The molecular size required varies according to the reaction step round the sodium pump cycle

Affinity labelling with MgATP analogues reveals coexisting Na+ and K+ forms of the alpha-subunits of Na+/K+-ATPase

To test the hypothesis that Na+/K+-ATPase works as an (alpha beta)2-diprotomer with interacting catalytic alpha-subunits, tryptic digestion of pig kidney enzyme, that had been inactivated with substitution-inert MgATP complex analogues, was performed. This led to the demonstration of coexisting C-terminal Na+-like 80-kDa as well as K+-like 60-kDa peptides and N-terminal 40-kDa peptides of the alpha-subunit. To localize the ATP binding sites on tryptic peptides, studies with radioactive MgATP complex analogues were performed: Co(NH3)4-8-N3-ATP specifically modified the E2ATP (low affinity) binding site of Na+/K+-ATPase with an inactivation rate constant (k2) of 12 x 10-3.min-1 at 37 degrees C and a dissociation constant (Kd) of 207 +/- 28 microm. Tryptic digestion of the [gamma32P]Co(NH3)4-8-N3-ATP-inactivated and photolabelled alpha-subunit (Mr = 100 kDa) led, in the absence of univalent cations, to a K+-like C-terminal 60-kDa fragment which was labelled in addition to an unlabelled Na+-like C-terminal 80-kDa fragment. Tryptic digestion of [alpha32P]-or [gamma32P]Cr(H2O)4ATP - bound to the E1ATP (high affinity) site - led to the labelling of a Na+-like 80-kDa fragment besides the immediate formation of an unlabelled K+-like N-terminal 40-kDa fragment and a C-terminal 60-kDa fragment. Because a labelled Na+-like 80-kDa fragment cannot result from an unlabelled K+-like 60-kDa fragment, and because unlabelled alpha-subunits did not show any catalytic activity, the findings are consistent with a situation in which Na+- and K+-like conformations are stabilized by tight binding of substitution-inert MgATP complex analogues to the E1ATP and E2ATP sites. Hence, all data are consistent with the hypothesis that ATP binding induces coexisting Na+ and K+ conformations within an (alphabeta)2-diprotomeric Na+/K+-ATPase.

Affinity labeling of two nucleotide sites on Na,K-ATPase using 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-[alpha-32P]diphosphate (TNP-8N3-[alpha-32P]ADP) as a photoactivatable probe. Label incorporation before and after blocking the high affinity ATP site with fluorescein isothiocyanate

ATP and its analogues act on the minimal functional unit of Na, K-ATPase, the alpha beta protomer, with high and low affinity effects. Fluorescein isothiocyanate (FITC) irreversibly blocks the high affinity, or catalytic, ATP site, and yet the surviving K+-phosphatase activity of soluble FITC-modified alphabeta protomers can be photoinactivated by 2'(3')-O-trinitrophenyl (TNP)-8N3-ADP (Ward, D. G., and Cavieres, J. D. (1998) J. Biol. Chem. 273, 14277-14284). We have now used TNP-8N3-[alpha-32P]ADP as a photoaffinity label for Na,K-ATPase. The native enzyme can be photolabeled at 5 microM TNP-8N3-[alpha-32P]ADP, and ATP or FITC treatment prevents labeling of the alpha chain. At 25 microM, however, TNP-8N3-[alpha-32P]ADP can be incorporated in the FITC-modified alpha chain, concurrently with the inactivation of the K+-phosphatase activity, to an extrapolated level of 0.5-1.2 mol of 32P-probe per mol of alpha chain. Photoinactivation and labeling are prevented by TNP-ADP, vanadate, or strophanthidin and are promoted by Na+ or Mg2+, but not K+. The cation effects suggest that the fluorescein-modified enzyme incorporates the TNP-8N3-[alpha-32P]ADP. Mg complex preferentially, and the free probe when in the E1 enzyme form and after occupation of a low-affinity Na+ site. Partial trypsinolysis reveals that the point of TNP-8N3-[alpha-32P]ADP attachment is on the C-terminal 58-kDa fragment of the FITC-modified alpha chain. The affinity labeling of the fluorescein enzyme by TNP-8N3-[alpha-32P]ADP endorses the view that two nucleotide sites can be occupied simultaneously in each alpha subunit of Na,K-ATPase.

Molecular distance measurements reveal an (alpha beta)2 dimeric structure of Na+/K+-ATPase. High affinity ATP binding site and K+-activated phosphatase reside on different alpha-subunits

ATP hydrolysis by Na+/K+-ATPase proceeds via the interaction of simultaneously existing and cooperating high (E1ATP) and low (E2ATP) substrate binding sites. It is unclear whether both ATP sites reside on the same or on different catalytic alpha-subunits. To answer this question, we looked for a fluorescent label for the E2ATP site that would be suitable for distance measurements by Förster energy transfer after affinity labeling of the E1ATP site by fluorescein 5'-isothiocyanate (FITC). Erythrosin 5'-isothiocyanate (ErITC) inactivated, in an E1ATP site-blocked enzyme (by FITC), the residual activity of the E2ATP site, namely K+-activated p-nitrophenylphosphatase in a concentration-dependent way that was ATP-protectable. The molar ratios of FITC/alpha-subunit of 0.6 and of ErITC/alpha-subunit of 0.48 indicate 2 ATP sites per (alpha beta)2 diprotomer. Measurements of Förster energy transfer between the FITC-labeled E1ATP and the ErITC-labeled or Co(NH3)4ATP-inactivated E2ATP sites gave a distance of 6.45 +/- 0.64 nm. This distance excludes 2 ATP sites per alpha-subunit since the diameter of alpha is 4-5 nm. Förster energy transfer between cardiac glycoside binding sites labeled with anthroylouabain and fluoresceinylethylenediamino ouabain gave a distance of 4.9 +/- 0.5 nm. Hence all data are consistent with the hypothesis that Na+/K+-ATPase in cellular membranes is an (alpha beta)2 diprotomer and works as a functional dimer (Thoenges, D., and Schoner, W. (1997) J. Biol. Chem. 272, 16315-16321).

Photoinactivation of fluorescein isothiocyanate-modified Na,K-ATPase by 2'(3')-O-(2,4,6-trinitrophenyl)8-azidoadenosine 5'-diphosphate. Abolition of E1 and E2 partial reactions by sequential block of high and low affinity nucleotide sites

The Na,K-ATPase activity of the sodium pump exhibits apparent multisite kinetics toward ATP, a feature that is inherent to the minimal enzyme unit, the alpha beta protomer. We have argued that this should arise from separate catalytic and noncatalytic sites on the alpha beta protomer as fluorescein isothiocyanate (FITC) blocks a high affinity ATP site on all alpha subunits and yet the modified Na, K-ATPase retains a low affinity response to nucleotides (Ward, D. G., and Cavieres, J. D. (1996) J. Biol. Chem. 271, 12317-12321). We now find that 2'(3')-O-(2,4,6-trinitrophenyl)8-azido-adenosine 5'-diphosphate (TNP-8N3-ADP), a high affinity photoactivatable analogue of ATP, can inhibit the K+-phosphatase activity of the FITC-modified enzyme during assays in dimmed light. The inhibition occurs with a Ki of 140 microM at 20 mM K+; it requires the adenine ring as 2'(3')-O-(2,4 6-trinitrophenyl) (TNP)-UDP or TNP-uridine are less potent and 2,4,6-trinitrobenzene-sulfonate is ineffective. Under irradiation with UV light, TNP-8N3-ADP inactivates the K+-phosphatase activity of the fluorescein-enzyme and also its phosphorylation by [32P]Pi. The photoinactivation process is stimulated by Na+ or Mg2+, and is inhibited by K+ or excess TNP-ADP. In the presence of 50 mM Na+ and 1 mM Mg2+, TNP-8N3-ADP photoinactivates with a K0.5 of 15 microM. Furthermore, TNP-8N3-ADP photoinactivates the FITC-modified, solubilized alpha beta protomers, even more effectively than the membrane-bound fluorescein-enzyme. These results strongly suggest that catalytic and allosteric ATP sites coexist on the alpha beta protomer of Na,K-ATPase.

Binding of 2'(3')-O-(2,4-6-trinitrophenyl) ADP to soluble alpha beta protomers of Na, K-ATPase modified with fluorescein isothiocyanate. Evidence for two distinct nucleotide sites

The overall reaction of well-defined solubilized protomers of Na,K-ATPase (one alpha plus one beta subunit) retains the dual ATP dependence observed with the membrane-bound enzyme, with distinctive ATP effects in the submicromolar and submillimolar ranges (Ward, D. G., and Cavieres, J. D. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 5332-5336). We have now found that the K+/-phosphatase activity of the alpha beta protomers is still inhibited by 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-diphosphate (TNP-ADP). What is most significant is that the TNP-ADP effect can be observed clearly with protomeric enzyme whose high affinity ATP site has been blocked covalently with fluorescein isothiocyanate. We conclude that nucleotides can bind at two discrete sites in each protomeric unit of Na,K-ATPase.

Na+/K(+)-ATPase with a blocked E1ATP site still allows backdoor phosphorylation of the E2ATP site

The role of simultaneously existing ATP-binding sites in the catalytic process of Na+/K(+)-ATPase is unclear. In order to learn whether blocking the E1ATP site affects the properties of the E2ATP site, the E1ATP site was inactivated by either fluorescein 5'-isothiocyanate, the non-phosphorylating Cr(H2O)4AdoPP[CH2]P or the phosphorylating Cr(H2O)4ATP. The properties of the remaining E2ATP site were studied by measuring 'backdoor phosphorylation' in the presence of ouabain, or K(+)-activated hydrolysis of p-nitrophenyl phosphate. The involvement of the E2ATP site was further tested by the effects of Co(NH3)4ATP, a specific inactivator of this site. When the E1ATP site was inactivated by fluorescein 5'-isothiocyanate or the non-phosphorylating Cr(H2O)4AdoPP[CH2]P, backdoor phosphorylation and the activity of K(+)-activated p-nitrophenylphosphatase remained unchanged. Both processes were lost, however, when the E2ATP site was additionally inactivated by Co(NH3)4ATP. Inactivation of the E1ATP site by fluorescein 5'-isothiocyanate or Cr(H2O)4AdoPP[CH2]P decreased the affinity of the p-nitrophenylphosphatase activity of the E2ATP site for the substrate p-nitrophenyl phosphate by four times. This is consistent with a former report showing that dephosphorylation in a fluorescein 5'-isothiocyanate-inactivated Na+/K(+)-ATPase has a lowered sensitivity for ATP [Scheiner-Bobis, G., Antonipillai, J. & Farley, R. A. (1993) Biochemistry 32, 9592-9599]. Inactivation of the E1ATP site by the phosphorylating Cr(H2O)4ATP, however, led to a loss of the property of the E2ATP site to hydrolyse K(+)-dependent p-nitrophenyl phosphate and to achieve backdoor phosphorylation. Evidently, ATP sites coexist in Na+/K(+)-ATPase, and binding of ATP to one site affects the property of the other site [Scheiner-Bobis, G., Esmann, M. & Schoner, W. (1989) Eur. J. Biochem. 183, 173-178]. Although the enzyme can be phosphorylated from both ATP sites, phosphorylation of the E1ATP site excludes the phosphorylation of the E2ATP site.

E1/E2 type cation transport ATPases: evidence for transient associations between protomers

E1/E2 type cation transport ATPases are known to exist in different conformeric states. Recent evidence characterizing these conformers in membrane is reviewed. A consensus view is proposed in which E2 conformers tend to form oligomeric complexes by lateral association between monomeric protomers and E1 conformers exhibit the opposite behaviour. It is suggested that transient associations between monomers during cation pump cycles may be a common feature of the ion translocation mechanism under physiological conditions.

Solubilized alpha beta Na,K-ATPase remains protomeric during turnover yet shows apparent negative cooperativity toward ATP

A prominent feature of the Na,K-ATPase reaction is an ATP dependence that suggests high- and low-affinity ATP requirements during the enzymic cycle. As only one ATP-binding domain has been identified in the alpha subunit and none has been identified in the beta subunit, it has seemed likely that the apparent negative cooperativity results from subunit interactions in an (alpha beta)2 diprotomer. To test this possibility, we have examined the behavior of solubilized alpha beta protomers of Na,K-ATPase down to 50 nM [gamma-32P]ATP. Active-enzyme analytical ultracentrifugation shows that the protomer is the active species and that no oligomerization occurs during turnover. However, we find that dual ATP effects can be clearly demonstrated and that nonhydrolyzable ATP analogs can stimulate the Na,K-ATPase activity of the soluble protomer. We conclude that the apparent negative cooperativity is inherent to the alpha beta protomer and that this should explain some of the complexities found with membrane-bound Na,K-ATPase and, perhaps, other P-type cation pumps.

Investigation of subunit interactions by radiation inactivation: the case of Na+/K+-ATPase

The target size of Na+/K+-ATPase has been determined by radiation inactivation. To interpret the results, we have performed Monte Carlo simulations of the inactivation process. This seems to be a general method for the interpretation of such studies. The simulation revealed that radiation inactivation can distinguish between monoprotomeric and multiprotomeric models of enzyme action only if the measured reaction requires the actual co-operation of, rather than the mere co-existence of, different protomeres.

Effects of ouabain on the rotational dynamics of renal Na,K-ATPase studied by saturation-transfer EPR

The interaction of a nitroxide spin-labeled derivative of ouabain with sheep kidney Na,K-ATPase and the motional behavior of the ouabain spin label-Na,K-ATPase complex have been studied by means of electron paramagnetic resonance (EPR) and saturation-transfer EPR (ST-EPR). Spin-labeled ouabain binds with high affinity to the Na,K-ATPase with concurrent inhibition of ATPase activity. Enzyme preparations retain 0.61 +/- 0.1 mol of bound ouabain spin label per mole of ATP-dependent phosphorylation sites, even after repeated centrifugation and resuspension of the purified ATPase-containing membrane fragments. The conventional EPR spectrum of the ouabain spin label bound to the ATPase consists almost entirely (greater than 99%) of a broad resonance at 0 degrees C, characteristic of a tightly bound spin label which is strongly immobilized by the protein backbone. Saturation-transfer EPR measurements of the spin-labeled ATPase preparations yield effective correlation times for the bound labels significantly longer than 100 microseconds at 0 degrees C. Since the conventional EPR measurements of the ouabain spin-labeled Na,K-ATPase indicated the label was strongly immobilized, these rotational correlation times most likely represent the motion of the protein itself rather than the independent motion of mobile spin probes relative to a slower moving protein. Additional ST-EPR measurements of ouabain spin-labeled Na,K-ATPase (a) cross-linked with glutaraldehyde and (b) crystallized in two-dimensional arrays indicated that the observed rotational correlation times predominantly represented the motion of large Na,K-ATPase-containing membrane fragments, as opposed to the motion of individual monomeric or dimeric polypeptides within the membrane fragment. The results suggest that the binding of spin-labeled ouabain to the ATPase induces the protein to form large aggregates, implying that cardiac glycoside induced enzyme aggregation may play a role in the mechanism of action of the cardiac glycosides in inhibiting the Na,K-ATPase.

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)

Phosphate binding and ATP-binding sites coexist in Na+/K(+)-transporting ATPase, as demonstrated by the inactivating MgPO4 complex analogue Co(NH3)4PO4

Tetrammine cobalt(III) phosphate [Co(NH3)4PO4] inactivates Na+/K(+)-ATPase in the E2 conformational state, dependent on time and concentration, according to Eqn (1): Co(NH3)4PO4 + E2 Kd in equilibrium E2.Co(NH3)4PO4k2----E'2.Co(NH3)4PO4. The inactivation rate constant k2 for the formation of a stable E'2.Co(NH3)4PO4 at 37 degrees C was 0.057 min-1; the dissociation constant, Kd = 300 microM. The activation energy for the inactivation process was 149 kJ/mol. ATP and the uncleavable adenosine 5'-[beta, gamma-methylene]triphosphate competed with Co(NH3)4PO4 for its binding site with Ks = 0.41 mM and 5 mM, respectively. MgPO4 competed with Co(NH3)4PO4 linearly, with Ks = 50 microM, as did phosphate (Ks = 16 mM) and Mg2+ (Ks = 160 microM). It is concluded that the MgPO4 analogue binds to the MgPO4-binding subsite of the low-affinity ATP-binding site (of the E2 conformation). Also, Na+ (Ks = 860 microM) protected the enzyme against inactivation in a competitive manner. From the intersecting (slope and intercept linear) noncompetitive effect of Na+ against the inactivation by Co(NH3)4PO4, apparent affinities of K+ for the free enzyme of 41 microM, and for the E.Co(NH3)4PO4 complex of 720 microM, were calculated. Binding of Co(NH3)4PO4 to the enzyme inactivated Na+/K(+)-ATPase and K(+)-activated phosphatase, and, moreover, prevented the occlusion of 86Rb+; however, the activity of the Na(+)-ATPase, the phosphorylation capacity of the high-affinity ATP-binding site and the ATP/ADP-exchange reaction remained unchanged. With Co(NH3)432PO4 a binding capacity of 135 pmol unit enzyme was found. Phosphorylation and complete inactivation of the enzyme with Co(NH3)432PO4 or the 32P-labelled tetramminecobalt ATP ([gamma-32P]Co(NH3)4ATP) at the low-affinity ATP-binding site, allowed (independent of the purity of the Na+/K(+)-ATPase preparation) a further incorporation of radioactivity from 32P-labelled tetraaquachromium(III) ATP ([gamma-32P]CrATP) to the high-affinity ATP-binding site with unchanged phosphorylation capacity. However, inactivation and phosphorylation of Na+/K(+)-ATPase by [gamma-32P]CrATP prevented the binding of Co(NH3)4 32PO4 or [gamma-32P]Co(NH3)4ATP to the enzyme. [gamma-32P]CO(NH3)4ATP and Co(NH3)432PO4 are mutually exclusive. The data are consistent with the assumption of a cooperation of catalytic subunits within an (alpha,beta)2-diprotomer, which change their interactions during the Na+/K(+)-pumping process. Our findings seem not to support a symmetrical Repke and Stein model of enzyme action.

Radiation inactivation analysis of sarcoplasmic reticulum Ca-ATPase in membrane-bound form and in detergent-solubilized monomeric states

The sarcoplasmic reticulum Ca-ATPase was subjected to target size analysis by radiation inactivation in various buffer conditions and after solubilization in monomeric form in non-ionic detergent and in SDS. The target size was also determined for Ca-ATPase in bidimensional crystals formed in the presence of decavanadate or lanthanide. The standardization obtained with defined monomers of Ca-ATPase shows that the target size of Ca-ATPase in the functional membrane-bound state may be ascribed to a single peptide chain, possibly with surrounding lipid. Further analysis of the radiation inactivation sizes of various partial reactions of the pump cycle, including phosphorylation and Ca2+ occlusion, indicated much smaller values than the target size pertaining to decomposition of the whole peptide chain. This is consistent with the existence of separate functional domains within a single peptide chain.