ADP- and K+-sensitive phosphorylated intermediate of Na,K-ATPase

Consensus, controversies, and conundrums of P4-ATPases: The emerging face of eukaryotic lipid flippases

The cryo-EM resolution revolution has heralded a new era in our understanding of eukaryotic lipid flippases with a rapidly growing number of high-resolution structures. Flippases belong to the P4 family of ATPases (type IV P-type ATPases) that largely follow the reaction cycle proposed for the more extensively studied cation-transporting P-type ATPases. However, unlike the canonical P-type ATPases, no flippase cargos are transported in the phosphorylation half-reaction. Instead of being released into the intracellular or extracellular milieu, lipid cargos are transported to their destination at the inner leaflet of the membrane. Recent flippase structures have revealed multiple conformational states during the lipid transport cycle. Nonetheless, critical conformational states capturing the lipid cargo "in transit" are still missing. In this review, we highlight the amazing structural advances of these lipid transporters, discuss various perspectives on catalytic and regulatory mechanisms in the literature, and shed light on future directions in further deciphering the detailed molecular mechanisms of lipid flipping.

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.

Structural and energetic analysis of metastable intermediate states in the E1P-E2P transition of Ca2+-ATPase

Ion pumps (or P-type ATPases) are membrane proteins, which transport ions through biological membranes against a concentration gradient, a function essential for many biological processes, such as muscle contraction, neurotransmission, and metabolism. Molecular mechanisms underlying active ion transport by ion pumps have been investigated by biochemical experiments and high-resolution structure analyses. Here, the transition of sarcoplasmic reticulum Ca²⁺-ATPase upon dissociation of Ca²⁺ is investigated using atomistic molecular dynamics simulations. We find intermediate structures along the pathway are stabilized by transient interactions between A- and P-domains as well as lipid molecules in the transmembrane helices.

Sarcoplasmic reticulum (SR) Ca²⁺-ATPase transports two Ca²⁺ ions from the cytoplasm to the SR lumen against a large concentration gradient. X-ray crystallography has revealed the atomic structures of the protein before and after the dissociation of Ca²⁺, while biochemical studies have suggested the existence of intermediate states in the transition between E1P⋅ADP⋅2Ca²⁺ and E2P. Here, we explore the pathway and free energy profile of the transition using atomistic molecular dynamics simulations with the mean-force string method and umbrella sampling. The simulations suggest that a series of structural changes accompany the ordered dissociation of ADP, the A-domain rotation, and the rearrangement of the transmembrane (TM) helices. The luminal gate then opens to release Ca²⁺ ions toward the SR lumen. Intermediate structures on the pathway are stabilized by transient sidechain interactions between the A- and P-domains. Lipid molecules between TM helices play a key role in the stabilization. Free energy profiles of the transition assuming different protonation states suggest rapid exchanges between Ca²⁺ ions and protons when the Ca²⁺ ions are released toward the SR lumen.

External Ion Access in the Na/K Pump: Kinetics of Na+, K+, and Quaternary Amine Interaction

Na/K pumps build essential ion gradients across the plasmalemma of animal cells by coupling the extrusion of three Na⁺, with the import of two K⁺ and the hydrolysis of one ATP molecule. The mechanisms of selectivity and competition between Na⁺, K⁺, and inhibitory amines remain unclear. We measured the effects of external tetrapropylammonium (TPA⁺) and ethylenediamine (EDA²⁺) on three different Na/K pump transport modes in voltage-clamped Xenopus oocytes: 1) outward pump current (I_P), 2) passive inward H⁺ current at negative voltages without Na⁺ or K⁺ (I_H), and 3) transient charge movement reporting the voltage-dependent extracellular binding/release of Na⁺ (Q_Na). Both amines competed with K⁺ to inhibit I_P. TPA⁺ inhibited I_H without competing with H⁺, whereas EDA²⁺ did not alter I_H at pH 7.6. TPA⁺ competed with Na⁺ in Q_Na measurements, reducing Na⁺-apparent affinity, evidenced by a ∼-75 mV shift in the charge-voltage curve (at 20 mM TPA⁺) without reduction of the total charge moved (Q_tot). In contrast, EDA²⁺ and K⁺ did not compete with Na⁺ to inhibit Q_Na; both reduced Q_tot without decreasing Na⁺-apparent affinity. EDA²⁺ (15 mM) right-shifted the charge-voltage curve by ∼+50 mV. Simultaneous occlusion of EDA²⁺ and Na⁺ by an E2P conformation unable to reach E1P was demonstrated by voltage-clamp fluorometry. Trypsinolysis experiments showed that EDA²⁺-bound pumps are much more proteolysis-resistant than Na⁺-, K⁺-, or TPA⁺-bound pumps, therefore uncovering unique EDA²⁺-bound conformations. K⁺ effects on Q_Na and I_H were also evaluated in pumps inhibited with beryllium fluoride, a phosphate mimic. K⁺ reduced Q_tot without shifting the charge-voltage curve, indicating noncompetitive effects, and partially inhibited I_H to the same extent as TPA⁺ in non-beryllium-fluorinated pumps. These results demonstrate that K⁺ interacts with beryllium-fluorinated pumps inducing conformational changes that alter Q_Na and I_H, suggesting that there are two external access pathways for proton transport by I_H.

Importance of a Potential Protein Kinase A Phosphorylation Site of Na+,K+-ATPase and Its Interaction Network for Na+ Binding

The molecular mechanism underlying PKA-mediated regulation of Na(+),K(+)-ATPase was explored in mutagenesis studies of the potential PKA site at Ser-938 and surrounding charged residues. The phosphomimetic mutations S938D/E interfered with Na(+) binding from the intracellular side of the membrane, whereas Na(+) binding from the extracellular side was unaffected. The reduction of Na(+) affinity is within the range expected for physiological regulation of the intracellular Na(+) concentration, thus supporting the hypothesis that PKA-mediated phosphorylation of Ser-938 regulates Na(+),K(+)-ATPase activity in vivo Ser-938 is located in the intracellular loop between transmembrane segments M8 and M9. An extended bonding network connects this loop with M10, the C terminus, and the Na(+) binding region. Charged residues Asp-997, Glu-998, Arg-1000, and Lys-1001 in M10, participating in this bonding network, are crucial to Na(+) interaction. Replacement of Arg-1005, also located in the vicinity of Ser-938, with alanine, lysine, methionine, or serine resulted in wild type-like Na(+) and K(+) affinities and catalytic turnover rate. However, when combined with the phosphomimetic mutation S938E only lysine substitution of Arg-1005 was compatible with Na(+),K(+)-ATPase function, and the Na(+) affinity of this double mutant was reduced even more than in single mutant S938E. This result indicates that the positive side chain of Arg-1005 or the lysine substituent plays a mechanistic role as interaction partner of phosphorylated Ser-938, transducing the phosphorylation signal into a reduced affinity of Na(+) site III. Electrostatic interaction of Glu-998 is of minor importance for the reduction of Na(+) affinity by phosphomimetic S938E as revealed by combining S938E with E998A.

Effects of oligomycin on transient currents carried by Na+ translocation of Bufo Na+/K(+)-ATPase expressed in Xenopus oocytes

Na(+)/K(+)-ATPase (NKA) exports 3Na(+) and imports 2K(+) at the expense of the hydrolysis of 1ATP under physiological conditions. In the absence of K(+), it can mediate electroneutral Na(+)/Na(+) exchange. In the electroneutral Na(+)/Na(+) exchange mode, NKA produces a transient current containing fast, medium and slow components in response to a sudden voltage step. These three components of the transient current demonstrate the sequential release of Na(+) ions from three binding sites. Our data from oocytes provide further experimental support for the existence of these components. Oligomycin is an NKA inhibitor that favors the 2Na(+)-occluded state without affecting the conformational state of the NKA. We studied the effects of oligomycin on both K(+)-activated currents and transient currents in wild-type Bufo NKA and a mutant form of Bufo NKA, NKA: G813A. Oligomycin blocked almost all of the K(+)-activated current, although the three components of the transient current showed different sensitivities to oligomycin. The oligomycin-inhibited charge movement measured using a P/4 protocol had a rate coefficient similar to the medium transient component. The fast component of the transient current elicited by a short voltage step also showed sensitivity to oligomycin. However, the slow component was not totally inhibited by oligomycin. Our results indicate that the second and third sodium ions might be released to the extracellular medium by a mechanism that is not shared by the first sodium ion.

The rapid-onset dystonia parkinsonism mutation D923N of the Na+, K+-ATPase alpha3 isoform disrupts Na+ interaction at the third Na+ site

Rapid-onset dystonia parkinsonism (RDP), a rare neurological disorder, is caused by mutation of the neuron-specific alpha3-isoform of Na(+), K(+)-ATPase. Here, we present the functional consequences of RDP mutation D923N. Relative to the wild type, the mutant exhibits a remarkable approximately 200-fold reduction of Na(+) affinity for activation of phosphorylation from ATP, reflecting a defective interaction of the E(1) form with intracellular Na(+). This is the largest effect on Na(+) affinity reported so far for any Na(+), K(+)-ATPase mutant. D923N also affects the interaction with extracellular Na(+) normally driving the E(1)P to E(2)P conformational transition backward. However, no impairment of K(+) binding was observed for D923N, leading to the conclusion that Asp(923) is specifically associated with the third Na(+) site that is selective toward Na(+). The crystal structure of the Na(+), K(+)-ATPase in E(2) form shows that Asp(923) is located in the cytoplasmic half of transmembrane helix M8 inside a putative transport channel, which is lined by residues from the transmembrane helices M5, M7, M8, and M10 and capped by the C terminus, recently found involved in recognition of the third Na(+) ion. Structural modeling of the E(1) form of Na(+), K(+)-ATPase based on the Ca(2+)-ATPase crystal structure is consistent with the hypothesis that Asp(923) contributes to a site binding the third Na(+) ion. These results in conjunction with our previous findings with other RDP mutants suggest that a selective defect in the handling of Na(+) may be a general feature of the RDP disorder.

The C terminus of Na+,K+-ATPase controls Na+ affinity on both sides of the membrane through Arg935

The Na(+),K(+)-ATPase C terminus has a unique location between transmembrane segments, appearing to participate in a network of interactions. We have examined the functional consequences of amino acid substitutions in this region and deletions of the C terminus of varying lengths. Assays revealing separately the mutational effects on internally and externally facing Na(+) sites, as well as E(1)-E(2) conformational changes, have been applied. The results pinpoint the two terminal tyrosines, Tyr(1017) and Tyr(1018), as well as putative interaction partners, Arg(935) in the loop between transmembrane segments M8 and M9 and Lys(768) in transmembrane segment M5, as crucial to Na(+) activation of phosphorylation of E(1), a partial reaction reflecting Na(+) interaction on the cytoplasmic side of the membrane. Tyr(1017), Tyr(1018), and Arg(935) are furthermore indispensable to Na(+) interaction on the extracellular side of the membrane, as revealed by inability of high Na(+) concentrations to drive the transition from E(1)P to E(2)P backwards toward E(1)P and inhibit Na(+)-ATPase activity in mutants. Lys(768) is not important for Na(+) binding from the external side of the membrane but is involved in stabilization of the E(2) form. These data demonstrate that the C terminus controls Na(+) affinity on both sides of the membrane and suggest that Arg(935) constitutes an important link between the C terminus and the third Na(+) site, involving an arginine-pi stacking interaction between Arg(935) and the C-terminal tyrosines. Lys(768) may interact preferentially with the C terminus in E(1) and E(1)P forms and with the loop between transmembrane segments M6 and M7 in E(2) and E(2)P forms.

Potassium as an intrinsic uncoupler of the plasma membrane H+-ATPase

The plant plasma membrane proton pump (H(+)-ATPase) is stimulated by potassium, but it has remained unclear whether potassium is actually transported by the pump or whether it serves other roles. We now show that K(+) is bound to the proton pump at a site involving Asp(617) in the cytoplasmic phosphorylation domain, from where it is unlikely to be transported. Binding of K(+) to this site can induce dephosphorylation of the phosphorylated E(1)P reaction cycle intermediate by a mechanism involving Glu(184) in the conserved TGES motif of the pump actuator domain. Our data identify K(+) as an intrinsic uncoupler of the proton pump and suggest a mechanism for control of the H(+)/ATP coupling ratio. K(+)-induced dephosphorylation of E(1)P may serve regulatory purposes and play a role in negative regulation of the transmembrane electrochemical gradient under cellular conditions where E(1)P is accumulating.

The identification of the sodium pump

The identification of the sodium potassium pump as a Na+, K+-ATPase is described.

Slippage and uncoupling in P-type cation pumps; implications for energy transduction mechanisms and regulation of metabolism

P-type ATPases couple scalar and vectorial events under optimized states. A number of procedures and conditions lead to uncoupling or slippage. A key branching point in the catalytic cycle is at the cation-bound form of E(1)-P, where isomerization to E(2)-P leads to coupled transport, and hydrolysis leads to uncoupled release of cations to the cis membrane surface. The phenomenon of slippage supports a channel model for active transport. Ability to occlude cations within the channel is essential for coupling. Uncoupling and slippage appear to be inherent properties of P-type cation pumps, and are significant contributors to standard metabolic rate. Heat production is favored in the uncoupled state. A number of disease conditions, include ageing, ischemia and cardiac failure, result in uncoupling of either the Ca(2+)-ATPase or Na(+)/K(+)-ATPase.

ATP-dependent silver transport across the basolateral membrane of rainbow trout gills

Silver has been shown to be extremely toxic to freshwater teleosts, acting to inhibit Na(+) uptake at the gills, due to the inactivation of branchial Na(+)/K(+)-ATPase activity. However, the gills are also a route by which silver may enter the fish. Therefore, this study focuses on the mechanism of transport of this nonessential metal across the basolateral membrane of the gill cell, using basolateral membrane vesicles (BLMV) prepared from the gills of freshwater rainbow trout. Uptake of silver by BLMV was via a carrier-mediated process, which was ATP-dependent, reached equilibium over time, and followed Michaelis-Menten kinetics, with maximal transport capacity (V(max)) of 14.3 +/- 5.5 (SE) nmol mg membrane protein(-1) min(-1) and an affinity (K(m)) of 62.6 +/- 43.7 microM, and was inhibited by 100 microM sodium orthovanadate (Na(3)VO(4)). The ionophore monensin (10 microM) released transported silver from the BLMV. Acylphosphate intermediates, of a 104 kDa size, were formed from the BLMV preparations in the presence of ATP plus Ag. These results demonstrate that there is a P-type ATPase present in the basolateral membrane of the gills of rainbow trout that can actively transport silver, a process which will remove this heavy metal from its site of toxic action, the gill.

Leucine 332 at the boundary between the fourth transmembrane segment and the cytoplasmic domain of Na+,K+-ATPase plays a pivotal role in the ion translocating conformational changes

Mutants Gly330-->Ala, Leu332-->Ala, Leu332-->Pro, and Pro780-->Ala of the alpha1-isoform of rat kidney Na+,K+-ATPase were expressed in COS-1 cells to active site concentrations between 20 and 70 pmol per mg of membrane protein. The functional properties of the mutants were characterized by titrations of Na+-, K+-, and ATP-dependencies of Na+,K+-ATPase activity as well as by a series of assays measuring the K+-dependence of the steady-state phosphoenzyme level, the kinetics of dephosphorylation under a variety of conditions, and the ADP-ATP exchange activity. In mutants Gly330-->Ala, Leu332-->Ala, and Leu332-->Pro, the molecular turnover number was reduced relative to that of the wild-type Na+,K+-ATPase, and the steady-state phosphoenzyme level was high even in the presence of several millimolar K+. At a low Na+ concentration in the absence of K+, mutants Leu332-->Pro and Gly330-->Ala displayed high ADP-ATP exchange activity and formed a high level of the ADP-sensitive phosphoenzyme (E1P). The phosphoenzyme decayed slowly following a jump in salt concentration and chase with ATP and K+. Hence, the conversion of E1P to the K+-sensitive phosphoenzyme (E2P) was inhibited in mutants Leu332-->Pro and Gly330-->Ala. In the Leu332-->Ala mutant, a high level of E2P was accumulated in the absence of K+, and the ADP-ATP exchange activity was low at low Na+ concentration in the absence of K+, but rose sharply on addition of K+. Dephosphorylation experiments indicated that in the Leu332-->Ala mutant K+ induced reversal of the phosphoenzyme interconversion, forming E1P from E2P. Leu332 is therefore a pivotal residue in the conformational change. Mutants Gly330-->Ala and Pro780-->Ala displayed reduced K+ affinities relative to the wild type, determined in three independent assays.

Kinetics of conformational changes associated with potassium binding to and release from Na+/K(+)-ATPase

The Na+/K(+)-ATPase functions in cells to couple energy from the hydrolysis of ATP to the transport Na+ out and K+ in. The fluorescent probe IAF (iodoacetamidofluorescein) covalently binds to this enzyme, reporting conformational changes without inhibiting enzyme activity. This paper describes experiments using dog kidney enzyme labeled with IAF to examine kinetics of conformational changes resulting from added Na+ and K+, measured in terms of steady-state and stopped-flow fluorescence changes. Kinetics of these fluorescence changes were examined as a function of temperature from two initial conditions: (a) enzyme in the high-fluorescence form (E(high)) was rapidly mixed with varying [K+]; and (b) enzyme in the low-fluorescence form (E(low)) was rapidly mixed with varying [ATP]. These experiments showed: (1) The rate constant for the fluorescence change from E(high) to E(low) was much larger than that for the opposite transition, E(low) to E(high); (2) the apparent free energy of activation (Ea(app)) for the two transitions were different (as estimated from Arrhenius plots); (3) under steady-state conditions, IAF fluorescence did not change when ATP was added to E(low)(K+) in the absence of Na+; (4) the apparent free energy of activation was independent of [K+] for the E(high) to E(low) transition (at 16.4 kcal/mol) but increased with [ATP] for the E(low) to E(high) transition; (5) Ea(app) for the E(low) to E(high) transition with 1 mM ATP was approximately the same as that in the absence of ATP (34 kcal/mol). These results can be interpreted as: (i) in the transition from E(low) to E(high), IAF reported a conformational change that occurred after K+ release to the intracellular side and which is involved in Na+ binding; (ii) Ea(app) increased with [ATP], while increasing the entropy of the transition state. Thus, ATP appeared to destabilize the enzyme during the transition from E(low) to E(high).

Phosphorylation/dephosphorylation of reconstituted shark Na+,K(+)-ATPase: one phosphorylation site per alpha beta protomer

In the present investigation reconstitution of Na+,K(+)-ATPase increases the number of phosphorylation sites (EP) of solubilized enzyme from 4.2 +/- 0.3 nmol/mg to 6.9 +/- 0.6 nmol/mg. The latter figure corresponds to one phosphorylation site per alpha beta-promoter. A cholesterol content > 10 mol% in the liposome bilayer and a high extracellular [Na+] are necessary to obtain this high value. Spontaneous dephosphorylation after maximum phosphorylation in Na+ is biphasic both in solubilized enzyme and after reconstitution. The rate of dephosphorylation compares with the specific hydrolytic Na(+)-ATPase activity measured at exactly identical conditions for all three preparations assuming parallel dephosphorylation of at least two phosphointermediates. The distribution of EP-species is found to vary among the three enzyme preparation used, i.e., membrane bound, solubilized, and reconstituted Na+,K(+)-ATPase, however in all the equilibrium is strongly poised away from the E1P-form.

Cholesterol modulation of molecular activity of reconstituted shark Na+,K(+)-ATPase

The cholesterol content of liposome bilayers has been varied between 0-40 mol% to study the effects on reconstituted Na+,K(+)-ATPase. The maximum hydrolytic activity of reconstituted Na+,K(+)-ATPase was increased by cholesterol at concentrations above 10 mol% for both the physiological Na+/K(+)-exchange reactions, as well as for the partial reactions Na+/Na(+)-exchange and uncoupled Na+ efflux. Omission of cholesterol from the liposome bilayer modified the activation by cytoplasmic Na+, indicating effects on both Vmax and on the Na(+)-affinity. Several other kinetic parameters were found to be strongly influenced as well, most notable the steady-state phosphorylation level, and the characteristics of the phosphorylation/dephosphorylation reactions. These results indicate that cholesterol interacts directly with the Na+,K(+)-ATPase as an essential effector perhaps by affecting its conformational mobility or monomer interaction.

Rapid kinetic analyses of the Na+/K(+)-ATPase distinguish among different criteria for conformational change

The Na+/K(+)-ATPase couples the hydrolysis of ATP to the transport of Na+ and K+ via a phosphorylated intermediate and conformational changes. In order to identify these conformational changes, we have probed the sequence of steps from EP(3Na+ in) to EP + 3Na+ out with three fluorescent probes (IAF: 5-iodoacetamidofluorescein; BIPM: N-[p-(2-benzimidazolyl)phenyl]maleimide; and RH421) and the sensitivity of their fluorescence change to oligomycin and divalent cations (Ca2+ and Mn2+). The magnitude (% delta F) and rate constant (k(obs)) of ATP-induced fluorescence changes were measured on a fluorescence stopped-flow apparatus, and yielded the following results. (a) With RH421, k(obs) and % delta F varied with [Na+] (maximal k(obs) = 100 s-1, K1/2 = 6 mM; % delta Fmax = 6%, K1/2 = 1 mM); these values are comparable to those previously reported using IAF-labeled enzyme (Pratap, P.R., Robinson, J.D. and Steinberg, M.I. (1991) Biochim. Biophys. Acta 1069, 288-298). (b) With BIPM-labeled enzyme k(obs) did not vary with [Na+] over the range tested, and was twice as high as the maximum k(obs) for RH421. (c) Treatment with oligomycin reduced k(obs) for all three probes to about the same level (approximately 1-2 s-1) while % delta Fmax was largely unaffected. (d) Replacing Mg2+ with Ca2+ had similar effects to treatment with oligomycin. (e) RH421 fluorescence change was completely abolished in the presence of oligomycin and Ca2+. (f) Replacing Mg2+ with Mn2+ decreased IAF fluorescence, i.e., put the enzyme in an E2-like form, reduced k(obs), and rendered oligomycin less effective in reducing k(obs). From these results, we conclude: (a) the release of the second/third Na+ is the rate-limiting step for the conformational change measured by IAF and charge transfer measured with RH421; (b) BIPM indicates an earlier step, either the deocclusion of Na+ and/or the release of the first Na+; (c) oligomycin blocks Na+ deocclusion, and this step is sensitive to the divalent cation used to activate enzyme phosphorylation; and (d) Ca2+ slows the step reported by IAF as well. These experiments indicate that a simple model with two conformations (E1 and E2) is insufficient to explain transient kinetic data.

Cis-allosteric effects of cytoplasmic Na+/K+ discrimination at varying pH. Low-affinity multisite inhibition of cytoplasmic K+ in reconstituted Na+/K(+)-ATPase engaged in uncoupled Na(+)-efflux

In liposomes with reconstituted shark Na+/K(+)-ATPase the effect of cytoplasmic K+ was investigated in the absence of extracellular alkali ions. During such conditions the Na+/K(+)-ATPase is engaged in the so called uncoupled Na+ efflux mode in which cytoplasmic Na+ activates and binds to the enzyme and becomes translocated without countertransport of K+ as in the physiological Na+/K+ exchange mode. In this uncoupled flux mode only low-affinity inhibition by K+cyt is found to be present. The inhibition pattern is consistent with a model in which cytoplasmic K+ exhibit mixed inhibition of Na+ activation, probably by binding at the three cytoplasmic loading sites on E1ATP (E1A). With determined intrinsic binding constants for cytoplasmic Na+ to this form of KS1, KS2, KS3 = 40 mM, 2 mM, 2 mM the inhibition pattern can be simulated assuming three K+cyt sites with equal affinity for Ki = 40 mM, similar to KS1 for the first Na+cyt site. The discrimination between cytoplasmic Na+ and K+ is therefore enhanced by allosteric interaction initiated from the cis-side due to binding of the first Na+, as opposed to K+, which induces the positive cooperatively in the successive Na+ bindings. pH is found to influence the pattern of K+cyt inhibition: A lowering of the pH potentiates the K+cyt inhibition, whereas at increased pH the inhibition is decreased and transformed into a pure competitive competition.

Role of protein conformation changes and transphosphorylations in the function of Na+/K(+)-transporting adenosine triphosphatase: an attempt at an integration into the Na+/K+ pump mechanism

The particular aim of the review on some basic facets of the mechanism of Na+/K(+)-transporting ATPase (Na/K-ATPase) has been to integrate the experimental findings concerning the Na(+)- and K(+)-elicited protein conformation changes and transphosphorylations into the perspective of an allosterically regulated, phosphoryl energy transferring enzyme. This has led the authors to the following summarizing evaluations. 1. The currently dominating hypothesis on a link between protein conformation changes ('E1 in equilibrium with E2') and Na+/K+ transport (the 'Albers-Post scheme') has been constructed from a variety of partial reactions and elementary steps, which, however, do not all unequivocally support the hypothesis. 2. The Na(+)- and K(+)-elicited protein conformation changes are inducible by a variety of other ligands and modulatory factors and therefore cannot be accepted as evidence for their direct participation in effecting cation translocation. 3. There is no evidence that the 'E1 in equilibrium with E2' protein conformation changes are moving Na+ and K+ across the plasma membrane. 4. The allosterically caused ER in equilibrium with ET ('E1 in equilibrium with E2') conformer transitions and the associated cation 'occlusion' in equilibrium with 'de-occlusion' processes regulate the actual catalytic power of an enzyme ensemble. 5. A host of experimental variables determines the proportion of functionally competent ER enzyme conformers and incompetent ET conformers so that any enzyme population, even at the start of a reaction, consists of an unknown mixture of these conformers. These circumstances account for the occurrence of contradictory observations and apparent failures in their comparability. 6. The modelling of the mechanism of the Na/K-ATPase and Na+/K+ pump from the results of reductionistically designed experiments requires the careful consideration of the physiological boundary conditions. 7. Na+ and K+ ligandation of Na/K-ATPase controls the geometry and chemical reactivity of the catalytic centre in the cycle of E1 in equilibrium with E2 state conversions. This is possibly effected by hinge-bending, concerted motions of three adjacent, intracellularly exposed peptide sequences, which shape open and closed forms of the catalytic centre in lock-and-key responses. 8. The Na(+)-dependent enzyme phosphorylation with ATP and the K(+)-dependent hydrolysis of the phosphoenzyme formed are integral steps in the transport mechanism of Na/K-ATPase, but the translocations of Na+ and K+ do not occur via a phosphate-cation symport mechanism.(ABSTRACT TRUNCATED AT 400 WORDS)

The reaction sequence of the Na+/K(+)-ATPase: rapid kinetic measurements distinguish between alternative schemes

Conformational changes between E1 and E2 enzyme forms of a dog kidney Na+/K(+)-ATPase preparation labeled with 5-iodoacetamidofluorescein were followed with a stopped-flow fluorimeter, in terms of the rate constant, kobs, and the steady-state magnitude, % delta F of fluorescence change. On rapid mixing of enzyme plus Mg2+ plus Na+ with saturating (0.5 mM) ATP in the absence of K+, kobs varied with Na+ concentration in the range 0-155 mM, with a K1/2 of 10 mM, while % delta F was relatively insensitive to Na+, with a K1/2 of 0.5 mM. Oligomycin reduced kobs by 98-99% for Na+ greater than or equal to 10 mM, but only by 50% for Na+ = 1 mM; % delta F was reduced at most by 20%. At 155 mM Na+, both kobs and % delta F changed if K+ was present with the enzyme. kobs decreased by 50% when K+ was increased from 0 to 0.2 mM, but increased when K+ was varied in the range 0.2-5 mM. K+ increased % delta F by a factor of 3 with a K1/2 of 0.3-0.5 mM as measured in both stopped-flow and steady-state experiments. These data are considered in terms of the derived presteady-state equations for two alternate schemes for the enzyme, with the E1P to E2P conformational change either preceding (Albers-Post) or following (Nørby-Yoda-Skou) Na+ transport and release. The analysis indicates that: (i) Na+ must be released before the conformational transition, from an E1 form; (ii) the step in which the second and/or third Na+ is released is rate-limiting, but this release is accelerated by Na+; and (iii) the release is also accelerated by K+ acting with low affinity (possibly at extracellular sites).

Kinetics of oligomycin inhibition and activation of Na+/K(+)-ATPase

Oligomycin inhibition of the maximal hydrolysis activity of ox brain Na+/K(+)-ATPase was studied at varying NaCl concentrations and it was found that for a given amount of live enzyme, the observed inhibition of a particular total oligomycin concentration decreased as the amount of added, (heat-) denatured enzyme increased. In the present article we derive a scale factor for the oligomycin concentration, i.e., the fraction of the total concentration of oligomycin which is free in solution, as a function of the enzyme concentration used. This fraction decreased linearly with the protein concentration and may attain quite small values. We also study the Na(+)-dependence of the hydrolysis rate at saturating substrate concentrations ([Mg2+] = [ATP] = 3 mM), in the presence as well as the absence of KCl, at various concentrations of oligomycin. These data may be explained if it is assumed that the sole effect of oligomycin is to confer upon the enzyme an increased affinity for Na+, i.e., oligomycin merely enhances the inhibitory effect of Na+ on the (maximal) activity seen at high Na(+)-concentrations. The increased Na(+)-affinity in the presence of oligomycin should result in activation of the hydrolysis rate measured under conditions where Na(+)-activation is predominant, i.e., at low Na(+)-concentration and sub-saturating substrate concentrations. This prediction is verified for both Na(+)-ATPase and for Na+/K(+)-ATPase. This proposed action of oligomycin seems to be corroborated also by other evidence discussed in the text.

CCCP activation of the reconstituted NaK-pump

In the NaK-ATPase proteoliposomes (PLs), the NaK-pump activity, Na+ uptake, and ATP hydrolysis were apparently enhanced by carbonyl cyanide m-chlorophenyl hydrazone (CCCP) and other ionophores without ion gradients. These ionophore effects were not cation specific. Without ionophores, the PL's ATPase activity fell to its steady-state value within 3 sec at 15 degrees C. This decrease in activity disappeared in the presence of CCCP. Since CCCP is believed to enhance proton mobility across the lipid bilayer and dissipate membrane potential (Vm), we postulated that a Vm build-up partially inhibits the PLs by changing the conformation of the NaK-pump, and that CCCP eliminated this partial inhibition. Since this activation required extracellular K+ and high ATP concentration in the PLs, CCCP must affect the conversion between the phosphorylated forms of NaK-ATPase (EP); this step has been suggested by Goldschlegger et al. (1987) to be the voltage-sensitive step (J. Physiol. (London) 387:331-355). Although cytoplasmic K+ accelerated the change of ADP- and K(+)-sensitive EP (E*P) to K(+)-sensitive ADP-insensitive EP (E2P), CCCP did not complete with cytoplasmic K+ when cytoplasmic Na+ was saturated. When the PLs were phosphorylated with 20 microM ATP and 20 microM palmitoyl CoA instead of with high concentration of ATP, CCCP increased the E*P content and decreased the ADP-sensitive K(+)-insensitive EP (E1P). The results described above suggest that CCCP affects the E1P to E*P change in the E1P----E*P----E2P conversion and that this reaction step is inhibited by Vm.

Cation sidedness in the phosphorylation step of Na+/K+-ATPase

Na+/K+ -ATPase, reconstituted into phospholipid vesicles, has been used to study the localisation of binding sites of ligands involved in the phosphorylation reaction. Inside-out oriented Na+/K+ -ATPase molecules are the only population in this system, which can be phosphorylated, as the rightside-out oriented as well as the non-incorporated enzyme molecules are inhibited by ouabain. In addition, the right-side-out oriented Na+/K+ -ATPase molecules have their ATP binding site intravesicularly and are thus not accessible to substrate added to the extravesicular medium. Functional binding sites for the following ligands have been demonstrated: (i) Potassium, acting at the extracellular side with high affinity (stimulating the dephosphorylation rate of the E2P conformation) and low affinity (inducing the non-phosphorylating E2K complex). (ii) Potassium, acting at the cytoplasmic side with both high and low affinity. The latter sites are also responsible for the formation of an E2K complex and complete with Na+ for its binding sites. (iii) Sodium at the cytoplasmic side responsible for stimulation of the phosphorylation reaction. (iv) Sodium (and amine buffers) at the extracellular side enhancing the phosphorylation level of Na+/K+ -ATPase where choline chloride has no effect. (v) Magnesium at the cytoplasmic side, stimulating the phosphorylation reaction and inhibiting it above optimal concentrations.

Conformational transitions and change translocation by the Na,K pump: comparison of optical and electrical transients elicited by ATP-concentration jumps

The electrogenic properties of the Na,K-ATPase were studied by correlating transient electrical events in the pump molecule with conformational transitions elicited by an ATP-concentration jump. Flat membrane fragments containing a high density (approximately 8000 microm(-2)) of oriented Na,K-ATPase molecules were bound to a planar lipid bilayer acting as a capacitive electrode. ATP was released in the medium from a photolabile inactive ATP derivative ("caged" ATP) by a 40-microsec light flash. Electrical signals resulting from transient charge movements in the protein under single-turnover conditions were recorded in the external measuring circuit. In parallel experiments carried out under virtually identical conditions, the fluorescence of membrane fragments containing Na,K-ATPase with covalently-bound 5-iodoacetamido-fluorescein (5-IAF) was monitored after the ATP-concentration jump. When the medium contained Na+, but no K+, the fluorescence of the 5-IAF-labeled protein decreases monotonously after release of ATP. In the experiments with membrane fragments bound to a planar bilayer, a transient pump current was observed which exhibited virtually the same time behavior as the fluorescence decay. This indicates that optical and electrical transients are governed by the same rate-limiting reaction step. Experiments with chymotrypsin-modified Na,K-ATPase suggest that both the fluorescence change as well as the charge movement are associated with the deocclusion of Na+ and release to the extracellular side. In experiments with Na+-free K+ media, a large inverse fluorescence change is observed after the ATP-concentration jump, but no charge translocation can be detected. This indicates that deocclusion of K+ is an electrically silent process.

Transient behaviour of the Na+/K+-pump: microscopic analysis of nonstationary ion-translocation

In recent years fast perturbation techniques have been applied for investigating the mechanism of the Na+/K+-pump. Experiments in which nonstationary pump-currents and ion fluxes are measured after a voltage or ATP-concentration jump yield kinetic information which cannot be obtained from ordinary steady-state experiments. In this paper a theoretical treatment is described by which transient pump-currents and ion fluxes can be analyzed in a unified way. The method is based on the assumption that the operation of the pump involves a sequence of conformational transitions and ion-binding and -release steps. The charge displacements associated with the individual reaction steps are described by a set of dielectric coefficients. The nonstationary behaviour of the Na+/K+-pump is analyzed on the basis of the Albers-Post reaction cycle. It is shown that the different studies of transient pump-currents and ion fluxes carried out so far lead to internally consistent conclusions with respect to the nature of the electrogenic steps of the transport cycle.

Apparent cooperativity of [3H]ouabain binding to myocytes obtained from guinea-pig heart

Kinetics of [3H]ouabain binding to intact cardiac cells were examined using myocytes obtained from guinea-pig heart. In intact cells, the use of excess unlabeled ouabain results in an under-estimation of nonspecific binding, presumably due to cytotoxic effects of the unlabeled glycoside; estimation of the specific binding, as that to rapidly releasing sites yields more accurate results. Specific [3H]ouabain binding to myocytes is promoted by an increase in Na+ influx, indicating that normal intracellular Na+ concentration is insufficient to fully stimulate glycoside binding. High concentrations of [3H]ouabain seem to increase the apparent affinity of binding sites for the glycoside via increases in intracellular Na+ concentration resulting from sodium-pump inhibition; hence the binding reaction may be regarded as having a novel type of cooperativity. This cooperativity has kinetics different from those of classical positive cooperativity based on binding-site interactions, and is apparent with toxic concentrations of the glycoside that cause marked increases in intracellular Na+ concentrations.

(Na+ + K+)-ATPase: confirmation of the three-pool model for the phosphointermediates of Na+-ATPase activity. Estimation of the enzyme-ATP dissociation rate constant

The dephosphorylation kinetics of acid-stable phosphointermediates of (Na+ + K+)-ATPase from ox brain, ox kidney and pig kidney was studied at 0 degree C. Experiments performed on brain enzyme phosphorylated at 0 degree C in the presence of 20-600 mM Na+, 1 mM Mg2+ and 25 microM [gamma-32P]ATP show that irrespectively of the EP-pool composition, which is determined by Na+ concentration, all phosphoenzyme is either ADP- or K+-sensitive. After phosphorylation of kidney enzymes at 0 degree C with 1 mM Mg2+, 25 microM [gamma-32P]ATP and 150-1000 mM Na+ the amounts of ADP- and K+-sensitive phosphoenzymes were determined by addition of 1 mM ATP + 2.5 mM ADP or 1 mM ATP + 20 mM K+. Similarly to the previously reported results on brain enzyme, both types of dephosphorylation curves have a fast and a slow phase, so that also for kidney enzymes a slow decay of a part of the phosphoenzyme, up to 80% at 1000 mM Na+, after addition of 1 mM ATP + 20 mM K+ is observed. The results obtained with the kidney enzymes seem therefore to reinforce previous doubts about the role played by E1 approximately P(Na3) as intermediate of (Na+ + K+)-ATPase activity. Furthermore, for both kidney enzymes the sum of ADP- and K+-sensitive phosphoenzymes is greater than E tot. In experiments on brain enzyme an estimate of dissociation rate constant for the enzyme-ATP complex, k-1, is obtained. k-1 varies between 1 and 4 s-1 and seems to depend on the ligands present during formation of the complex. The highest values are found for enzyme-ATP complex formed in the presence of Na+ or Tris+. The results confirm the validity of the three-pool model in describing dephosphorylation kinetics of phosphointermediates of Na+-ATPase activity.