Proton-activated rubidium transport catalyzed by the sodium pump

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.

Steady-state analysis of enzymes with non-Michaelis-Menten kinetics: The transport mechanism of Na+/K+-ATPase

Procedures to define kinetic mechanisms from catalytic activity measurements that obey the Michaelis-Menten equation are well established. In contrast, analytical tools for enzymes displaying non-Michaelis-Menten kinetics are underdeveloped, and transient-state measurements, when feasible, are therefore preferred in kinetic studies. Of note, transient-state determinations evaluate only partial reactions, and these might not participate in the reaction cycle. Here, we provide a general procedure to characterize kinetic mechanisms from steady-state determinations. We described non-Michaelis-Menten kinetics with equations containing parameters equivalent to k_cat and K_m and modeled the underlying mechanism by an approach similar to that used under Michaelis-Menten kinetics. The procedure enabled us to evaluate whether Na⁺/K⁺-ATPase uses the same sites to alternatively transport Na⁺ and K⁺ This ping-pong mechanism is supported by transient-state studies but contradicted to date by steady-state analyses claiming that the release of one cationic species as product requires the binding of the other (ternary-complex mechanism). To derive robust conclusions about the Na⁺/K⁺-ATPase transport mechanism, we did not rely on ATPase activity measurements alone. During the catalytic cycle, the transported cations become transitorily occluded (i.e. trapped within the enzyme). We employed radioactive isotopes to quantify occluded cations under steady-state conditions. We replaced K⁺ with Rb⁺ because ⁴²K⁺ has a short half-life, and previous studies showed that K⁺- and Rb⁺-occluded reaction intermediates are similar. We derived conclusions regarding the rate of Rb⁺ deocclusion that were verified by direct measurements. Our results validated the ping-pong mechanism and proved that Rb⁺ deocclusion is accelerated when Na⁺ binds to an allosteric, nonspecific site, leading to a 2-fold increase in ATPase activity.

Energy-sensitive regulation of Na+/K+-ATPase by Janus kinase 2

Janus kinase 2 (JAK2) contributes to intracellular signaling of leptin and erythropoietin, hormones protecting cells during energy depletion. The present study explores whether JAK2 is activated by energy depletion and regulates Na(+)/K(+)-ATPase, the major energy-consuming pump. In Jurkat cells, JAK2 activity was determined by radioactive kinase assay, phosphorylated JAK2 detected by Western blotting, ATP levels measured by luciferase assay, as well as Na(+)/K(+)-ATPase α1-subunit transcript and protein abundance determined by real-time PCR and Western blotting, respectively. Ouabain-sensitive K(+)-induced currents (Ipump) were measured by whole cell patch clamp. Ipump was further determined by dual-electrode voltage clamp in Xenopus oocytes injected with cRNA-encoding JAK2, active (V617F)JAK2, or inactive (K882E)JAK2. As a result, in Jurkat T cells, JAK2 activity significantly increased following energy depletion by sodium azide (NaN3) or 2,4- dinitro phenol (DNP). DNP- and NaN3-induced decrease of cellular ATP was significantly augmented by JAK2 inhibitor AG490 and blunted by Na(+)/K(+)-ATPase inhibitor ouabain. DNP decreased and AG490 enhanced Ipump as well as Na(+)/K(+)-ATPase α1-subunit transcript and protein abundance. The α1-subunit transcript levels were also enhanced by signal transducer and activator of transcription-5 inhibitor CAS 285986-31-4. In Xenopus oocytes, Ipump was significantly decreased by expression of JAK2 and (V617F)JAK2 but not of (K882E)JAK2, effects again reversed by AG490. In (V617F)JAK2-expressing Xenopus oocytes, neither DNP nor NaN3 resulted in further decline of Ipump. In Xenopus oocytes, the effect of (V617F)JAK2 on Ipump was not prevented by inhibition of transcription with actinomycin. In conclusion, JAK2 is a novel energy-sensing kinase that curtails energy consumption by downregulating Na(+)/K(+)-ATPase expression and activity.

Alternative cycling modes of the Na(+)/K(+)-ATPase in the presence of either Na(+) or Rb(+)

A comprehensive study of the interaction between Na(+) and K(+) with the Na(+)/K(+)-ATPase requires dissecting the incidence of alternative cycling modes on activity measurements in which one or both of these cations are absent. With this aim, we used membrane fragments containing pig-kidney Na(+)/K(+)-ATPase to perform measurements, at 25°C and pH=7.4, of ATPase activity and steady-state levels of (i) intermediates containing occluded Rb(+) at different [Rb(+)] in media lacking Na(+), and (ii) phosphorylated intermediates at different [Na(+)] in media lacking Rb(+). Most relevant results are: (1) Rb(+) can be occluded through an ATPasic cycling mode that takes place in the absence of Na(+) ions, (2) the kinetic behavior of the phosphoenzyme formed by ATP in the absence of Na(+) is different from the one that is formed with Na(+), and (3) binding of Na(+) to transport sites during catalysis is not at random unless rapid equilibrium holds.

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.

Proton diet for the sodium pump

In the absence of Na(+) and K(+) ions the Na,K-ATPase shows a pH-dependent ATP hydrolysis that can be inhibited by ouabain. At pH 7.2 this activity is 5% of the maximal under physiological conditions. It could be inferred that this activity is associated with H(+) transport in both directions across the membrane and facilitates an H-only mode of the sodium pump under such unphysiological conditions. By the analysis of experiments with reconstituted proteoliposomes an overall electroneutral transport mode has been proven. The stoichiometry was determined to be 2 H(+)/2 H(+)/1 ATP and is comparable to what is known from the closely related H,K-ATPase. By time-resolved ATP-concentration jump experiments it was found that at no time was the third, Na(+)-specific binding site of the pump occupied by protons. A modified Post-Albers pump cycle is proposed, with H(+) ions as congeners for Na(+) and K(+), by which all experiments performed can be explained.

Proton paths in the sarcoplasmic reticulum Ca(2+) -ATPase

The sarcoplasmic reticulum Ca(2+)-ATPase (SERCA1a) pumps Ca(2+) and countertransport protons. Proton pathways in the Ca(2+) bound and Ca(2+)-free states are suggested based on an analysis of crystal structures to which water molecules were added. The pathways are indicated by chains of water molecules that interact favorably with the protein. In the Ca(2+) bound state Ca(2)E1, one of the proposed Ca(2+) entry paths is suggested to operate additionally or alternatively as proton pathway. In analogs of the ADP-insensitive phosphoenzyme E2P and in the Ca(2+)-free state E2, the proton path leads between transmembrane helices M5 to M8 from the lumenal side of the protein to the Ca(2+) binding residues Glu-771, Asp-800 and Glu-908. The proton path is different from suggested Ca(2+) dissociation pathways. We suggest that separate proton and Ca(2+) pathways enable rapid (partial) neutralization of the empty cation binding sites. For this reason, transient protonation of empty cation binding sites and separate pathways for different ions are advantageous for P-type ATPases in general.

Functional differences between alpha subunit isoforms of the rat Na,K-ATPase expressed in Xenopus oocytes

The functional properties of the three most widely distributed alpha subunit isoforms of the Na,K-ATPase are not well known, particularly concerning the voltage dependence of their activity and cation binding kinetics. We measured the electrogenic activity generated by Na,K-ATPases resulting from co-expression of the rat alpha1, alpha2* or alpha3* subunits with the rat beta1 subunit in Xenopus oocytes; alpha2* and alpha3* are ouabain-resistant mutants of the alpha2 and alpha3 isoform, which allowed selective inhibition of the endogenous Na(+),K(+)-pump of the oocyte. In oocytes expressing the three isoforms of the alpha subunit, K(+) induced robust outward currents that were largely ouabain-sensitive. In addition, ouabain-sensitive inward currents were recorded for all three isoforms in sodium-free and potassium-free acid solutions. The very similar voltage dependence of the Na(+),K(+)-pump activity observed in the absence of extracellular Na(+) indicated a similar stoichiometry of the transported cations by the three isoforms. The affinity for extracellular K(+) was slightly lower for the alpha2* and alpha3* than for the alpha1 isoform. The alpha2* isoform was, however, more sensitive to voltage-dependent inhibition by extracellular Na(+), indicating a higher affinity of the extracellular Na(+) site in this isoform. We measured and controlled [Na(+)](i) using a co-expressed amiloride-sensitive Na(+) channel. The intracellular affinity for Na(+) was slightly higher in the alpha2* than in the alpha1 or alpha3* isoforms. These results suggest that the alpha2 isoform could have an activity that is strongly dependent upon [Na(+)](o) and [K(+)](o). These concentrations could selectively modulate its activity when large variations are present, for instance in the narrow intercellular spaces of brain or muscle tissues.

Bufo marinus bladder H-K-ATPase carries out electroneutral ion transport

Bufo marinus bladder H-K-ATPase belongs to the Na-K-ATPase and H-K-ATPase subfamily of oligomeric P-type ATPases and is closely related to rat and human nongastric H-K-ATPases. It has been demonstrated that this ATPase transports K(+) into the cell in exchange for protons and sodium ions, but the stoichiometry of this cation exchange is not yet known. We studied the electrogenic properties of B. marinus bladder H-K-ATPase expressed in Xenopus laevis oocytes. In a HEPES-buffered solution, K(+) activation of the H-K-ATPase induced a slow-onset inward current that reached an amplitude of approximately 20 nA after 1-2 min. When measurements were performed in a solution containing 25 mM HCO at a PCO(2) of 40 Torr, the negative current activated by K(+) was reduced. In noninjected oocytes, intracellular alkalization activated an inward current similar to that due to B. marinus H-K-ATPase. We conclude that the transport activity of the nongastric B. marinus H-K-ATPase is not intrinsically electrogenic but that the inward current observed in oocytes expressing this ion pump is secondary to intracellular alkalization induced by proton transport.

Tissue-specific distribution and modulatory role of the gamma subunit of the Na,K-ATPase

The Na,K-ATPase comprises a catalytic alpha subunit and a glycosylated beta subunit. Another membrane polypeptide, gamma, first described by Forbush et al. (Forbush, B., III, Kaplan, J. H., and Hoffman, J. F. (1978) Biochemistry 17, 3667-3676) associates with alpha and beta in purified kidney enzyme preparations. In this study, we have used a polyclonal anti-gamma antiserum to define the tissue specificity and topology of gamma and to address the question of whether gamma has a functional role. The trypsin sensitivity of the amino terminus of the gamma subunit in intact right-side-out pig kidney microsomes has confirmed that it is a type I membrane protein with an extracellular amino terminus. Western blot analysis shows that gamma subunit protein is present only in membranes from kidney tubules (rat, dog, pig) and not those from axolemma, heart, red blood cells, kidney glomeruli, cultured glomerular cells, alpha1-transfected HeLa cells, all derived from the same (rat) species, nor from three cultured cell lines derived from tubules of the kidney, namely NRK-52E (rat), LLC-PK (pig), or MDCK (dog). To gain insight into gamma function, the effects of the anti-gamma serum on the kinetic behavior of rat kidney sodium pumps was examined. The following evidence suggests that gamma stabilizes E1 conformation(s) of the enzyme and that anti-gamma counteracts this effect: (i) anti-gamma inhibits Na,K-ATPase, and the inhibition increases at acidic pH under which condition the E2(K) --> E1 phase of the reaction sequence becomes more rate-limiting, (ii) the oligomycin-stimulated increase in the level of phosphoenzyme was greater in the presence of anti-gamma indicating that the antibody shifts the E1 left and right arrow left and right arrow E2P equilibria toward E2P, and (iii) when the Na+-ATPase reaction is assayed with the Na+ concentration reduced to levels (</=2 mM) which limit the rate of the E1 --> --> E2P transition, anti-gamma is stimulatory. These observations taken together with evidence that the pig gamma subunit, which migrates as a doublet on polyacrylamide gels, is sensitive to digestion by trypsin, and that Rb+ ions partially protect it against this effect, indicate that the gamma subunit is a tissue-specific regulator which shifts the steady-state equilibria toward E1. Accordingly, binding of anti-gamma disrupts alphabeta-gamma interactions and counteracts these modulatory effects of the gamma subunit.

Adaptation to low-K+ media increases H(+)-K(+)-ATPase but not H(+)-ATPase-mediated pHi recovery in OMCD1 cells

Studies in rat and rabbit outer medullary collecting duct of inner stripe origin (OMCDis) suggest that both H(+)-ATPase and H(+)-K(+)-ATPase participate in H+ secretion. However, the relative contributions of these transporters, and, in particular, that of H(+)-K(+)-ATPase to K+ absorption have not been defined precisely. The present study was designed to delineate more clearly the response of these two transporters to hypokalemia and acidosis in a newly developed mouse OMCD1 cell line. In cells grown in normal K+ (5 mM) media, intracellular pH (pHi) recovery was similar either in the presence or absence of K+ in the perfusate (delta pHi/min = 0.014 +/- 0.001 vs. 0.017 +/- 0.003, not significant). The inhibitory effects of Sch-28080 (10 microM) and bafilomycin A1 (10 nM) on pHi recovery were evident only in the presence and absence of K+ in the perfusate, respectively. In cells grown in low-K+ (2.5 mM) media to simulate chronic hypokalemia, pHi recovery was significantly faster than in cells grown in normal K+ media (delta pHi/min = 0.045 +/- 0.01 vs. 0.014 +/- 0.001, P < 0.01) and was inhibited specifically by Sch-28080, not by bafilomycin A1. In contrast, in cells preconditioned to low pH (7.0) to simulate chronic acidosis, the enhanced pHi recovery was abolished by bafilomycin A1 but not by Sch-28080. 86Rb+ uptake, when used as a K+ congener, was inhibited by Sch-28080. The K(m) for 86Rb+ uptake (H(+)-K(+)-ATPase activity) and the 50% inhibitory concentration for Sch-28080 were 270 and 5.0 microM, respectively. These studies provide evidence that, in morphologically homogeneous OMCD1 cells, 1) both H(+)-K(+)-ATPase and H(+)-ATPase participate in pHi regulation, 2) the H(+)-K(+)-ATPase is selectively upregulated by preconditioning in low-K+ media, and 3) conversely, preconditioning in low-pH media stimulates only the H(+)-ATPase. Thus, in OMCDis, the H(+)-K(+)-ATPase and H(+)-ATPase respond selectively and independently to chronic hypokalemia and acidosis, respectively.

Sidedness of the effect of amines on the steady-state phosphorylation level of reconstituted Na+/K+-ATPase

The sidedness of the effects of several amines on the steady-state phosphorylation level of rabbit kidney Na+/K+-ATPase has been studied with the enzyme incorporated in phosphatidylcholine-cholesterol containing proteoliposomes. The presence of ouabain prevented phosphorylation of non-incorporated or rightside-out incorporated enzyme, so that only the inside-out incorporated Na+/K+-ATPase molecules were studied. Addition of either Na+ or several amines to the extracellular side of the enzyme led to an enhancement of the steady-state phosphorylation level obtained with optimal concentrations of Na+, Mg2+ and ATP at the cytosolic side. The series imidazole greater than Na+ greater than triallylamine greater than Tris greater than ethylenediamine showed a decrease in affinity. Histidine, sorbitol and choline chloride had no effect at the extracellular side. This means that in addition to the well-known cytosolic ligands either Na+ or a positively charged amine buffer has to be present extracellularly in order to obtain an optimal phosphorylation level. At the cytoplasmic side the tested amines exerted different effects. (i) Imidazole and triallylamine enhanced the steady-state phosphorylation level when the extracellular conditions were optimal (saturating amine concentration). (ii) Tris and ethylenediamine decreased the steady-state phosphorylation level and (iii) histidine had no effect. The cytoplasmic effects of the amine compounds correlate with those described by Schuurmans Stekhoven et al. (Biochim. Biophys. Acta 937 (1988) 161-171) for the unsided preparation. The extracellular effects, however, are apparently masked in experiments with fragmented enzyme preparations and are assumed to be potentiating effects which make the enzyme ready for phosphorylation upon a cytoplasmic trigger (e.g. Na+).

Some total and partial reactions of Na+/K+-ATPase using ATP and acetyl phosphate as a substrate

Acetyl phosphate, as a substrate of (Na+ + K+)-ATPase, was further characterized by comparing its effects with those of ATP on some total and partial reactions carried out by the enzyme. In the absence of Mg2+ acetyl phosphate could not induce disocclusion (release) of Rb+ from E2(Rb); nor did it affect the acceleration of Rb+ release by non-limiting concentrations of ADP. In K+-free solutions and at pH 7.4 sodium ions were essential for ATP hydrolysis by (Na+ + K+)-ATPase; when acetyl phosphate was the substrate a hydrolysis (inhibited by ouabain) was observed in the presence and absence of Na+. In liposomes with (Na+ + K+)-ATPase incorporated and exposed to extravesicular (intracellular) Na+, acetyl phosphate could sustain a ouabain-sensitive Rb+ efflux; the levels of that flux were similar to those obtained with micromolar concentrations of ATP. When the liposomes were incubated in the absence of extravesicular Na+ a ouabain-sensitive Rb+ efflux could not be detected with either substrate. Native (Na+ + K+)-ATPase was phosphorylated at 0 degrees C in the presence of NaCl (50 mM for ATP and 10 mM for acetyl phosphate); after phosphorylation had been stopped by simultaneous addition of excess trans-1,2-diaminocyclohexane-N,N,N',N' tetraacetic acid and 1 M NaCl net synthesis of ATP by addition of ADP was obtained with both phosphoenzymes. The present results show that acetyl phosphate can fuel the overall cycle of cation translocation by (Na+ + K+)-ATPase acting only at the catalytic substrate site; this takes place via the formation of phosphorylated intermediates which can lead to ATP synthesis in a way which is indistinguishable from that obtained with ATP.

Nonexclusive solute transport thru protein channels. Model of the Na,K ATPase complex and similar channels as general transport routes

In earlier work this author put forward a model of the Na,K ATPase complex as a general transport channel. Detailed treatment was limited to anion and monovalent cation transport. Here the functional mechanisms of the Na,K ATPase and similar protein channels as transport routes for all ionic fluxes and also amino acid, sugar and other solutes are presented. Anions, monosaccharide -OH groups and amino acid carboxyls bind to common arginyls and lose hydration water. They combine with cations which bind to adjacent side chain carboxyls, forming neutral ion pairs or positively charged complexes which have minimums in size, hydration and free polar groups. The smaller size and polarity facilitate entry into the tight, structured water channel of some 8-10 A outer bore. Solute fluxes depend on membrane redox activity which maintains channel sulfhydryls in reduced state required for proper transport. ATP binding at channels contributes to transport conformation while ATP hydrolysis gives high efflux of Na+, H+ and Ca2+ as phosphate ion pairs. This cation efflux current clears cations from inner membrane sites, increases negative potential and provides Na+ and H+ about the outer combining sites, while maintaining their inward gradients. Binding of many agents widens the outer bore to give larger, less selective influx.

Buffer, pH, and ionic strength effects on the (Na+ + K+)-ATPase

A dog kidney (Na+ + K+)-ATPase preparation also catalyzes K+-independent and K+-activated phosphatase reactions with p-nitrophenyl phosphate as substrate. K+-independent activity increases with declining pH over the range 7.5 to 5.8, whereas the other two activities decrease. The increased K+-independent activity is similar with imidazole, histidine, and several Good buffers, and is thus attributable to free H+, probably by affecting enzyme conformations rather than by changing affinity for Mg2+ or substrate or by H+ occupying specific K+-sites. The decrease in K+-phosphatase and (Na+ + K+)-ATPase activities with pH also occurs similarly with those buffers, and is not due to changes in apparent affinity for substrate or for cation activators. However, the Good buffers Pipes and ADA inhibit the K+-independent phosphatase reaction strongly, the K+-activated reaction moderately, and the (Na+ + K+)-ATPase reaction little; both contain two acidic groups, unlike the other buffers tested. Inhibition of the phosphatase reaction by Pipes is associated with a decreased apparent affinity for K+ and an increased sensitivity to inhibition by Na+ and ADP, consistent with Pipes hindering conformational transitions to the E2 enzyme forms required for phosphatase hydrolytic activity.

Free protons do not substitute for sodium ions in buffer-mediated phosphorylation of (Na+ + K+)-ATPase

In view of our recent finding of imidazole-activation of the phosphorylation of (Na+ + K+)-ATPase and the suggestion by others of an activating role of protons, in lieu of sodium ions, in the overall hydrolytic and phosphorylation processes of the enzyme, we have investigated the effect of pH on the phosphorylation process. No indication of proton activation is found. Rather, phosphorylation at low pH in the absence of Na+ is dependent on the buffer concentration. Imidazole-H+ stimulated phosphorylation at pH 5 reaches the same maximal steady-state level as Na+-stimulated phosphorylation. Low pH also elicits Tris-H+ stimulated phosphorylation, but due to a simultaneous inhibitory effect of this buffer the maximal steady-state level is no more than 50% of the Na+-stimulated phosphorylation level. Protons inhibit rather than activate phosphorylation. Upon decreasing the pH from 7 to 5, we observe for all ligands, whether activating or inhibiting phosphorylation (ATP, Na+, protonated imidazole, Mg2+ and K+), a decrease in affinity (largest for Mg2+) and a decrease in the maximal steady-state phosphorylation capacity. The effects of Na+ and imidazole-H+ on the phosphorylation step have been compared with those on the E2----E1 conformational change, which leads to the phosphorylation step. The different pH-dependence of the affinities for Na+ and protonated buffer in the E2----E1 transition suggests that there are separate activation sites with different pK values for Na+ and the buffer cation. The above findings rule out a role of free protons as a substitution for Na+ in the phosphorylation process.

Kinetics of (Na+ + K+)-ATPase: analysis of the influence of Na+ and K+ by steady-state kinetics

The influence of Na+ and K+ on the steady-state kinetics at 37 degrees C of (Na+ + K+)-ATPase was investigated. From an analysis of the dependence of slopes and intercepts (from double-reciprocal plots or from Hanes plots) of the primary data on Na+ and K+ concentrations a detailed model for the interaction of the cations with the individual steps in the mechanism may be inferred and a set of intrinsic (i.e. cation independent) rate constants and cation dissociation constants are obtained. A comparison of the rate constants with those obtained from an analogous analysis of Na+-ATPase kinetics (preceding paper) provides evidence that the ATP hydrolysis proceeds through a series of intermediates, all of which are kinetically different from those responsible for the Na+-ATPase activity. The complete model for the enzyme thus involves two distinct, but doubly connected, hydrolysis cycles. The model derived for (Na+ + K+)-ATPase has the following properties: The empty, substrate free, enzyme form is the K+-bound form E2K. Na+ (Kd = 9 mM) and MgATP (Kd = 0.48 mM), in that order, must be bound to it in order to effect K+ release. Thus Na+ and K+ are simultaneously present on the enzyme in part of the reaction cycle. Each enzyme unit has three equivalent and independent Na+ sites. K+ binding to high-affinity sites (Kd = 1.4 mM) on the presumed phosphorylated intermediate is preceded by release of Na+ from low-affinity sites (Kd = 430 mM). The stoichiometry is variable, and may be Na:K:ATP = 3:2:1. To the extent that the transport properties of the enzyme are reflected in the kinetic ATPase model, these properties are in accord with one of the models shown by Sachs ((1980) J. Physiol. 302, 219-240) to give a quantitative fit of transport data for red blood cells.