1
|
Peluffo RD, Hernández JA. The Na +,K +-ATPase and its stoichiometric ratio: some thermodynamic speculations. Biophys Rev 2023; 15:539-552. [PMID: 37681108 PMCID: PMC10480117 DOI: 10.1007/s12551-023-01082-5] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/19/2023] [Accepted: 06/18/2023] [Indexed: 09/09/2023] Open
Abstract
Almost seventy years after its discovery, the sodium-potassium adenosine triphosphatase (the sodium pump) located in the cell plasma membrane remains a source of novel mechanistic and physiologic findings. A noteworthy feature of this enzyme/transporter is its robust stoichiometric ratio under physiological conditions: it sequentially counter-transports three sodium ions and two potassium ions against their electrochemical potential gradients per each hydrolyzed ATP molecule. Here we summarize some present knowledge about the sodium pump and its physiological roles, and speculate whether energetic constraints may have played a role in the evolutionary selection of its characteristic stoichiometric ratio.
Collapse
Affiliation(s)
- R. Daniel Peluffo
- Group of Biophysical Chemistry, Department of Biological Sciences, CENUR Litoral Norte, Universidad de La República, Rivera 1350, CP: 50000 Salto, Uruguay
| | - Julio A. Hernández
- Biophysics and Systems Biology Section, Department of Cell and Molecular Biology, Facultad de Ciencias, Universidad de La República, Iguá 4225, CP: 11400 Montevideo, Uruguay
| |
Collapse
|
2
|
External Ion Access in the Na/K Pump: Kinetics of Na +, K +, and Quaternary Amine Interaction. Biophys J 2019; 115:361-374. [PMID: 30021111 DOI: 10.1016/j.bpj.2018.06.007] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2018] [Revised: 05/18/2018] [Accepted: 06/06/2018] [Indexed: 11/23/2022] Open
Abstract
Na/K pumps build essential ion gradients across the plasmalemma of animal cells by coupling the extrusion of three Na+, with the import of two K+ and the hydrolysis of one ATP molecule. The mechanisms of selectivity and competition between Na+, K+, and inhibitory amines remain unclear. We measured the effects of external tetrapropylammonium (TPA+) and ethylenediamine (EDA2+) on three different Na/K pump transport modes in voltage-clamped Xenopus oocytes: 1) outward pump current (IP), 2) passive inward H+ current at negative voltages without Na+ or K+ (IH), and 3) transient charge movement reporting the voltage-dependent extracellular binding/release of Na+ (QNa). Both amines competed with K+ to inhibit IP. TPA+ inhibited IH without competing with H+, whereas EDA2+ did not alter IH at pH 7.6. TPA+ competed with Na+ in QNa measurements, reducing Na+-apparent affinity, evidenced by a ∼-75 mV shift in the charge-voltage curve (at 20 mM TPA+) without reduction of the total charge moved (Qtot). In contrast, EDA2+ and K+ did not compete with Na+ to inhibit QNa; both reduced Qtot without decreasing Na+-apparent affinity. EDA2+ (15 mM) right-shifted the charge-voltage curve by ∼+50 mV. Simultaneous occlusion of EDA2+ and Na+ by an E2P conformation unable to reach E1P was demonstrated by voltage-clamp fluorometry. Trypsinolysis experiments showed that EDA2+-bound pumps are much more proteolysis-resistant than Na+-, K+-, or TPA+-bound pumps, therefore uncovering unique EDA2+-bound conformations. K+ effects on QNa and IH were also evaluated in pumps inhibited with beryllium fluoride, a phosphate mimic. K+ reduced Qtot without shifting the charge-voltage curve, indicating noncompetitive effects, and partially inhibited IH to the same extent as TPA+ in non-beryllium-fluorinated pumps. These results demonstrate that K+ interacts with beryllium-fluorinated pumps inducing conformational changes that alter QNa and IH, suggesting that there are two external access pathways for proton transport by IH.
Collapse
|
3
|
Meyer DJ, Gatto C, Artigas P. Na/K Pump Mutations Associated with Primary Hyperaldosteronism Cause Loss of Function. Biochemistry 2019; 58:1774-1785. [PMID: 30811176 DOI: 10.1021/acs.biochem.9b00051] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Primary hyperaldosteronism (Conn's syndrome), a common cause of secondary hypertension, is frequently produced by unilateral aldosterone-producing adenomas that carry mutations in ion-transporting genes, including ATP1A1, encoding the Na/K pump's α1 subunit. Whether Na/K pump mutant-mediated inward currents are required to depolarize the cell and increase aldosterone production remains unclear, as such currents were observed in four out of five mutants described so far. Here, we use electrophysiology and uptake of the K+ congener 86Rb+, to characterize the effects of eight additional Na/K pump mutations in transmembrane segments TM1 (delM102-L103, delL103-L104, and delM102-I106), TM4 (delI322-I325 and I327S), and TM9 (delF956-E961, delF959-E961, and delE960-L964), expressed in Xenopus oocytes. All deletion mutants induced abnormal inward currents of different amplitudes at physiological voltages, while I327S lacked such currents. A detailed functional characterization revealed that I327S significantly reduces intracellular Na+ affinity without altering affinity for external K+. 86Rb+-uptake experiments show that I327S dramatically impairs function under physiological concentrations of Na+ and K+. Since Na/K pumps in the adrenal cortex may be formed by association of α1 with β3 instead of β1 subunits, we evaluated whether G99R (another mutant without inward currents when associated with β1) would show inward currents when associated with β3. We found that the kinetic characteristics of either mutant or wild-type α1β3 pumps expressed in Xenopus oocytes to be indistinguishable from those of α1β1 pumps. The observed functional consequences of each hyperaldosteronism mutant point to the loss of Na/K pump function as the common feature of all mutants, which is sufficient to induce hyperaldosteronism.
Collapse
Affiliation(s)
- Dylan J Meyer
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research , Texas Tech University Health Sciences Center , Lubbock , Texas 79430 , United States
| | - Craig Gatto
- School of Biological Sciences , Illinois State University , Normal , Illinois 61790 , United States
| | - Pablo Artigas
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research , Texas Tech University Health Sciences Center , Lubbock , Texas 79430 , United States
| |
Collapse
|
4
|
Garcia A, Pratap PR, Lüpfert C, Cornelius F, Jacquemin D, Lev B, Allen TW, Clarke RJ. The voltage-sensitive dye RH421 detects a Na + ,K + -ATPase conformational change at the membrane surface. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2017; 1859:813-823. [DOI: 10.1016/j.bbamem.2017.01.022] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/19/2016] [Revised: 01/09/2017] [Accepted: 01/19/2017] [Indexed: 10/20/2022]
|
5
|
Lu FM, Deisl C, Hilgemann DW. Profound regulation of Na/K pump activity by transient elevations of cytoplasmic calcium in murine cardiac myocytes. eLife 2016; 5. [PMID: 27627745 PMCID: PMC5050017 DOI: 10.7554/elife.19267] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2016] [Accepted: 09/09/2016] [Indexed: 01/06/2023] Open
Abstract
Small changes of Na/K pump activity regulate internal Ca release in cardiac myocytes via Na/Ca exchange. We now show conversely that transient elevations of cytoplasmic Ca strongly regulate cardiac Na/K pumps. When cytoplasmic Na is submaximal, Na/K pump currents decay rapidly during extracellular K application and multiple results suggest that an inactivation mechanism is involved. Brief activation of Ca influx by reverse Na/Ca exchange enhances pump currents and attenuates current decay, while repeated Ca elevations suppress pump currents. Pump current enhancement reverses over 3 min, and results are similar in myocytes lacking the regulatory protein, phospholemman. Classical signaling mechanisms, including Ca-activated protein kinases and reactive oxygen, are evidently not involved. Electrogenic signals mediated by intramembrane movement of hydrophobic ions, such as hexyltriphenylphosphonium (C6TPP), increase and decrease in parallel with pump currents. Thus, transient Ca elevation and Na/K pump inactivation cause opposing sarcolemma changes that may affect diverse membrane processes.
Collapse
Affiliation(s)
- Fang-Min Lu
- Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, United States
| | - Christine Deisl
- Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, United States
| | - Donald W Hilgemann
- Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, United States
| |
Collapse
|
6
|
Stanley CM, Gagnon DG, Bernal A, Meyer DJ, Rosenthal JJ, Artigas P. Importance of the Voltage Dependence of Cardiac Na/K ATPase Isozymes. Biophys J 2016; 109:1852-62. [PMID: 26536262 DOI: 10.1016/j.bpj.2015.09.015] [Citation(s) in RCA: 32] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2015] [Revised: 08/11/2015] [Accepted: 09/10/2015] [Indexed: 11/25/2022] Open
Abstract
Cardiac cells express more than one isoform of the Na, K-ATPase (NKA), the heteromeric enzyme that creates the Na(+) and K(+) gradients across the plasmalemma. Cardiac isozymes contain one catalytic α-subunit isoform (α1, α2, or α3) associated with an auxiliary β-subunit isoform (β1 or β2). Past studies using biochemical approaches have revealed minor kinetic differences between isozymes formed by different α-β isoform combinations; these results make it difficult to understand the physiological requirement for multiple isoforms. In intact cells, however, NKA enzymes operate in a more complex environment, which includes a substantial transmembrane potential. We evaluated the voltage dependence of human cardiac NKA isozymes expressed in Xenopus oocytes, and of native NKA isozymes in rat ventricular myocytes, using normal mammalian physiological concentrations of Na(+)o and K(+)o. We demonstrate that although α1 and α3 pumps are functional at all physiologically relevant voltages, α2β1 pumps and α2β2 pumps are inhibited by ∼75% and ∼95%, respectively, at resting membrane potentials, and only activate appreciably upon depolarization. Furthermore, phospholemman (FXYD1) inhibits pump function without significantly altering the pump's voltage dependence. Our observations provide a simple explanation for the physiological relevance of the α2 subunit (∼20% of total α subunits in rat ventricle): they act as a reserve and are recruited into action for extra pumping during the long-lasting cardiac action potential, where most of the Na(+) entry occurs. This strong voltage dependence of α2 pumps also helps explain how cardiotonic steroids, which block NKA pumps, can be a beneficial treatment for heart failure: by only inhibiting the α2 pumps, they selectively reduce NKA activity during the cardiac action potential, leading to an increase in systolic Ca(2+), due to reduced extrusion through the Na/Ca exchanger, without affecting resting Na(+) and Ca(2+) concentrations.
Collapse
Affiliation(s)
- Christopher M Stanley
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas
| | - Dominique G Gagnon
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas; Department of Physics, Texas Tech University, Lubbock, Texas
| | - Adam Bernal
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas
| | - Dylan J Meyer
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas
| | - Joshua J Rosenthal
- Universidad de Puerto Rico, Recinto de Ciencias Médicas, Instituto de Neurobiología, San Juan, Puerto Rico
| | - Pablo Artigas
- Department of Cell Physiology and Molecular Biophysics, Center for Membrane Protein Research, Texas Tech University Health Sciences Center, Lubbock, Texas.
| |
Collapse
|
7
|
Hover S, King B, Hall B, Loundras EA, Taqi H, Daly J, Dallas M, Peers C, Schnettler E, McKimmie C, Kohl A, Barr JN, Mankouri J. Modulation of Potassium Channels Inhibits Bunyavirus Infection. J Biol Chem 2015; 291:3411-22. [PMID: 26677217 PMCID: PMC4751384 DOI: 10.1074/jbc.m115.692673] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2015] [Indexed: 11/06/2022] Open
Abstract
Bunyaviruses are considered to be emerging pathogens facilitated by the segmented nature of their genome that allows reassortment between different species to generate novel viruses with altered pathogenicity. Bunyaviruses are transmitted via a diverse range of arthropod vectors, as well as rodents, and have established a global disease range with massive importance in healthcare, animal welfare, and economics. There are no vaccines or anti-viral therapies available to treat human bunyavirus infections and so development of new anti-viral strategies is urgently required. Bunyamwera virus (BUNV; genus Orthobunyavirus) is the model bunyavirus, sharing aspects of its molecular and cellular biology with all Bunyaviridae family members. Here, we show for the first time that BUNV activates and requires cellular potassium (K(+)) channels to infect cells. Time of addition assays using K(+) channel modulating agents demonstrated that K(+) channel function is critical to events shortly after virus entry but prior to viral RNA synthesis/replication. A similar K(+) channel dependence was identified for other bunyaviruses namely Schmallenberg virus (Orthobunyavirus) as well as the more distantly related Hazara virus (Nairovirus). Using a rational pharmacological screening regimen, two-pore domain K(+) channels (K2P) were identified as the K(+) channel family mediating BUNV K(+) channel dependence. As several K2P channel modulators are currently in clinical use, our work suggests they may represent a new and safe drug class for the treatment of potentially lethal bunyavirus disease.
Collapse
Affiliation(s)
- Samantha Hover
- From the School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT
| | - Barnabas King
- School of Life Sciences, University of Nottingham, Nottingham NG7 2UH
| | - Bradley Hall
- From the School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT
| | - Eleni-Anna Loundras
- From the School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT
| | - Hussah Taqi
- From the School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT
| | - Janet Daly
- Veterinary Medicine and Science, University of Nottingham, Nottingham NG7 2RD
| | - Mark Dallas
- School of Pharmacy, University of Reading, Reading RG6 6AP, and
| | - Chris Peers
- From the School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT
| | - Esther Schnettler
- MRC-University of Glasgow Centre for Virus Research, Scotland, Glasgow G61 1QH, United Kingdom
| | - Clive McKimmie
- MRC-University of Glasgow Centre for Virus Research, Scotland, Glasgow G61 1QH, United Kingdom
| | - Alain Kohl
- MRC-University of Glasgow Centre for Virus Research, Scotland, Glasgow G61 1QH, United Kingdom
| | - John N Barr
- From the School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT,
| | - Jamel Mankouri
- From the School of Molecular and Cellular Biology, University of Leeds, Leeds LS2 9JT,
| |
Collapse
|
8
|
Hansen UP, Rauh O, Schroeder I. A simple recipe for setting up the flux equations of cyclic and linear reaction schemes of ion transport with a high number of states: The arrow scheme. Channels (Austin) 2015; 10:119-38. [PMID: 26646356 DOI: 10.1080/19336950.2015.1120391] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022] Open
Abstract
The calculation of flux equations or current-voltage relationships in reaction kinetic models with a high number of states can be very cumbersome. Here, a recipe based on an arrow scheme is presented, which yields a straightforward access to the minimum form of the flux equations and the occupation probability of the involved states in cyclic and linear reaction schemes. This is extremely simple for cyclic schemes without branches. If branches are involved, the effort of setting up the equations is a little bit higher. However, also here a straightforward recipe making use of so-called reserve factors is provided for implementing the branches into the cyclic scheme, thus enabling also a simple treatment of such cases.
Collapse
Affiliation(s)
- Ulf-Peter Hansen
- a Department of Structural Biology , University of Kiel , Kiel , Germany
| | - Oliver Rauh
- b Plant Membrane Biophysics , Technical University of Darmstadt , Darmstadt , Germany
| | - Indra Schroeder
- b Plant Membrane Biophysics , Technical University of Darmstadt , Darmstadt , Germany
| |
Collapse
|
9
|
Abstract
The Na+/K+-ATPase restores sodium
(Na+) and potassium (K+)
electrochemical gradients dissipated by action potentials and ion-coupled transport
processes. As ions are transported, they become transiently trapped between
intracellular and extracellular gates. Once the external gate opens, three
Na+ ions are released, followed by the binding and
occlusion of two K+ ions. While the mechanisms of
Na+ release have been well characterized by the study of
transient Na+ currents, smaller and faster transient
currents mediated by external K+ have been more difficult to
study. Here we show that external K+ ions travelling to
their binding sites sense only a small fraction of the electric field as they
rapidly and simultaneously become occluded. Consistent with these results, molecular
dynamics simulations of a pump model show a wide water-filled access channel
connecting the binding site to the external solution. These results suggest a
mechanism of K+ gating different from that of
Na+ occlusion. During transport by the
Na+/K+-ATPase,
Na+ and K+ ions become occluded
between intra- and extracellular gates. Here Castillo et al. measure transient
electrical signals arising from K+ occlusion and use molecular
simulations to describe a K+ gating mechanism fundamentally
different to that of Na+.
Collapse
|
10
|
Abstract
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.
Collapse
Affiliation(s)
| | - Hans-Jürgen Apell
- Department of Biology, University of Konstanz, 78464 Konstanz, Germany
| |
Collapse
|
11
|
Mares LJ, Garcia A, Rasmussen HH, Cornelius F, Mahmmoud YA, Berlin JR, Lev B, Allen TW, Clarke RJ. Identification of electric-field-dependent steps in the Na(+),K(+)-pump cycle. Biophys J 2015; 107:1352-63. [PMID: 25229143 DOI: 10.1016/j.bpj.2014.05.054] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/25/2014] [Revised: 04/30/2014] [Accepted: 05/16/2014] [Indexed: 01/25/2023] Open
Abstract
The charge-transporting activity of the Na(+),K(+)-ATPase depends on its surrounding electric field. To isolate which steps of the enzyme's reaction cycle involve charge movement, we have investigated the response of the voltage-sensitive fluorescent probe RH421 to interaction of the protein with BTEA (benzyltriethylammonium), which binds from the extracellular medium to the Na(+),K(+)-ATPase's transport sites in competition with Na(+) and K(+), but is not occluded within the protein. We find that only the occludable ions Na(+), K(+), Rb(+), and Cs(+) cause a drop in RH421 fluorescence. We conclude that RH421 detects intramembrane electric field strength changes arising from charge transport associated with conformational changes occluding the transported ions within the protein, not the electric fields of the bound ions themselves. This appears at first to conflict with electrophysiological studies suggesting extracellular Na(+) or K(+) binding in a high field access channel is a major electrogenic reaction of the Na(+),K(+)-ATPase. All results can be explained consistently if ion occlusion involves local deformations in the lipid membrane surrounding the protein occurring simultaneously with conformational changes necessary for ion occlusion. The most likely origin of the RH421 fluorescence response is a change in membrane dipole potential caused by membrane deformation.
Collapse
Affiliation(s)
- Laura J Mares
- School of Chemistry, University of Sydney, Sydney, Australia
| | - Alvaro Garcia
- Department of Cardiology, Royal North Shore Hospital, Sydney, Australia; Kolling Institute, University of Sydney, Sydney, Australia
| | - Helge H Rasmussen
- Department of Cardiology, Royal North Shore Hospital, Sydney, Australia; Kolling Institute, University of Sydney, Sydney, Australia
| | | | | | - Joshua R Berlin
- Department of Pharmacology and Physiology, Rutgers University, Newark, New Jersey
| | - Bogdan Lev
- School of Applied Science and Health Innovations Research Institute, REMIT University, Melbourne, Australia
| | - Toby W Allen
- School of Applied Science and Health Innovations Research Institute, REMIT University, Melbourne, Australia
| | - Ronald J Clarke
- School of Chemistry, University of Sydney, Sydney, Australia.
| |
Collapse
|
12
|
Peluffo RD, Berlin JR. Membrane potential-dependent inhibition of the Na+,K+-ATPase by para-nitrobenzyltriethylammonium bromide. Mol Pharmacol 2012; 82:1-8. [PMID: 22456853 DOI: 10.1124/mol.111.077008] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Membrane potential (V(M))-dependent inhibitors of the Na(+),K(+)-ATPase are a new class of compounds that may have inherent advantages over currently available drugs targeting this enzyme. However, two questions remain unanswered regarding these inhibitors: (1) what is the mechanism of V(M)-dependent Na(+),K(+)-ATPase inhibition, and (2) is their binding affinity high enough to consider them as possible lead compounds? To address these questions, we investigated how a recently synthesized V(M)-dependent Na(+),K(+)-ATPase inhibitor, para-nitrobenzyltriethylamine (pNBTEA), binds to the enzyme by measuring the extracellular pNBTEA concentration and V(M) dependence of ouabain-sensitive transient charge movements in whole-cell patch-clamped rat cardiac ventricular myocytes. By analyzing the kinetics of charge movements and the steady-state distribution of charge, we show that the V(M)-dependent properties of pNBTEA binding differ from those for extracellular Na(+) and K(+) binding, even though inhibitor binding is competitive with extracellular K(+). The data were also fit to specific models for pNBTEA binding to show that pNBTEA binding is a rate-limiting V(M)-dependent reaction that, in light of homology models for the Na(+),K(+)-ATPase, we interpret as a transfer reaction of pNBTEA from a peripheral binding site in the enzyme to a site near the known K(+) coordination sites buried within the transmembrane helices of the enzyme. These models also suggest that binding occurs with an apparent affinity of 7 μM. This apparent binding affinity suggests that high-affinity V(M)-dependent Na(+),K(+)-ATPase inhibitors should be feasible to design and test as specific enzyme inhibitors.
Collapse
Affiliation(s)
- R Daniel Peluffo
- Department of Pharmacology and Physiology, UMDNJ-New Jersey Medical School, 185 S. Orange Ave., Newark, NJ 07101-1709, USA
| | | |
Collapse
|
13
|
Peluffo RD, González-Lebrero RM, Kaufman SB, Kortagere S, Orban B, Rossi RC, Berlin JR. Quaternary benzyltriethylammonium ion binding to the Na,K-ATPase: a tool to investigate extracellular K+ binding reactions. Biochemistry 2009; 48:8105-19. [PMID: 19621894 DOI: 10.1021/bi900687u] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
This study examined how the quaternary organic ammonium ion, benzyltriethylamine (BTEA), binds to the Na,K-ATPase to produce membrane potential (V(M))-dependent inhibition and tested the prediction that such a V(M)-dependent inhibitor would display electrogenic binding kinetics. BTEA competitively inhibited K(+) activation of Na,K-ATPase activity and steady-state (86)Rb(+) occlusion. The initial rate of (86)Rb(+) occlusion was decreased by BTEA to a similar degree whether it was added to the enzyme prior to or simultaneously with Rb(+), a demonstration that BTEA inhibits the Na,K-ATPase without being occluded. Several BTEA structural analogues reversibly inhibited Na,K-pump current, but none blocked current in a V(M)-dependent manner except BTEA and its para-nitro derivative, pNBTEA. Under conditions that promoted electroneutral K(+)-K(+) exchange by the Na,K-ATPase, step changes in V(M) elicited pNBTEA-activated ouabain-sensitive transient currents that had similarities to those produced with the K(+) congener, Tl(+). pNBTEA- and Tl(+)-dependent transient currents both displayed saturation of charge moved at extreme negative and positive V(M), equivalence of charge moved during and after step changes in V(M), and similar apparent valence. The rate constant (k(tot)) for Tl(+)-dependent transient current asymptotically approached a minimum value at positive V(M). In contrast, k(tot) for pNBTEA-dependent transient current was a "U"-shaped function of V(M) with a minimum value near 0 mV. Homology models of the Na,K-ATPase alpha subunit suggested that quaternary amines can bind to two extracellularly accessible sites, one of them located at K(+) binding sites positioned between transmembrane helices 4, 5, and 6. Altogether, these data revealed important information about electrogenic ion binding reactions of the Na,K-ATPase that are not directly measurable during ion transport by this enzyme.
Collapse
Affiliation(s)
- R Daniel Peluffo
- Department of Pharmacology and Physiology, UMDNJ-New Jersey Medical School, Newark, New Jersey 07101-1709, USA
| | | | | | | | | | | | | |
Collapse
|
14
|
Altered Na+ transport after an intracellular alpha-subunit deletion reveals strict external sequential release of Na+ from the Na/K pump. Proc Natl Acad Sci U S A 2009; 106:15507-12. [PMID: 19706387 DOI: 10.1073/pnas.0903752106] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022] Open
Abstract
The Na/K pump actively exports 3 Na(+) in exchange for 2 K(+) across the plasmalemma of animal cells. As in other P-type ATPases, pump function is more effective when the relative affinity for transported ions is altered as the ion binding sites alternate between opposite sides of the membrane. Deletion of the five C-terminal residues from the alpha-subunit diminishes internal Na(+) (Na(i)(+)) affinity approximately 25-fold [Morth et al. (2007) Nature 450:1043-1049]. Because external Na(+) (Na(o)(+)) binding is voltage-dependent, we studied the reactions involving this process by using two-electrode and inside-out patch voltage clamp in normal and truncated (DeltaKESYY) Xenopus-alpha1 pumps expressed in oocytes. We observed that DeltaKESYY (i) decreased both Na(o)(+) and Na(i)(+) apparent affinities in the absence of K(o)(+), and (ii) did not affect apparent Na(o)(+) affinity at high K(o)(+). These results support a model of strict sequential external release of Na(+) ions, where the Na(+)-exclusive site releases Na(+) before the sites shared with K(+) and the DeltaKESYY deletion only reduces Na(o)(+) affinity at the shared sites. Moreover, at nonsaturating K(o)(+), DeltaKESYY induced an inward flow of Na(+) through Na/K pumps at negative potentials. Guanidinium(+) can also permeate truncated pumps, whereas N-methyl-D-glucamine cannot. Because guanidinium(o)(+) can also traverse normal Na/K pumps in the absence of both Na(o)(+) and K(o)(+) and can also inhibit Na/K pump currents in a Na(+)-like voltage-dependent manner, we conclude that the normal pathway transited by the first externally released Na(+) is large enough to accommodate guanidinium(+).
Collapse
|
15
|
Reifenberger MS, Arnett KL, Gatto C, Milanick MA. Extracellular terbium and divalent cation effects on the red blood cell Na pump and chrysoidine effects on the renal Na pump. Blood Cells Mol Dis 2007; 39:7-13. [PMID: 17459735 DOI: 10.1016/j.bcmd.2007.02.012] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2007] [Accepted: 02/19/2007] [Indexed: 11/20/2022]
Abstract
We examined the effect of extracellular terbium (Tb(3+)) and divalent metal cations (Ca(2+), Sr(2+), and Ba(2+)) on (86)Rb(+) influx into rabbit and human red blood cells. We found that Tb(3+) at 15 and 25 microM was a non-competitive inhibitor of (86)Rb(+) influx suggesting that Tb(3+) is not binding to the transport site. This result reduces the usefulness of Tb(3+) as a potential probe for the E(out) conformation (the conformation with the transport site facing extracellularly). Ba(2+), Sr(2+) and Ca(2+), at concentrations >50 mM, had minimal effects on Rb(+) influx into red blood cells (1 mM Rb-out). This suggests that the outside transport site is very specific for monovalent cations over divalent cations, in contrast to the inside transport site. We also found that chrysoidine (4-phenylazo-m-phenylenediamine) competes with Na(+) for ATPase activity and K(+) for pNPPase activity suggesting it is binding to the E(in) conformation. Chrysoidine and similar compounds may be useful as optical probes of the E(in) conformation.
Collapse
Affiliation(s)
- Matthew S Reifenberger
- Department of Medical Pharmacology and Physiology, School of Medicine, Dalton Cardiovascular Research Center, University of Missouri, Columbia, MO 65211, USA
| | | | | | | |
Collapse
|
16
|
Gatto C, Helms JB, Prasse MC, Huang SY, Zou X, Arnett KL, Milanick MA. Similarities and differences between organic cation inhibition of the Na,K-ATPase and PMCA. Biochemistry 2006; 45:13331-45. [PMID: 17073454 DOI: 10.1021/bi060667j] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
The effects of three classes of organic cations on the inhibition of the plasma membrane Ca pump (PMCA) were determined and compared to inhibition of the Na pump. Quaternary amines (tetramethylammonium, tetraethylammonium, and tetrapropylammonium, TMA, TEA, and TPA, respectively) did not inhibit PMCA. This is not to imply that PMCA is inherently selective against monovalent cations because guanidine and tetramethylguanidine inhibited PMCA by competing with Ca(2+). The divalent organic cation, ethyl diamine, inhibited PMCA but was not competitive with Ca(2+). In contrast, propyl diamine did compete with Ca(2+) and was about 10-fold more potent than butyl diamine in inhibiting PMCA. For the Na pump, both TEA and TPA inhibited, but TMA did not. TEA, guanidine, and tetramethylguanidine inhibition was competitive with Na(+) for ATPase activation and with K(+) for pNPPase activation, both of which are cytoplasmic substrate cation effects. Thus, these findings are consistent with TEA, guanidine, and tetramethylguanidine inhibiting from the cytoplasmic side of the Na pump; in contrast, we have previously shown that TPA did not inhibit from the cytoplasmic side. The divalent alkane diamines ethyl, propyl, and butyl diamine all inhibited the Na pump and all competed at the intracellular surface. The order of potency was ED > PD > BD consistent with an optimal size for binding; similarly, for the quaternary amines TMA is apparently too small to make appropriate contacts, and TPA is too large. Homology models based upon the high-resolution SERCA structure are included to contextualize the kinetic observations.
Collapse
Affiliation(s)
- Craig Gatto
- Division of Biomedical Sciences, Department of Biological Sciences, Illinois State University, Normal, Illinois 61790-4120, USA
| | | | | | | | | | | | | |
Collapse
|
17
|
Gatto C, Helms JB, Prasse MC, Arnett KL, Milanick MA. Kinetic characterization of tetrapropylammonium inhibition reveals how ATP and Pi alter access to the Na+-K+-ATPase transport site. Am J Physiol Cell Physiol 2005; 289:C302-11. [PMID: 15788490 DOI: 10.1152/ajpcell.00043.2005] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Current models of the Na(+)-K(+)-ATPase reaction cycle have ATP binding with low affinity to the K(+)-occluded form and accelerating K(+) deocclusion, presumably by opening the inside gate. Implicit in this situation is that ATP binds after closing the extracellular gate and thus predicts that ATP binding and extracellular cation binding to be mutually exclusive. We tested this hypothesis. Accordingly, we needed a cation that binds outside and not inside, and we determined that tetrapropylammonium (TPA) behaves as such. TPA competed with K(+) (and not Na(+)) for ATPase, TPA was unable to prevent phosphoenzyme (EP) formation even at low Na(+), and TPA decreased the rate of EP hydrolysis in a K(+)-competitive manner. Having established that TPA binding is a measurement of extracellular access, we next determined that TPA and inorganic phosphate (P(i)) were not mutually exclusive inhibitors of para-nitrophenylphosphatase (pNPPase) activity, implying that when P(i) is bound, the transport site has extracellular access. Surprisingly, we found that ATP and TPA also were not mutually exclusive inhibitors of pNPPase activity, implying that when the cation transport site has extracellular access, ATP can still bind. This is consistent with a model in which ATP speeds up the conformational changes that lead to intracellular or extracellular access, but that ATP binding is not, by itself, the trigger that causes opening of the cation site to the cytoplasm.
Collapse
Affiliation(s)
- Craig Gatto
- Division of Biomedical Sciences, Cell Biology & Physiology Section, Department of Biological Sciences, Illinois State University, 210 Julian Hall, Campus Box 4120, Normal, IL 61790-4120, USA.
| | | | | | | | | |
Collapse
|