Fluorescence study on cardiac glycoside binding to the Na,K-pump. Ouabain binding is associated with movement of electrical charge

Displacement of the Na+/K+ pump's transmembrane domains demonstrates conserved conformational changes in P-type 2 ATPases

Cellular survival requires the ion gradients built by the Na⁺/K⁺ pump, an ATPase that alternates between two major conformations (E1 and E2). Here we use state-specific engineered-disulfide cross-linking to demonstrate that transmembrane segment 2 (M2) of the pump's α-subunit moves in directions that are inconsistent with distances observed in existing crystal structures of the Na⁺/K⁺ pump in E1 and E2. We characterize this movement with voltage-clamp fluorometry in single-cysteine mutants. Most mutants in the M1-M2 loop produced state-dependent fluorescence changes upon labeling with tetramethylrhodamine-6-maleimide (TMRM), which were due to quenching by multiple endogenous tryptophans. To avoid complications arising from multiple potential quenchers, we analyzed quenching of TMRM conjugated to R977C (in the static M9-M10 loop) by tryptophans introduced, one at a time, in M1-M2. This approach showed that tryptophans introduced in M2 quench TMRM only in E2, with D126W and L130W on the same helix producing the largest fluorescence changes. These observations indicate that M2 moves outward as Na⁺ is deoccluded from the E1 conformation, a mechanism consistent with cross-linking results and with proposals for other P-type 2 ATPases.

Probing the extracellular access channel of the Na,K-ATPase

When the Na,K-ATPase pumps at each turnover two K(+) ions into the cytoplasm, this translocation consists of several reaction steps. First, the ions diffuse consecutively from the extracellular phase through an access pathway to the binding sites where they are coordinated. In the next step, the enzyme is dephosphorylated and the ions are occluded inside the membrane domain. The subsequent transition to the E1 conformation produces a deocclusion of the binding sites to the cytoplasmic side of the membrane and allows in the last steps ion dissociation and diffusion to the aqueous phase. The interaction and competition of K(+) with various quaternary organic ammonium ions have been used to gain insight into the molecular mechanism of the ion binding process from the extracellular side in the P-E2 conformation of the enzyme. Using the electrochromic styryl dye RH421, evidence has been obtained that the access pathway consists of a wide and water-filled funnel-like part that is accessible also for bulky cations such as the benzyltriethylammonium ion, and a narrow part that permits passage only of small cations such as K(+) and NH4(+) in a distinct electrogenic way. Benzyltriethylammonium ions inhibit K(+) binding in a competitive manner that can be explained by a stopper-like function at the interface between the wide and narrow parts of the access pathway. In contrast to other quaternary organic ammonium ions, benzyltriethylammonium ions show a specific binding to the ion pump in a position inside the access pathway where it blocks effectively the access to the binding sites.

A structural rearrangement of the Na+/K+-ATPase traps ouabain within the external ion permeation pathway

With the use of the energy of ATP hydrolysis, the Na+/K+-ATPase is able to transport across the cell membrane Na+ and K+ against their electrochemical gradients. The enzyme is strongly inhibited by ouabain and its derivatives, some that are therapeutically used for patients with heart failure (cardiotonic steroids). Using lanthanide resonance energy transfer, we trace here the conformational changes occurring on the external side of functional Na+/K+-ATPases induced by the binding of ouabain. Changes in donor/acceptor pair distances are mainly observed within the α subunit of the enzyme. To derive a structural model matching the experimental lanthanide resonance energy transfer distances measured with bound ouabain, we carried out molecular dynamics simulations with energy restraints applied simultaneously using a novel methodology with multiple non-interacting fragments. The restrained simulation, initiated from the X-ray structure of the E2(2K+) state, became strikingly similar to the X-ray structure of the sodium-bound state. The final model shows that ouabain is trapped within the external ion permeation pathway of the pump.

New crystal structures of PII-type ATPases: excitement continues

P-type ATPases are ATP-powered ion pumps, classified into five subfamilies (PI-PV). Of these, PII-type ATPases, including Ca2+-ATPase, Na+,K+-ATPase and gastric H+,K+-ATPase, among others, have been the most intensively studied. Best understood structurally and biochemically is Ca2+-ATPase from sarcoplasmic reticulum of fast twitch skeletal muscle (sarco(endo)plasmic reticulum Ca2+-ATPase 1a, SERCA1a). Since publication of the first crystal structure in 2000, it has continuously been a source of excitement, as crystal structures for new reaction intermediates always show large structural changes. Crystal structures now exist for most of the reaction intermediates, almost covering the entire reaction cycle. This year the crystal structure of a missing link, the E1·Mg2+ state, finally appeared, bringing another surprise: bound sarcolipin (SLN). The current status of two other important PII-type ATPases, Na+,K+-ATPase and H+,K+-ATPase, is also briefly described.

First crystal structures of Na+,K+-ATPase: new light on the oldest ion pump

Na(+),K(+)-adenosine triphosphatase (NKA) is the first P-type ion translocating adenosine triphosphatase (ATPase) ever identified, and the significance of this class of proteins was highlighted by the 1997 Nobel Prize in Chemistry awarded to Jens C. Skou for the discovery in 1957. More than half a century passed between the initial identification and the publication of a high-resolution crystal structure of NKA. Although the new crystal structures provided many surprises and insights, structural biology on this system remains challenging, as NKA is a very difficult protein to crystallize. Here we explain the reasons behind the challenges, introduce a mechanism that governs the function, and summarize current knowledge of NKA structure in comparison with another member of the P-type ATPase family, Ca(2+)-ATPase.

The dynamic relationships between the three events that release individual Na⁺ ions from the Na⁺/K⁺-ATPase

Na(+)/K(+) pumps move net charge through the cell membrane by mediating unequal exchange of intracellular Na(+) and extracellular K(+). Most charge moves during transitions that release Na(+) to the cell exterior. When pumps are constrained to bind and release only Na(+), a membrane voltage-step redistributes pumps among conformations with zero, one, two or three bound Na(+), thereby transiently generating current. By applying rapid voltage steps to squid giant axons, we previously identified three components in such transient currents, with distinct relaxation speeds: fast (which nearly parallels the voltage-jump time course), medium speed (τ(m)=0.2-0.5 ms) and slow (τ(s)=1-10 ms). Here we show that these three components are tightly correlated, both in their magnitudes and in the time courses of their changes. The correlations reveal the dynamics of the conformational rearrangements that release three Na(+) to the exterior (or sequester them into their binding sites) one at a time, in an obligatorily sequential manner.

Ouabain binding site in a functioning Na+/K+ ATPase

The Na(+)/K(+) ATPase is an almost ubiquitous integral membrane protein within the animal kingdom. It is also the selective target for cardiotonic derivatives, widely prescribed inhibitors for patients with heart failure. Functional studies revealed that ouabain-sensitive residues distributed widely throughout the primary sequence of the protein. Recently, structural work has brought some consensus to the functional observations. Here, we use a spectroscopic approach to estimate distances between a fluorescent ouabain and a lanthanide binding tag (LBT), which was introduced at five different positions in the Na(+)/K(+) ATPase sequence. These five normally functional LBT-Na(+)/K(+) ATPase constructs were expressed in the cell membrane of Xenopus laevis oocytes, operating under physiological internal and external ion conditions. The spectroscopic data suggest two mutually exclusive distances between the LBT and the fluorescent ouabain. From the estimated distances and using homology models of the LBT-Na(+)/K(+) ATPase constructs, approximate ouabain positions could be determined. Our results suggest that ouabain binds at two sites along the ion permeation pathway of the Na(+)/K(+) ATPase. The external site (low apparent affinity) occupies the same region as previous structural findings. The high apparent affinity site is, however, slightly deeper toward the intracellular end of the protein. Interestingly, in both cases the lactone ring faces outward. We propose a sequential ouabain binding mechanism that is consistent with all functional and structural studies.

Confining the sodium pump in a phosphoenzyme form: the effect of lead(II) ions

The effect of Pb(2+) ions on the Na(+),K(+)-ATPase was investigated in detail by means of steady-state fluorescence spectroscopy. Experiments were performed by using the electrochromic styryl dye RH421. It is shown that Pb(2+) ions can bind reversibly to the protein and do not affect the Na(+) and K(+) binding affinities in the E(1) and P-E(2) conformations of the enzyme. The pH titrations indicate that lead(II) favors binding of one H(+) to the P-E(2) conformation in the absence of K(+). A model scheme is proposed that accounts for the experimental results obtained for backdoor phosphorylation of the enzyme in the presence of Pb(2+) ions. Taken together, our results clearly indicate that Pb(2+) bound to the enzyme stabilizes an E(2)-type conformation. In particular, under conditions that promote enzyme phosphorylation, Pb(2+) ions are able to confine the Na(+),K(+)-ATPase into a phosphorylated E(2) state.

ATP binding equilibria of the Na(+),K(+)-ATPase

Reported values of the dissociation constant, K(d), of ATP with the E1 conformation of the Na(+),K(+)-ATPase fall in two distinct ranges depending on how it is measured. Equilibrium binding studies yield values of 0.1-0.6 microM, whereas presteady-state kinetic studies yield values of 3-14 microM. It is unacceptable that K(d) varies with the experimental method of its determination. Using simulations of the expected equilibrium behavior for different binding models based on thermodynamic data obtained from isothermal titration calorimetry we show that this apparent discrepancy can be explained in part by the presence in presteady-state kinetic studies of excess Mg(2+) ions, which compete with the enzyme for the available ATP. Another important contributing factor is an inaccurate assumption in the majority of presteady-state kinetic studies of a rapid relaxation of the ATP binding reaction on the time scale of the subsequent phosphorylation. However, these two factors alone are insufficient to explain the previously observed presteady-state kinetic behavior. In addition one must assume that there are two E1-ATP binding equilibria. Because crystal structures of P-type ATPases indicate only a single bound ATP per alpha-subunit, the only explanation consistent with both crystal structural and kinetic data is that the enzyme exists as an (alphabeta)(2) diprotomer, with protein-protein interactions between adjacent alpha-subunits producing two ATP affinities. We propose that in equilibrium measurements the measured K(d) is due to binding of ATP to one alpha-subunit, whereas in presteady-state kinetic studies, the measured apparent K(d) is due to the binding of ATP to both alpha-subunits within the diprotomer.

Effect of clotrimazole on the pump cycle of the Na,K-ATPase

The effect of the antimycotic drug clotrimazole (CLT) on the Na,K-ATPase was investigated using fluorescence and electrical measurements. The results obtained by steady-state fluorescence experiments with the electrochromic styryl dye RH421 were combined with those achieved by a pre-steady-state method based on fast solution exchange on a solid supported membrane that adsorbs the protein. Both techniques are suitable for monitoring the electrogenic steps of the pump cycle and are in general complementary, yielding distinct kinetic information. The experiments show clearly that CLT affects specific partial reactions of the pump cycle of the Na,K-ATPase with an affinity in the low micromolar range and in a reversible manner. All results can be consistently explained by proposing the CLT-promoted formation of an ion-occluded-CLT-bound conformational E(2) state, E(2)(CLT)(X(2)) that acts as a "dead-end" side track of the pump cycle, where X stands for H+ or K+. Na+ binding, enzyme phosphorylation, and Na+ transport were not affected by CLT, and at high CLT concentrations approximately (1/3) of the enzyme remained active in the physiological transport mode. The presence of Na+ and K+ destabilized the inactivated form of the Na,K-ATPase.

Sodium flux ratio in Na/K pump-channels opened by palytoxin

Palytoxin binds to Na⁺/K⁺ pumps in the plasma membrane of animal cells and opens an electrodiffusive cation pathway through the pumps. We investigated properties of the palytoxin-opened channels by recording macroscopic and microscopic currents in cell bodies of neurons from the giant fiber lobe, and by simultaneously measuring net current and ²²Na⁺ efflux in voltage-clamped, internally dialyzed giant axons of the squid Loligo pealei. The conductance of single palytoxin-bound “pump-channels” in outside-out patches was ∼7 pS in symmetrical 500 mM [Na⁺], comparable to findings in other cells. In these high-[Na⁺], K⁺-free solutions, with 5 mM cytoplasmic [ATP], the K_0.5 for palytoxin action was ∼70 pM. The pump-channels were ∼40–50 times less permeable to N-methyl-d-glucamine (NMG⁺) than to Na⁺. The reversal potential of palytoxin-elicited current under biionic conditions, with the same concentration of a different permeant cation on each side of the membrane, was independent of the concentration of those ions over the range 55–550 mM. In giant axons, the Ussing flux ratio exponent (n') for Na⁺ movements through palytoxin-bound pump-channels, over a 100–400 mM range of external [Na⁺] and 0 to −40 mV range of membrane potentials, averaged 1.05 ± 0.02 (n = 28). These findings are consistent with occupancy of palytoxin-bound Na⁺/K⁺ pump-channels either by a single Na⁺ ion or by two Na⁺ ions as might be anticipated from other work; idiosyncratic constraints are needed if the two Na⁺ ions occupy a single-file pore, but not if they occupy side-by-side binding sites, as observed in related structures, and if only one of the sites is readily accessible from both sides of the membrane.

Palytoxin-induced effects on partial reactions of the Na,K-ATPase

The interaction of palytoxin with the Na,K-ATPase was studied by the electrochromic styryl dye RH421, which monitors the amount of ions in the membrane domain of the pump. The toxin affected the pump function in the state P-E2, independently of the type of phosphorylation (ATP or inorganic phosphate). The palytoxin-induced modification of the protein consisted of two steps: toxin binding and a subsequent conformational change into a transmembrane ion channel. At 20 degrees C, the rate-limiting reaction had a forward rate constant of 10(5) M(-1)s(-1) and a backward rate constant of about 10(-3) s(-1). In the palytoxin-modified state, the binding affinity for Na+ and H+ was increased and reached values between those obtained in the E1 and P-E2 conformation under physiological conditions. Even under saturating palytoxin concentrations, the ATPase activity was not completely inhibited. In the Na/K mode, approximately 50% of the enzyme remained active in the average, and in the Na-only mode 25%. The experimental findings indicate that an additional exit from the inhibited state exists. An obvious reaction pathway is a slow dephosphorylation of the palytoxin-inhibited state with a time constant of approximately 100 s. Analysis of the effect of blockers of the extracellular and cytoplasmic access channels, TPA+ and Br2-Titu3+, respectively, showed that both access channels are part of the ion pathway in the palytoxin-modified protein. All experiments can be explained by an extension of the Post-Albers cycle, in which three additional states were added that branch off in the P-E2 state and lead to states in which the open-channel conformation is introduced and returns into the pump cycle in the occluded E2 state. The previously suggested molecular model for the channel state of the Na,K-ATPase as a conformation in which both gates between binding sites and aqueous phases are simultaneously in their open state is supported by this study.

Ouabain affinity determining residues lie close to the Na/K pump ion pathway

The Na/K pump establishes essential ion concentration gradients across animal cell membranes. Cardiotonic steroids, such as ouabain, are specific inhibitors of the Na/K pump. We exploited the marine toxin, palytoxin, to probe both the ion translocation pathway through the Na/K pump and the site of its interaction with ouabain. Palytoxin uncouples the pump's gates, which normally open strictly alternately, thus allowing both gates to sometimes be open, so transforming the pump into an ion channel. Palytoxin therefore permits electrophysiological analysis of even a single Na/K pump. We used outside-out patch recording of Xenopus alpha1beta3 Na/K pumps, which were made ouabain-resistant by point mutation, after expressing them in Xenopus oocytes. Endogenous, ouabain-sensitive, Xenopus alpha1beta3 Na/K pumps were silenced by continuous exposure to ouabain. We found that side-chain charge of two residues at either end of the alpha subunit's first extracellular loop, known to make a major contribution to ouabain affinity, strongly influenced conductance of single palytoxin-bound pump-channels by an electrostatic mechanism. The effects were mimicked by modification of cysteines introduced at those two positions with variously charged methanethiosulfonate reagents. The consequences of these modifications demonstrate that both residues lie in a wide vestibule near the mouth of the pump's ion pathway. Bound ouabain protects the site with the strongest influence on conductance from methanethiosulfonate modification, while leaving the site with the weaker influence unprotected. The results suggest a method for mapping the footprint of bound cardiotonic steroid on the extracellular surface of the Na/K pump.

Large diameter of palytoxin-induced Na/K pump channels and modulation of palytoxin interaction by Na/K pump ligands

Palytoxin binds to Na/K pumps to generate nonselective cation channels whose pore likely comprises at least part of the pump's ion translocation pathway. We systematically analyzed palytoxin's interactions with native human Na/K pumps in outside-out patches from HEK293 cells over a broad range of ionic and nucleotide conditions, and with or without cardiotonic steroids. With 5 mM internal (pipette) [MgATP], palytoxin activated the conductance with an apparent affinity that was highest for Na⁺-containing (K⁺-free) external and internal solutions, lowest for K⁺-containing (Na⁺-free) external and internal solutions, and intermediate for the mixed external Na⁺/internal K⁺, and external K⁺/internal Na⁺ conditions; with Na⁺ solutions and MgATP, the mean dwell time of palytoxin on the Na/K pump was about one day. With Na⁺ solutions, the apparent affinity for palytoxin action was low after equilibration of patches with nucleotide-free pipette solution. That apparent affinity was increased in two phases as the equilibrating [MgATP] was raised over the submicromolar, and submillimolar, ranges, but was increased by pipette MgAMPPNP in a single phase, over the submillimolar range; the apparent affinity at saturating [MgAMPPNP] remained ∼30-fold lower than at saturating [MgATP]. After palytoxin washout, the conductance decay that reflects palytoxin unbinding was accelerated by cardiotonic steroid. When Na/K pumps were preincubated with cardiotonic steroid, subsequent activation of palytoxin-induced conductance was greatly slowed, even after washout of the cardiotonic steroid, but activation could still be accelerated by increasing palytoxin concentration. These results indicate that palytoxin and a cardiotonic steroid can simultaneously occupy the same Na/K pump, each destabilizing the other. The palytoxin-induced channels were permeable to several large organic cations, including N-methyl-d-glucamine⁺, suggesting that the narrowest section of the pore must be ∼7.5 Å wide. Enhanced understanding of palytoxin action now allows its use for examining the structures and mechanisms of the gates that occlude/deocclude transported ions during the normal Na/K pump cycle.

E2P phosphoforms of Na,K-ATPase. I. Comparison of phosphointermediates formed from ATP and Pi by their reactivity toward hydroxylamine and vanadate

The properties of Na,K-ATPase phosphoenzymes formed either from ATP in the presence of Mg2+ and Na+ or from Pi in the absence of alkali cations were investigated by biochemical methods and spectrofluorometry employing the styryl dye RH421. We characterized the phosphoenzyme species by their reaction to N-methyl hydroxylamine, which attacks specifically the protein-phosphate bond. We studied reactions of the phospho- and dephospho-enzymes with vanadate, which is a transition-state analogue of phosphate in this enzyme. On the basis of substantial differences in the properties of the phosphoenzyme species formed either from ATP or Pi, especially in their reactivity to N-methyl hydroxylamine, it is suggested that the two phosphoenzyme species are two subconformations of the E2P phosphoform. Analysis of the RH421 fluorescence responses under a variety of experimental conditions and comparing different enzyme sources suggested that the increase of RH421 fluorescence induced by inorganic phosphate in the absence of alkali cations is associated with the formation of the covalent acyl-phosphate bond.

Ion binding and permeation at the GABA transporter GAT1

This study addresses the binding of ions and the permeation of substrates during function of the GABA transporter GAT1. GAT1 was expressed in Xenopus oocytes and studied electrophysiologically as well as with [3H]GABA flux; GAT1 was also expressed in mammalian cells and studied with [3H]GABA and [3H]tiagabine binding. Voltage jumps, Na+ and Cl- concentration jumps, and exposure to high-affinity blockers (NO-05-711 and SKF-100330A) all produce capacitive charge movements. Occlusive interactions among these three types of perturbations show that they all measure the same population of charges. The concentration dependences of the charge movements reveal (1) that two Na+ ions interact with the transporter even in the absence of GABA, and (2) that Cl- facilitates the binding of Na+. Comparison between the charge movements and the transport-associated current shows that this initial Na(+)-transporter interaction limits the overall transport rate when [GABA] is saturating. However, two classes of manipulation--treatment with high-affinity uptake blockers and the W68L mutation-"lock" Na+ onto the transporter by slowing or preventing the subsequent events that release the substrates to the intracellular medium. The Na+ substitutes Li+ and Cs+ do not support charge movements, but they can permeate the transporter in an uncoupled manner. Our results (1) support the hypothesis that efficient removal of synaptic transmitter by the GABA transporter GAT1 depends on the previous binding of Na+ and Cl-, and (2) indicate the important role of the conserved putative transmembrane domain 1 in interactions with the permeant substrates.

Kinetics of the phosphorylation of Na,K-ATPase by inorganic phosphate detected by a fluorescence method

Phosphorylation by Pi of the Na,K-ATPase from rabbit kidney in the absence of Na+ ions but in the presence of Mg2+ ions has been studied. In the absence of K+ ions, unphosphorylated and phosphorylated states induce different fluorescence levels in the membrane-bound styryl dye RH421, and hence transitions between the two states were monitored. Transient kinetic studies of phosphorylation were initiated by manual addition of Pi or by photochemical release of Pi from 1-(2-nitrophenyl)ethyl phosphate (caged Pi) using laser flash photolysis at 308 nm. Equilibrium studies of phosphorylation showed that the apparent Km for Pi was 23.0 +/- 0.3 microM (mean +/- sem) at pH 7.1 and 21 degrees C. The dye fluorescence increased in a biphasic manner on addition of 500 microM Pi to the enzyme: a rapid phase (t 1/2 < 1 s) and a slower exponential phase at 0.059 +/- 0.003 s-1. The rate of the rapid phase was studied by fast concentration-jump experiments and exhibited first-order kinetics in Pi up to 60 microM. Fluorescence records vs time were exponential, and a plot of the rate constant versus [Pi] had a slope of 1.47 x 10(5) M-1 s-1 and ordinate [Pi] = 0) intercept of 3.1 s-1. Addition of 50 mM NaCl to the phosphorylated enzyme induced an exponential decay in the dye fluorescence from which a rate constant of 0.10 +/- 0.005 s-1 was determined. These data were interpreted in terms of transformations between conformational states E1 and E2, and the phosphorylated state P-E2 defined in the Post-Albers mechanism of the Na,K-ATPase [Läuger, P., (1991) Electrogenic Ion Pumps, Sinauer Associates Inc., Sunderland, MA] as follows: [formula: see text] The RH421 fluorescence of state P-E2 was studied over the pH range 6-8.5. Fluorescence was greatest at pH 8.5 and lowest at pH 6.0 in a simple binding isotherm with pK 7.5. The apparent Km for Pi rose cooperatively with increasing pH (pKa 8.6 and a Hill coefficient of 2). Therefore in the absence of monovalent metal ions, occupation of the cation (K+) binding sites by protons promotes phosphorylation by Pi.

Interaction of the fluorescent probe RH421 with ribulose-1,5-bisphosphate carboxylase/oxygenase and with Na+,K(+)-ATPase membrane fragments

Fluorescence titrations have shown that the voltage-sensitive probe RH421 interacts with the water-soluble protein ribulose-1,5-bisphosphate carboxylase/oxygenase and with Na+,K(+)-ATPase membrane fragments. The probe exhibits significantly different fluorescence excitation spectra in pure lipid and pure protein environments. Experiments with a range of polyamino acids showed interactions of the probe with tyrosine, lysine and arginine residues. At saturating RH421 concentrations (> or = microM) the probe quenches 60-75% of the total tryptophan fluorescence of the Na+,K(+)-ATPase preparation. Inhibition of the hydrolytic activity of the Na+,K(+)-ATPase occurs at RH421 concentrations in the micromolar range. This may be due to a probe-induced change in membrane fluidity. The sensitivity of the probe towards conformational changes of the Na+,K(+)-ATPase decreases hyperbolically as one increases the probe concentration. The decrease in sensitivity correlates well with association of the probe in the vicinity of membrane protein, as measured by tryptophan quenching. These results have important practical consequences for the application of RH421 as a voltage indicator in membrane preparations. Based on these and previously reported results, the fluorescent response of RH421 to the ATP-induced conformational change of the Na+,K+-ATPase is consistent with either a redistribution of dye from the liquid-crystalline lipid matrix into the vicinity of membrane protein or a reorganisation of the lipids surrounding the protein into a more rigid structure caused by the conformational change of the protein.

Electrogenicity of the sodium transport pathway in the Na,K-ATPase probed by charge-pulse experiments

A charge-pulse technique was designed to measure charge movements in the Na-transport mode of the Na,K-ATPase in membrane fragments adsorbed to a planar lipid bilayer with high time resolution. 1) Na+ transport was measured as a function of membrane potential, and 2) voltage-dependent extracellular ion binding and release were analyzed as a function of Na+ concentration and membrane potential. The results could be fitted and explained on the basis of a Post-Albers cycle by simulations with a mathematical model. The minimal reaction sequence explaining the electrogenicity of the pump consists of the following steps: (Na3)E1-P <--> P-E2(Na3) <--> P-E2(Na2) <--> P-E2(Na) <--> P-E2. The conformational change, E1 to E2, is electrogenic (beta 0 < or = 0.1) and the rate-limiting step of forward Na+ transport with a rate constant of 25 s-1 (T = 20 degrees C). The first ion release step, P-E2(Na3) <--> P-E2(Na2), is the major charge translocating process (delta 0 = 0.65). It is probably accompanied by a protein relaxation in which the access structure between aqueous phase and binding site reduces the dielectric distance. The release of the subsequent Na+ ions has a significantly lower dielectric coefficient (delta1 = delta 2 = 0.2). Compared with other partial reactions, the ion release rates are fast (1400 s-1, 700 s-1, and 4000 s-1). On the basis of these findings, a refined electrostatic model of the transport cycle is proposed.

Investigation of ion binding to the cytoplasmic binding sites of the Na,K-pump

A dual-wavelength fluorimeter was constructed, which used two light emitting diodes (LEDs) to excite the fluorescence dye RH 421 alternately with two different wavelengths. The ratio of the emissions at the two excitation wavelengths provided a drift-insensitive signal, which allowed detection of very small changes of the fluorescence intensity. Those small changes were induced by ion binding and release in conformation E1 of the Na,K-ATPase. Titration experiments were performed to determine equilibrium dissociation constants (+/- standard deviation) for each step in the complete binding and release sequence: 0.12 +/- 0.01 mM (E2(K2)<==>KE1), 0.08 +/- 0.01 mM (KE1<==>E1A), 3.0 +/- 0.2 mM (NaE1<==>E1), 5.2 +/- 0.4 mM (Na2E1<==>NaE1) and 6.5 +/- 0.4 mM (Na3E1<==>Na2E1) at pH 7.2 and T = 16 degrees C. These numbers show that the affinities of the binding sites exposed to the cytoplasm, are higher for K+ than for Na+ ions, similar to what was found on the extracellular side. The physiological requirement for extrusion of Na+ from the cytoplasm, and for import of K+ from the extracellular medium seems to be facilitated not by favorable binding affinities in state E1 but by the two ATP-driven reaction steps of the cycle, E2(K2) + ATP-->K2E1.ATP and Na3E1.ATP<==>(Na3) E1-P, which border the ion exchange reactions at the binding sites in conformation E1.

Fluorescence spectroscopic studies on equilibrium dipole-relaxational dynamics of Na,K-ATPase

Intramolecular dynamics in Na,K-ATPase molecules have been studied by ultraviolet fluorescence spectroscopic methods: determination of temperature-dependent shifts in steady-state spectra, site-selective red-edge effects and their temperature dependence, and time-resolved emission decay as a function of excitation and emission wavelengths. The combination of these methods allows the characterization of the dipolar-relaxational mobility in the environment of the tryptophan residues. Our results show that the mean dipolar-relaxational time is of the order of one nanosecond at room temperature. This is much faster than what is usually observed in globular proteins. The fast dynamics of the protein dipoles are rapid enough so that the dipoles are in dielectric equilibrium during the slower ion transfer processes; this may have important functional consequences.