Structural organization and energy transduction mechanism of Na+,K+-ATPase studied with transition metal-catalyzed oxidative cleavage

Spanning BODIPY fluorescence with self-assembled micellar clusters

BODIPY dyes possess favorable optical properties for a variety of applications including in vivo and in vitro diagnostics. However, their utilization might be limited by their water insolubility and incompatibility with chemical modifications, resulting in low aggregation stability. Here, we outline the route for addressing this issue. We have demonstrated two approaches, based on dye entrapment in micellar coordination clusters (MCCs); this provides a general solution for water solubility as well as aggregation stability of the seven BODIPY derivatives. These derivatives have various bulky aromatic substituents in the 2,3,5,6- and meso-positions and can rotate relative to a dipyrrin core, which also provides molecular rotor properties. The molecular structural features and the presence of aromatic groups allows BODIPY dyes to be used as "supporting molecules", thus promoting micelle-micelle interaction and micellar network stabilization. In the second approach, self-micellization, following BODIPY use, leads to MCC formation without the use of any mediators, including chelators and/or metal ions. In both approaches, BODIPY exhibits an excellent optical response, at a concentration beyond its solubilization limit in aqueous media and without undesired crystallization. The suggested approaches represent systems used to encapsulate BODIPY in a capsule-based surfactant environment, enabling one to track the aggregation of BODIPY; these approaches represent an alternative system to study and apply BODIPY's molecular rotor properties. The stabilized compounds, i.e., the BODIPY-loaded MCCs, provide a unique feature of permeability to hydrophilic ligand-switching proteins such as BSA; they exhibit a bright "turn-on" fluorescence signal within the clusters via macromolecular complexation, thus expanding the possibilities of water-soluble BODIPY-loaded MCCs utilization for functional indicators.

Serine phosphorylation regulates the P-type potassium pump KdpFABC

KdpFABC is an ATP-dependent K⁺ pump that ensures bacterial survival in K⁺-deficient environments. Whereas transcriptional activation of kdpFABC expression is well studied, a mechanism for down-regulation when K⁺ levels are restored has not been described. Here, we show that KdpFABC is inhibited when cells return to a K⁺-rich environment. The mechanism of inhibition involves phosphorylation of Ser162 on KdpB, which can be reversed in vitro by treatment with serine phosphatase. Mutating Ser162 to Alanine produces constitutive activity, whereas the phosphomimetic Ser162Asp mutation inactivates the pump. Analyses of the transport cycle show that serine phosphorylation abolishes the K⁺-dependence of ATP hydrolysis and blocks the catalytic cycle after formation of the aspartyl phosphate intermediate (E1~P). This regulatory mechanism is unique amongst P-type pumps and this study furthers our understanding of how bacteria control potassium homeostasis to maintain cell volume and osmotic potential.

iTRAQ quantitative clinical proteomics revealed role of Na(+)K(+)-ATPase and its correlation with deamidation in vascular dementia

Dementia is a major public health burden characterized by impaired cognition and loss of function. There are limited treatment options due to inadequate understanding of its pathophysiology and underlying causative mechanisms. Discovery-driven iTRAQ-based quantitative proteomics techniques were applied on frozen brain samples to profile the proteome from vascular dementia (VaD) and age-matched nondementia controls to elucidate the perturbed pathways contributing to pathophysiology of VaD. The iTRAQ quantitative data revealed significant up-regulation of protein-l-isoaspartate O-methyltransferase and sodium-potassium transporting ATPase, while post-translational modification analysis suggested deamidation of catalytic and regulatory subunits of sodium-potassium transporting ATPase. Spontaneous protein deamidation of labile asparagines, generating abnormal l-isoaspartyl residues, is associated with cell aging and dementia due to Alzheimer's disease and may be a cause of neurodegeneration. As ion channel proteins play important roles in cellular signaling processes, alterations in their function by deamidation may lead to perturbations in membrane excitability and neuronal function. Structural modeling of sodium-potassium transporting ATPase revealed the close proximity of these deamidated residues to the catalytic site during E2P confirmation. The deamidated residues may disrupt electrostatic interaction during E1 phosphorylation, which may affect ion transport and signal transduction. Our findings suggest impaired regulation and compromised activity of ion channel proteins contribute to the pathophysiology of VaD.

The effect of vitamin C on morphine self-administration in rats

BACKGROUND

Recent studies have shown that addiction may be caused by abnormality of neurotransmission in the brain. Two neurotransmitters that involve into morphine addiction are dopamine and glutamate. The glutamatergic and dopaminergic systems are also involved in morphine tolerance and morphine withdrawal syndrome signs. Ascorbic acid (AA), as the antioxidant releases from the glutamatergic neurons, modulates the action of the dopamine and glutamate systems. In this study, the effect of AA on morphine self-administration and morphine withdrawal symptoms has been investigated.

MATERIALS AND METHODS

Male Wistar rats (250 - 300g) were anesthetized with ketamine (11%) and xailazine (15%). The cannula was inserted into the right jugular vein, and it was fixed subcutaneously on the skull. After surgery the animals were placed in individual home cages, and they were allowed to recover from the operation for five days, before the test. The animals were subjected to self-administration morphine for12 consecutive days, two-hour/sessions. The number of infusions and number of active and passive lever pressings were recorded.

RESULTS

An intra peritoneal injection of Ascorbic acid (AA) (400 mg/kg, i.p.), 30 minutes before morphine self-administration, produced a significant decrease in 12 days self-administration of morphine and withdrawal syndrome signs (P < 0.05). The morphine withdrawal signs (MWS) were recorded after naloxone precipitation, which decreased significantly with the injection of AA (400,700mg/kg), (<0.05). The number of self-infusions and the number of active lever pressings had significantly decreased after AA injection (P < 0.05).

CONCLUSION

The chronic administration of AA may prevent the development of tolerance and physical dependence on morphine self-administration via the glutamatergic system.

Cryo-TEM structural analysis of conjugated nonionic engineered-micelles

Conjugated engineered-micelles, i.e. micelles that are composed of nonionic detergents and hydrophobic chelators and subsequently conjugated in the presence of divalent metal ions, have been shown to be remarkably suited to the task of membrane protein purification, maintaining these proteins in their native state. They also efficiently solubilize highly hydrophobic antibiotics. To date, however, the morphological changes induced in the initially spherical or ellipsoidal micelles by conjugation have not been explored. In this study, the very rapid sample-vitrification protocol of cryogenic transmission electron microscopy (cryo-TEM) has been used to capture structural transformations that engineered-micelles undergo immediately following conjugation with the [(bathophenanthroline)3:Fe(2+)] hydrophobic complex. We found that condensed thread-like aggregates are formed when the detergents used are: octyl β-D-glucopyranoside (OG), octyl β-D-thioglucopyranoside (OTG) or pentaethylene glycol monododecyl ether (C12E5). However, with β-D-maltoside (DM), n-dodecyl β-D-maltoside (DDM) or β-D-glucopyranoside (DDG), lamellar structures, some of which appear as stacked lamellae or multilamellar vesicles (MLV's), were observed. Such architectural changes occur under very mild conditions i.e. low detergent concentration, no temperature or pH alterations and without the presence of any precipitants such as PEG or ammonium sulfate.

Structural analysis of 2D crystals of gastric H+,K+-ATPase in different states of the transport cycle

The H+,K+-ATPase uses ATP to pump protons across the gastric membrane. We used electron crystallography and limited trypsin proteolysis to study conformational changes in the H+,K+-ATPase. Well-ordered 2D crystals were obtained with detergent-solubilized H+,K+-ATPase at low pH in the absence of nucleotides, E1 state, and in the presence of fluoroaluminate and ADP, mimicking the E1PADP state. Projection maps obtained with frozen-hydrated two-dimensional crystals of the H+,K+-ATPase in these two states looked very similar, suggesting only small conformational changes during the transition from the E1 to the E1P x ADP state. This result differs from the X-ray crystal structures of the related ATPase SERCA, which revealed substantially different conformations in the E1 and E1P x ADP states. To further characterize the conformational changes in the H+,K+-ATPase during its transport cycle, we performed limited proteolysis with trypsin. All examined states of the H+,K+-ATPase, including the E1 and E1P x ADP states present in the 2D crystals,showed characteristic differences in the digestion patterns. While the results from the limited proteolysis experiments thus show that the H+,K+-ATPase adopts distinct conformations during different stages of the transport cycle, the projection maps indicate that the structural rearrangements in the H+,K+-ATPase are much smaller than those observed in the related SERCA ATPase.

Osmotic stress and viscous retardation of the Na,K-ATPase ion pump

The transport function of the Na pump (Na,K-ATPase) in cellular ion homeostasis involves both nucleotide binding reactions in the cytoplasm and alternating aqueous exposure of inward- and outward-facing ion binding sites. An osmotically active, nonpenetrating polymer (poly(ethyleneglycol); PEG) and a modifier of the aqueous viscosity (glycerol) were used to probe the overall and partial enzymatic reactions of membranous Na,K-ATPase from shark salt glands. Both inhibit the steady-state Na,K-ATPase as well as Na-ATPase activity, whereas the K(+)-dependent phosphatase activity is little affected by up to 50% of either. Both Na,K-ATPase and Na-ATPase activities are inversely proportional to the viscosity of glycerol solutions in which the membranes are suspended, in accordance with Kramers' theory for strong coupling of fluctuations at the active site to solvent mobility in the aqueous environment. PEG decreases the affinity for Tl(+) (a congener for K(+)), whereas glycerol increases that for the nucleotides ATP and ADP in the presence of NaCl but has little effect on the affinity for Tl(+). From the dependence on osmotic stress induced by PEG, the aqueous activation volume for the Na,K-ATPase reaction is estimated to be approximately 5-6 nm(3) (i.e., approximately 180 water molecules), approximately half this for Na-ATPase, and essentially zero for p-nitrophenol phosphatase. The change in aqueous hydrated volume associated with the binding of Tl(+) is in the region of 9 nm(3). Analysis of 15 crystal structures of the homologous Ca-ATPase reveals an increase in PEG-inaccessible water space of approximately 22 nm(3) between the E(1)-nucleotide bound forms and the E(2)-thapsigargin forms, showing that the experimental activation volumes for Na,K-ATPase are of a magnitude comparable to the overall change in hydration between the major E(1) and E(2) conformations of the Ca-ATPase.

Site-Specific Cross-Linking of Proteins through Tyrosine Hexahistidine Tags

The genetic addition of hexahistidine (H(6)) tags is widely used to isolate recombinant proteins by immobilized metal-affinity chromatography (IMAC). Addition of a tyrosine residue to H(6) tags enabled proteins to be covalently cross-linked under mild conditions in a manner similar to the natural, site-specific cross-linking of tyrosines into dityrosine. A series of seven hexahistidine tags with tyrosines placed in various positions (H(6)Y tags) were added to the amino terminus of the I28 immunoglobulin domain of the human cardiac titin. The H(6)Y-tagged I28 dimerized in the presence of excess Ni(2+) with a K(D) of 200 microM. Treatment of Ni(2+)-dimerized H(6)Y-I28 with an oxidant, monoperoxyphthalic acid (MMPP) or sodium sulfite, resulted in covalent protein multimerization through chelated Ni(2+)-catalyzed cross-linking of the Y residues engineered into the H(6) tag. The protein oligomerization was observed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE). The presence of dityrosine in the cross-linked proteins was confirmed by fluorescence emission at 410 nm. Proteins lacking the Y residue in the H(6) tag treated with the same oxidative conditions did not cross-link or exhibit dityrosine fluorescence, despite the presence of an endogenous Y residue. The method may have potential uses in other protein conjugation applications such as protein labeling and interfacial immobilization of proteins on artificial surfaces.

Ascorbic acid decreases morphine self-administration and withdrawal symptoms in rats

Recent studies have indicated that the glutamatergic system is involved in the motivational aspects during the initiation of drug self-administration. Ascorbic acid (AA), an antioxidant vitamin, is released from glutamatergic neurons, and it modulates the synaptic action of dopamine and glutamate. In this study the AA effects on the self-administration of morphine and on the morphine withdrawal syndrome have been investigated. Wistar rats were allowed to self-administer morphine (1 mg/infusion) during 10 consecutive days for 2 h/session. The number of lever pressings was recorded. An intrapritoneal AA injection (500 mg/kg, i.p.), 30 min before morphine self-administration produced a significant decrease in the initiation of morphine self administration during all sessions. After the last test session morphine withdrawal symptom signs (MWS) were recorded after naloxone precipitation. Most of MWS (but not all) were decreased by AA application. In conclusion, AA may change the motivational processes underlying the morphine self-administration.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

Role of strategic cysteine residues in oxidative damage to the yeast plasma membrane H(+)-ATPase caused by Fe- and Cu-containing Fenton reagents

Damage caused to Saccharomyces cerevisiae SY4 plasma membrane H(+)-ATPase by Fe- and Cu-Fenton reagents was determined in secretory vesicles containing enzyme in which Cys residues were replaced singly or in pairs by Ala. Cys-221 situated in a beta-sheet domain between M2 and M3 segments, phosphorylation domain-located Cys-409 and Cys-532 situated at the ATP-binding site play a role in the inactivation. In the presence of all three residues the enzyme exhibited a certain basic inactivation, which did not change when Cys-532 was replaced with Ala. In mutants having intact Cys-532 but lacking one or both other cysteines, replacement of Cys-221 with Ala led to lower inactivation, suggesting that Cys-221 may serve as a target for metal-catalyzed oxidation and intact Cys-532 promotes this target role of Cys-221. In contrast, the absence of Cys-409 caused higher inactivation by Fe-Fenton. Cys-532 thus seems to serve as a target for Fe-Fenton, intact Cys-409 causing a conformational change that makes Cys-532 less accessible to oxidation. The mutant lacking both Cys-221 and Cys-409 is more sensitive to Fe-Fenton than to Cu-Fenton and the absence of both Cys residues thus seems to expose presumable extra Fe-binding sites. These data and those on protection by ATP, ADP, 1,4-dithiothreitol and deferrioxamine B point to complex interactions between individual parts of the enzyme molecule that determine its sensitivity towards Fenton reagents. ATPase fragmentation caused by the two reagents differed in that the Fe-Fenton reagent produced in Western blot "smears" whereas the Cu-Fenton reagent produced defined fragments.

Structure and mechanism of Na,K-ATPase: functional sites and their interactions

The cell membrane Na,K-ATPase is a member of the P-type family of active cation transport proteins. Recently the molecular structure of the related sarcoplasmic reticulum Ca-ATPase in an E1 conformation has been determined at 2.6 A resolution. Furthermore, theoretical models of the Ca-ATPase in E2 conformations are available. As a result of these developments, these structural data have allowed construction of homology models that address the central questions of mechanism of active cation transport by all P-type cation pumps. This review relates recent evidence on functional sites of Na,K-ATPase for the substrate (ATP), the essential cofactor (Mg(2+) ions), and the transported cations (Na(+) and K(+)) to the molecular structure. The essential elements of the Ca-ATPase structure, including 10 transmembrane helices and well-defined N, P, and A cytoplasmic domains, are common to all PII-type pumps such as Na,K-ATPase and H,K-ATPases. However, for Na,K-ATPase and H,K-ATPase, which consist of both alpha- and beta-subunits, there may be some detailed differences in regions of subunit interactions. Mutagenesis, proteolytic cleavage, and transition metal-catalyzed oxidative cleavages are providing much evidence about residues involved in binding of Na(+), K(+), ATP, and Mg(2+) ions and changes accompanying E1-E2 or E1-P-E2-P conformational transitions. We discuss this evidence in relation to N, P, and A cytoplasmic domain interactions, and long-range interactions between the active site and the Na(+) and K(+) sites in the transmembrane segments, for the different steps of the catalytic cycle.

Investigating the energy transduction mechanism of P-type ATPases with Fe2+-catalyzed oxidative cleavage

This paper discusses specific oxidative cleavage of renal Na(+),K(+)-ATPase and gastric H(+),K(+)-ATPase, catalyzed by bound Fe(2+) or the complex ATP-Fe(2+), and its implication for the energy transduction mechanism of P-type ATPases. The cleavage technique provides information on the spatial organization of the proteins in different conformations and, since ATP-Fe(2+) substitutes for ATP-Mg(2+) in activating ATPase activity, on Mg(2+)-ligating residues in different conformations. The experiments predict the existence of large movements of N, P, and A cytoplasmic domains accompanying E(1)-E(2) and E(1) P-E(2)P conformational transitions-open in E(1) conformations and closed in E(2) conformations. These features fit well with the Ca(2+)-ATPase crystal structures in E(1) or E(2) conformations and also provide evidence on ATP and Mg(2+) binding sites that is not available from the structures. By combining information from cleavage experiments with molecular modeling, based on the Ca(2+)-ATPase structure, features such as an N to P domain interaction in an E(1). ATP-Mg(2+) conformation can be inferred. The organization of the N, P, and A domains and the ATP and Mg(2+) binding sites in the different conformational states appears to be essentially similar for Na(+),K(+)-ATPase, H(+),K(+)-ATPase, and Ca(2+)-ATPase. The oxidative cleavage technique may be a valuable tool to investigate long-range interactions that transduce the free energy of hydrolysis of ATP to active cation movements in P-type ATPases.

Importance of conserved Thr214 in domain A of the Na+,K+ -ATPase for stabilization of the phosphoryl transition state complex in E2P dephosphorylation

Thr(214) of the highly conserved (214)TGES sequence in domain A of the Na(+),K(+)-ATPase was replaced with alanine, and the mutant was compared functionally with the previously characterized domain A mutant Gly(263) --> Ala. Thr(214) --> Ala displayed a conspicuous 150-fold reduction of the apparent vanadate affinity for inhibition of ATPase activity, which could not simply be explained by the observed shifts of the conformational equilibria in favor of E(1) and E(1)P. The intrinsic vanadate affinity of the E(2) form and the effect on the apparent vanadate affinity of displacement of the E(1)-E(2) equilibrium were determined in a phosphorylation assay that allows the enzyme-vanadate complex to be formed under equilibrium conditions. When the E(2) form prevailed, Thr(214) --> Ala retained a reduced vanadate affinity relative to wild type, whereas the affinity of Gly(263) --> Ala became wild type-like. Thus, mutation of Thr(214) affected the intrinsic affinity of E(2) for vanadate. Furthermore, Thr(214) --> Ala showed at least a 5-fold reduced E(2)P dephosphorylation rate relative to wild type in the presence of saturating concentrations of K(+) and Mg(2+). Because vanadate is a phosphoryl transition state analog, it is proposed that defective binding of the phosphoryl transition state complex (transition state destabilization) causes the inability to catalyze E(2)P dephosphorylation properly. By contrast, the phosphorylation site in the E(1) form was unaffected in Thr(214) --> Ala. Replacement of the glutamate, Glu(216), of (214)TGES with alanine was incompatible with cell viability, indicating a very low transport activity or expression level. Our results support the hypothesis that domain A is isolated in the E(1) form, but contributes to make up the catalytic site in the E(2) and E(2)P conformations.

Molecular architecture of the phosphorylation region of the yeast plasma membrane H+-ATPase

The molecular architecture of the yeast plasma membrane H(+)-ATPase phosphorylation region was explored by Fe(2+)-catalyzed cleavage. An ATP-Mg(2+).Fe(2+) complex was found to act as an affinity cleavage reagent in the presence of dithiothreitol/H(2)O(2). Selective enzyme cleavage required bound adenine nucleotide, either ATP or ADP, in the presence of Mg(2+). The fragment profile included a predominant N-terminal 61-kDa fragment, a minor 37-kDa fragment, and three prominent C-terminal fragments of 39, 36, and 30 kDa. The 61-kDa N-terminal and 39-kDa C-terminal fragments were predicted to originate from cleavage within the conserved MLT(558)GDAVG sequence. The 37-kDa fragment was consistent with cleavage within the S4/M4 sequence PVGLPA(340)V, while the 30-kDa and 36-kDa C-terminal fragments appeared to originate from cleavage in or around sequences D(646)TGIAVE and DMPGS(595)ELADF, respectively. The latter are spatially close to the highly conserved motif GD(634)GVND(638)APSL and conserved residues Thr(558) and Lys(615), which have been implicated in coordinating Mg(2+) and ATP. Overall, these results demonstrate that Fe(2+) associated with ATP and Mg(2+) acts as an affinity cleavage agent of the H(+)-ATPase with backbone cleavage occurring in conserved regions known to coordinate metal-nucleotide complexes. This study provides support for a three-dimensional organization of the phosphorylation region of the yeast plasma membrane H(+)-ATPase that is consistent with, but not identical to, typical P-type enzymes.

Biochemistry of Na,K-ATPase

The Na,K-ATPase or sodium pump carries out the coupled extrusion and uptake of Na and K ions across the plasma membranes of cells of most higher eukaryotes. It is a member of the P-type ATPase superfamily. This heterodimeric integral membrane protein is composed of a 100-kDa alpha-subunit with ten transmembrane segments and a heavily glycosylated beta subunit of about 55 kDa, which is a type II membrane protein. Current ideas on how the protein achieves active transport are based on a fusion of results of transport physiology, protein chemistry, and heterologous expression of mutant proteins. Recently acquired high resolution structural information provides an important new avenue for a more complete understanding of this protein. In this review, the current status of knowledge of Na,K-ATPase is discussed, and areas where there is still considerable uncertainty are highlighted.

The ATP-Mg2+ binding site and cytoplasmic domain interactions of Na+,K+-ATPase investigated with Fe2+-catalyzed oxidative cleavage and molecular modeling

This work utilizes Fe(2+)-catalyzed cleavages and molecular modeling to obtain insight into conformations of cytoplasmic domains and ATP-Mg(2+) binding sites of Na(+),K(+)-ATPase. In E(1) conformations the ATP-Fe(2+) complex mediates specific cleavages at 712VNDS (P domain) and near 440VAGDA (N domain). In E(2)(K), ATP-Fe(2+) mediates cleavages near 212TGES (A domain), near 440VAGDA, and between residues 460-490 (N domain). Cleavages at high ATP-Fe(2+) concentrations do not support suggestions for two ATP sites. A new reagent, fluorescein-DTPA, has been synthesized. The fluorescein-DTPA-Fe(2+) complex mediates cleavages similar to those mediated by ATP-Fe(2+). The data suggest the existence of N to P domain interactions in E(1)Na, with bound ATP-Fe(2+) or fluorescein-DPTA-Fe(2+), A-N, and A-P interactions in E(2)(K), and provide testable constraints for model building. Molecular models based on the Ca(2+)-ATPase structure are consistent with the predictions. Specifically, high-affinity ATP-Mg(2+) binding in E(1) is explained with the N domain tilted ca. 80 degrees toward the P domain, by comparison with well-separated N and P domains in the Ca-ATPase crystal structure. With ATP-Mg(2+) docked, bound Mg(2+) is close to both D710 (in 710DGVNDS) and D443 (in 440VAGDASE). D710 is known to be crucial for Mg(2+) binding. The cleavage and modeling data imply that D443 could also be a candidate for Mg(2+) binding. Comparison of E(1).ATP,Mg(2+) and E(2) models suggests an explanation of the high or low ATP affinities, respectively. We propose a scheme of ATP-Mg(2+) and Mg(2+) binding and N, P, and A domain interactions in the different conformations of the catalytic cycle.

New insights into the role of the N terminus in conformational transitions of the Na,K-ATPase

The deletion of 32 residues from the N terminus of the alpha1 catalytic subunit of the rat Na,K-ATPase (mutant alpha1M32) shifts the E(1)/E(2) conformational equilibrium toward E(1), and the combination of this deletion with mutation E233K in the M2-M3 loop acts synergistically to shift the conformation further toward E(1) (Boxenbaum, N., Daly, S. E., Javaid, Z. Z., Lane, L. K., and Blostein, R. (1998) J. Biol. Chem. 273, 23086-23092). To delimit the region of the cytoplasmic N terminus involved in these interactions, the consequences of a series of N-terminal deletions of alpha1 beyond Delta32 were evaluated. Criteria to assess shifts in conformational equilibrium were based on effects of perturbation of the entire catalytic cycle ((i) sensitivity to vanadate inhibition, (ii) K(+) sensitivity of Na-ATPase measured at micromolar ATP, (iii) changes in K'(ATP), and (iv) catalytic turnover), as well as estimates of the rates of the conformational transitions of phospho- and dephosphoenzyme (E(1)P --> E(2)P and E(2)(K(+)) --> E(1) + K(+)). The results show that, compared with alpha1M32, the deletion of up to 40 residues (alpha1M40) further shifts the poise toward E(1). Remarkably, further deletions (mutants alpha1M46, alpha1M49, and alpha1M56) reverse the effect, such that these mutants increasingly resemble the wild type alpha1. These results suggest novel intramolecular interactions involving domains within the N terminus that impact the manner in which the N terminus/M2-M3 loop regulatory domain interacts with the M4-M5 catalytic loop to effect E(1) <--> E(2) transitions.

Selective Fe2+-catalyzed oxidative cleavage of gastric H+,K+-ATPase: implications for the energy transduction mechanism of P-type cation pumps

In the presence of ascorbate/H(2)O(2), Fe(2+) ions or the ATP-Fe(2+) complex catalyze selective cleavage of the alpha subunit of gastric H(+),K(+)-ATPase. The electrophoretic mobilities of the fragments and dependence of the cleavage patterns on E(1) and E(2) conformational states are essentially identical to those described previously for renal Na(+),K(+)-ATPase. The cleavage pattern of H(+),K(+)-ATPase by Fe(2+) ions is consistent with the existence of two Fe(2+) sites: site 1 within highly conserved sequences in the P and A domains, and site 2 at the cytoplasmic entrance to trans-membrane segments M3 and M1. The change in the pattern of cleavage catalyzed by Fe(2+) or the ATP-Fe(2+) complex induced by different ligands provides evidence for large conformational movements of the N, P, and A cytoplasmic domains of the enzyme. The results are consistent with the Ca(2+)-ATPase crystal structure (Protein Data Bank identification code; Toyoshima, C., Nakasako, M., Nomura, H., and Ogawa, H. (2000) Nature 405, 647-655), an E(1)Ca(2+) conformation, and a theoretical model of Ca(2+)-ATPase in an E(2) conformation (Protein Data Bank identification code ). Thus, it can be presumed that the movements of N, P, and A cytoplasmic domains, associated with the E(1) <--> E(2) transitions, are similar in all P-type ATPases. Fe(2+)-catalyzed cleavage patterns also reveal sequences involved in phosphate, Mg(2+), and ATP binding, which have not yet been shown in crystal structures, as well as changes which occur in E(1) <--> E(2) transitions, and subconformations induced by H(+),K(+)-ATPase-specific ligands such as SCH28080.