm light. The voltage sensitivity of ANNINE 5 was tested with the use of Na+,K+-ATPase membrane fragments. As long as ANNINE 5 is excited on the far red edge of its visible absorption band, it shows a similar sensitivity to RH421 in detecting charge-translocating reactions triggered by ATP phosphorylation. Unfortunately the wavelengths necessary for ANNINE 5 excitation are in a region where the Hg lamps routinely used in stopped-flow apparatus have no significant lines available for excitation.

Kinetic characterization of tetrapropylammonium inhibition reveals how ATP and Pi alter access to the Na+-K+-ATPase transport site

Current models of the Na(+)-K(+)-ATPase reaction cycle have ATP binding with low affinity to the K(+)-occluded form and accelerating K(+) deocclusion, presumably by opening the inside gate. Implicit in this situation is that ATP binds after closing the extracellular gate and thus predicts that ATP binding and extracellular cation binding to be mutually exclusive. We tested this hypothesis. Accordingly, we needed a cation that binds outside and not inside, and we determined that tetrapropylammonium (TPA) behaves as such. TPA competed with K(+) (and not Na(+)) for ATPase, TPA was unable to prevent phosphoenzyme (EP) formation even at low Na(+), and TPA decreased the rate of EP hydrolysis in a K(+)-competitive manner. Having established that TPA binding is a measurement of extracellular access, we next determined that TPA and inorganic phosphate (P(i)) were not mutually exclusive inhibitors of para-nitrophenylphosphatase (pNPPase) activity, implying that when P(i) is bound, the transport site has extracellular access. Surprisingly, we found that ATP and TPA also were not mutually exclusive inhibitors of pNPPase activity, implying that when the cation transport site has extracellular access, ATP can still bind. This is consistent with a model in which ATP speeds up the conformational changes that lead to intracellular or extracellular access, but that ATP binding is not, by itself, the trigger that causes opening of the cation site to the cytoplasm.

Identification of potential regulatory sites of the Na+,K+-ATPase by kinetic analysis

Kinetic models are presented that allow the Na(+),K(+)-ATPase steady-state turnover number to be estimated at given intra- and extracellular concentrations of Na(+), K(+), and ATP. Based on experimental transient kinetic data, the models utilize either three or four steps of the Albers-Post scheme, that is, E(2) --> E(1), E(1) --> E(2)P (or E(1) --> E(1)P and E(1)P --> E(2)P), and E(2)P --> E(2), which are the major rate-determining steps of the enzyme cycle. On the time scale of these reactions, the faster binding steps of Na(+), K(+), and ATP to the enzyme are considered to be in equilibrium. Each model was tested by comparing calculations of the steady-state turnover from rate constants and equilibrium constants for the individual partial reactions with published experimental data of the steady-state activity at varying Na(+) and K(+) concentrations. To provide reasonable agreement between the calculations and the experimental data, it was found that Na(+)/K(+) competition for cytoplasmic binding sites was an essential feature required in the model. The activity was also very dependent on the degree of K(+)-induced stimulation of the reverse reaction E(1) --> E(2). Taking into account the physiological substrate concentrations, the models allow the most likely potential sites of short-term Na(+),K(+)-ATPase regulation to be identified. These were found to be (a) the cytoplasmic Na(+) and K(+) binding sites, via changes in Na(+) or K(+) concentration or their dissociation constants, (b) ATP phosphorylation (as a substrate), via a change in its rate constant, and (c) the position of the E(2)<==>E(1) equilibrium.

Modulation of Na,K-ATPase by phospholipids and cholesterol. II. Steady-state and presteady-state kinetics

The effects of phospholipid acyl chain length (n(c)) and cholesterol on several partial reactions of Na,K-ATPase reconstituted into liposomes of defined lipid composition are described. This regards the E(1)/E(2) equilibrium, the phosphoenzyme level, and the K(+)-deocclusion reaction. In addition, the lipid effects on some steady-state properties were investigated. Finally, the effects of cholesterol on the temperature sensitivity of the phosphorylation and spontaneous dephosphorylation reactions were investigated. The fatty acid and cholesterol composition of the native Na,K-ATPase membrane preparation showed a remarkable similarity to the lipid composition known to support maximum hydrolytic capacity as determined from in vitro experiments. The main rate-determining step of the Na,K-ATPase reaction, the E(2) --> E(1) reaction, as well as several other partial reactions were accelerated by cholesterol. This regards the phosphorylation by ATP as well as the E(1) - P --> E(2)-P reaction. Moreover, cholesterol shifted the E(1)/E(2) equilibrium toward the E(1) conformation and increased the K(+)-deocclusion rate. Finally, cholesterol significantly affected the temperature sensitivity of the spontaneous dephosphorylation reaction and the phosphorylation by ATP. The effects of cholesterol were not completely equivalent to those induced by increasing the phospholipid acyl chain length, indicating that the cholesterol effects are not entirely caused by increasing the hydrophobic bilayer thickness, which indicates an additional mechanism of action on the Na,K-ATPase.

Effects of palytoxin on cation occlusion and phosphorylation of the (Na+,K+)-ATPase

Palytoxin (PTX) inhibits the (Na(+) + K+)-driven pump and simultaneously opens channels that are equally permeable to Na+ and K+ in red cells and other cell membranes. In an effort to understand the mechanism by which PTX induces these fluxes, we have studied the effects of PTX on: 1) K+ and Na+ occlusion by the pump protein; 2) phosphorylation and dephosphorylation of the enzyme when a phosphoenzyme is formed from ATP and from P(i); and 3) p-nitro phenyl phosphatase (p-NPPase) activity associated with the (Na+, K+)-ATPase. We have found that palytoxin 1) increases the rate of deocclusion of K+(Rb+) in a time- and concentration-dependent manner, whereas Na+ occluded in the presence of oligomycin is unaffected by the toxin; 2) makes phosphorylation from P(i) insensitive to K+, and 3) stimulates the p-NPPase activity. The results are consistent with the notion that PTX produces a conformation of the Na+, K(+)-pump that resembles the one observed when ATP is bound to its low-affinity binding site. Further, they suggest that the channels that are formed by PTX might arise as a consequence of a perturbation in the ATPase structure, leading to the loss of control of the outside "gate" of the enzyme and hence to an uncoupling of the ion transport from the catalytic function of the ATPase.

Thapsigargin and dimethyl sulfoxide activate medium P(i)<-->HOH oxygen exchange catalyzed by sarcoplasmic reticulum Ca2+-ATPase

Thapsigargin is a potent inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase. It binds the Ca(2+)-free E2 conformation in the picomolar range, supposedly resulting in a largely catalytically inactive species. We now find that thapsigargin has little effect on medium P(i) <--> HOH oxygen exchange and that this activity is greatly stimulated (up to 30-fold) in the presence of 30% (v/v) Me(2)SO. Assuming a simple two-step mechanism, we have evaluated the effect of thapsigargin and Me(2)SO on the four rate constants governing the reaction of P(i) with Ca(2+)-ATPase. The principal effect of thapsigargin alone is to stimulate EP hydrolysis (k(-2)), whereas that of Me(2)SO is to greatly retard P(i) dissociation (k(-1)), accounting for its well known effect on increasing the apparent affinity for P(i). These effects persist when the agents are used in combination and substantially account for the activated oxygen exchange (v(exchange) = k(-2)[EP]). Kinetic simulations show that the overall rate constant for the formation of EP is very fast (approximately 300 s(-1)) when the exchange is maximal. Thapsigargin greatly stabilizes Ca(2+)-ATPase against denaturation in detergent in the absence of Ca(2+), as revealed by glutaraldehyde cross-linking, suggesting that the membrane helices lock together. It seems that the reactions at the phosphorylation site, associated with the activated exchange reaction, are occurring without much movement of the transport site helices, and we suggest that they may be associated solely with an occluded H+ state.

Breakdown of Na+/K+-exchanging ATPase phosphoenzymes formed from ATP and from inorganic phosphate during Na+-ATPase activity

The reactivity towards Na+ and K+ of Na+/K+-ATPase phosphoenzymes formed from ATP and Pi during Na+-ATPase turnover and that obtained from Pi in the absence of ATP, Na+ and K+ was studied. The phosphoenzyme formed from Pi in the absence of cycling and with no Na+ or K+ in the medium showed a biphasic time-dependent breakdown. The fast component, 96% of the total EP, had a decay rate of about 4 s(-1) in K+-free 130 mm Na+, and was 40% inhibited by 20 mm K+. The slow component, about 0.14 s(-1), was K+ insensitive. Values for the time-dependent breakdown of the phosphoenzymes obtained from ATP and from Pi during Na+-ATPase activity were indistinguishable from each other. In K+-free medium containing 130 mm Na+, the decays followed a single exponential with a rate constant of 0.45 s(-1). The addition of 20 mm K+ markedly increased the decays and made them biphasic. The fast components had a rate of approximately 220 s-1 and accounted for 92-93% of the total phosphoenzyme. The slow components decayed at a rate of about 47-53 s(-1). A second group of experiments examined the reactivity towards Na+ of the E2P forms obtained with ATP and Pi when the enzyme was cycling. In both cases, the rate of dephosphorylation was a biphasic function of [Na+]: inhibition at low [Na+], with a minimum at about 5 mm Na+, followed by recovery at higher [Na+]. Although qualitatively similar, the phosphoenzyme formed from Pi showed slightly less inhibition and more pronounced recovery. These results indicate that forward and backward phosphorylation during Na+-ATPase turnover share the same intermediates.

P(3)-[2-(4-hydroxyphenyl)-2-oxo]ethyl ATP for the rapid activation of the Na(+),K(+)-ATPase

P(3)-[2-(4-hydroxyphenyl)-2-oxo]ethyl ATP (pHP-caged ATP) has been investigated for its application as a phototrigger for the rapid activation of electrogenic ion pumps. The yield of ATP after irradiation with a XeCl excimer laser (lambda = 308 nm) was determined at pH 6.0-7.5. For comparison, the photolytic yields of P(3)-[1-(2-nitrophenyl)]ethyl ATP (NPE-caged ATP) and P(3)-[1, 2-diphenyl-2-oxo]ethyl ATP (desyl-caged ATP) were also measured. It was shown that at lambda = 308 nm pHP-caged ATP is superior to the other caged ATP derivatives investigated in terms of yield of ATP after irradiation. Using time-resolved single-wavelength IR spectroscopy, we determined a lower limit of 10(6) s(-1) for the rate constant of release of ATP from pHP-caged ATP at pH 7.0. Like NPE-caged ATP, pHP-caged ATP and desyl-caged ATP bind to the Na(+), K(+)-ATPase and act as competitive inhibitors of ATPase function. Using pHP-caged ATP, we investigated the charge translocation kinetics of the Na(+),K(+)-ATPase at pH 6.2-7.4. The kinetic parameters obtained from the electrical measurements are compared to those obtained with a technique that does not require caged ATP, namely parallel stopped-flow experiments using the voltage-sensitive dye RH421. It is shown that the two techniques yield identical results, provided the inhibitory properties of the caged compound are taken into account. Our results demonstrate that under physiological (pH 7.0) and slightly basic (pH 7.5) or acidic (pH 6. 0) conditions, pHP-caged ATP is a rapid, effective, and biocompatible phototrigger for ATP-driven biological systems.

Rate determination in phosphorylation of shark rectal Na,K-ATPase by ATP: temperature sensitivity and effects of ADP

Phosphorylation of shark rectal Na,K-ATPase by ATP in the presence of Na(+) was characterized by chemical quench experiments and by stopped-flow RH421 fluorescence. The appearance of acid-stable phosphoenzyme was faster than the rate of fluorescence increase, suggesting that of the two acid-stable phosphoenzymes formed, RH421 exclusively detects formation of E(2)-P, which follows formation of E(1)-P. The stopped-flow RH421 fluorescence response to ATP phosphorylation was biphasic, with a major fast phase with k(obs) approximately 90 s(-1) and a minor slow phase with a k(obs) of approximately 9 s(-1) (20 degrees C, pH 7.4). The observed rate constants for both the slow and the fast phase could be fitted with identical second-degree functions of the ATP concentration with apparent binding constants of approximately 3.1 x 10(7) M(-1) and 1. 8 x 10(5) M(-1), respectively. Increasing [ADP] decreased k(obs) for the rate of the RH421 fluorescence response to ATP phosphorylation. This could be accounted for by the reaction of ADP with the initially formed E(1)-P followed by a conformational change to E(2)-P. The biphasic stopped-flow RH421 responses to ATP phosphorylation could be simulated, assuming that in the absence of K(+) the highly fluorescent E(2)-P is slowly transformed into the "K(+)-insensitive" E'(2)-P subconformation forming a side branch of the main cycle.

Hofmeister effects of anions on the kinetics of partial reactions of the Na+,K+-ATPase

The effects of lyotropic anions, particularly perchlorate, on the kinetics of partial reactions of the Na+,K+-ATPase from pig kidney were investigated by two different kinetic techniques: stopped flow in combination with the fluorescent label RH421 and a stationary electrical relaxation technique. It was found that 130 mM NaClO4 caused an increase in the Kd values of both the high- and low-affinity ATP-binding sites, from values of 7.0 (+/- 0.6) microM and 143 (+/- 17) microM in 130 mM NaCl solution to values of 42 (+/- 3) microM and 660 (+/- 100) microM in 130 mM NaClO4 (pH 7.4, 24 degrees C). The half-saturating concentration of the Na+-binding sites on the E1 conformation was found to decrease from 8-10 mM in NaCl to 2.5-3.5 mM in NaClO4 solution. The rate of equilibration of the reaction, E1P(Na+)3 left arrow over right arrow E2P + 3Na+, decreased from 393 (+/- 51) s-1 in NaCl solution to 114 (+/- 15) s-1 in NaClO4. This decrease is attributed predominantly to an inhibition of the E1P(Na+)3 --> E2P(Na+)3 transition. The effects can be explained in terms of electrostatic interactions due to perchlorate binding within the membrane and/or protein matrix of the Na+,K+-ATPase membrane fragments and alteration of the local electric field strength experienced by the protein. The kinetic results obtained support the conclusion that the conformational transition E1P(Na+)3 --> E2P(Na+)3 is a major charge translocating step of the pump cycle.

The energy transduction mechanism of Na,K-ATPase studied with iron-catalyzed oxidative cleavage

This paper extends our recent report on specific iron-catalyzed oxidative cleavages of renal Na,K-ATPase and effects of E1 left arrow over right arrow E2 conformational transitions (Goldshleger, R. , and Karlish, S. J. D. (1997) Proc. Natl. Acad. Sci. U. S. A. 94, 9596-9601). The experiments indicate that only peptide bonds close to a bound Fe2+ ion are cleaved, and provide evidence on proximity of the different cleavage positions in the native enzyme. A sequence HFIH near trans-membrane segment M3 appears to be involved in Fe2+ binding. Previously we hypothesized that E2 and E1 conformations are characterized by formation or relaxation of interactions within the alpha subunit at or near highly conserved sequences, TGES in the minor cytoplasmic loop and CSDK, MVTGD, and VNDSPALKK in the major cytoplasmic loop. This concept has been tested by examining iron-catalyzed cleavage in both non-phosphorylated and phosphorylated conformations and effects of phosphate, vanadate, and ouabain. The results imply that both E1 left arrow over right arrow E2 and E1P left arrow over right arrow E2P transitions are indeed associated with formation and relaxation of interactions between cytoplasmic domains, comprising the minor loop plus N-terminal tail leading into M1 and major loop, respectively. Furthermore, it appears that either non-covalently or covalently bound phosphate bind near CSDK and MVTGD, and Mg2+ ions may bind to residues within TGES and VNDSPALKK and to bound phosphate. Thus cytoplasmic domain interactions seem to occur within or near the active site. We discuss the relationship between structural changes in the cytoplasmic domain and movements of trans-membrane segments that lead to cation transport. Presumably conformation-dependent formation and relaxation of domain interactions underlie energy transduction in all P-type pumps.