Bending of a lipid membrane edge by annexin A5 trimers

Plasma membrane damage occurs in healthy cells and more frequently in cancer cells where high growth rates and metastasis result in frequent membrane damage. The annexin family of proteins plays a key role in membrane repair. Annexins are recruited at the membrane injury site by Ca+2 and repair the damaged membrane in concert with several other proteins. Annexin A4 (ANXA4) and ANXA5 form trimers at the bilayer surface, and previous simulations show that the trimers induce high local negative membrane curvature on a flat bilayer. The membrane-curvature-inducing property of ANXA5 is presumed to be vital to the membrane repair mechanism. A previously proposed descriptive model hypothesizes that ANXA5-mediated curvature force is utilized at the free edge of the membrane at a wound site to pull the wound edges together, resulting in the formation of a "neck"-shaped structure, which, when combined with a constriction force exerted by ANXA6, leads to membrane repair. The molecular details and mechanisms of repair remain unknown, in part because the membrane edge is a transient structure that is difficult to investigate both experimentally and computationally. For the first time, we investigate the impact of ANXA5 near a membrane edge, which is modeled by a bicelle under periodic boundary conditions. ANXA5 trimers induce local curvature on the membrane leading to global bending of the bicelle. The global curvature depends on the density of annexins on the bicelle, and the curvature increases with the ANXA5 concentration until it reaches a plateau. The simulations suggest that not only do annexins induce local membrane curvature, but they can change the overall shape of a free-standing membrane. We also demonstrate that ANXA5 trimers reduce the rate of phosphatidylserine lipid diffusion from the cytoplasmic to the exoplasmic leaflet along the edge of the bicelle. In this way, membrane-bound annexins can potentially delay the apoptotic signal triggered by the presence of phosphatidylserine lipids in the outer leaflet, thus biding time for repair of the membrane hole. Our findings provide new insights into the role of ANXA5 at the edges of the membrane (the injury site) and support the curvature-constriction model of membrane repair.

Phospholamban inhibits the cardiac calcium pump by interrupting an allosteric activation pathway

Phospholamban (PLB) is a transmembrane micropeptide that regulates the sarcoplasmic reticulum Ca2+-ATPase (SERCA) in cardiac muscle, but the physical mechanism of this regulation remains poorly understood. PLB reduces the Ca2+ sensitivity of active SERCA, increasing the Ca2+ concentration required for pump cycling. However, PLB does not decrease Ca2+ binding to SERCA when ATP is absent, suggesting PLB does not inhibit SERCA Ca2+ affinity. The prevailing explanation for these seemingly conflicting results is that PLB slows transitions in the SERCA enzymatic cycle associated with Ca2+ binding, altering transport Ca2+ dependence without actually affecting the equilibrium binding affinity of the Ca2+-coordinating sites. Here, we consider another hypothesis, that measurements of Ca2+ binding in the absence of ATP overlook important allosteric effects of nucleotide binding that increase SERCA Ca2+ binding affinity. We speculated that PLB inhibits SERCA by reversing this allostery. To test this, we used a fluorescent SERCA biosensor to quantify the Ca2+ affinity of non-cycling SERCA in the presence and absence of a non-hydrolyzable ATP-analog, AMPPCP. Nucleotide activation increased SERCA Ca2+ affinity, and this effect was reversed by co-expression of PLB. Interestingly, PLB had no effect on Ca2+ affinity in the absence of nucleotide. These results reconcile the previous conflicting observations from ATPase assays versus Ca2+ binding assays. Moreover, structural analysis of SERCA revealed a novel allosteric pathway connecting the ATP- and Ca2+-binding sites. We propose this pathway is disrupted by PLB binding. Thus, PLB reduces the equilibrium Ca2+ affinity of SERCA by interrupting allosteric activation of the pump by ATP.

Sensing membrane voltage by reorientation of dipolar transmembrane peptides

Membrane voltage plays a vital role in the behavior and functions of the lipid bilayer membrane. For instance, it regulates the exchange of molecules across the membrane through transmembrane proteins such as ion channels. In this paper, we study the membrane voltage-sensing mechanism, which entails the reorientation of α-helices with a change in the membrane voltage. We consider a helix having a large electrical macrodipole embedded in a lipid bilayer as a model system. We performed extensive molecular dynamics simulations to study the effect of variation of membrane voltage on the tilt angle of peptides and ascertain the optimal parameters for designing such a voltage-sensing peptide. A theoretical model for the system is also developed to investigate the interplay of competing effects of hydrophobic mismatch and dipole-electric field coupling on the tilt of the peptide and further explore the parameter space. This work opens the possibility for the design and fabrication of artificial dipolar membrane voltage-sensing elements for biomedical applications.

Specific protonation of acidic residues confers K+ selectivity to the gastric proton pump

The gastric proton pump (H+,K+-ATPase) transports a proton into the stomach lumen for every K+ ion exchanged in the opposite direction. In the lumen-facing state of the pump (E2), the pump selectively binds K+ despite the presence of a 10-fold higher concentration of Na+. The molecular basis for the ion selectivity of the pump is unknown. Using molecular dynamics simulations, free energy calculations, and Na+ and K+-dependent ATPase activity assays, we demonstrate that the K+ selectivity of the pump depends upon the simultaneous protonation of the acidic residues E343 and E795 in the ion-binding site. We also show that when E936 is protonated, the pump becomes Na+ sensitive. The protonation-mimetic mutant E936Q exhibits weak Na+-activated ATPase activity. A 2.5-Å resolution cryo-EM structure of the E936Q mutant in the K+-occluded E2-Pi form shows, however, no significant structural difference compared with wildtype except less-than-ideal coordination of K+ in the mutant. The selectivity toward a specific ion correlates with a more rigid and less fluctuating ion-binding site. Despite being exposed to a pH of 1, the fundamental principle driving the K+ ion selectivity of H+,K+-ATPase is similar to that of Na+,K+-ATPase: the ionization states of the acidic residues in the ion-binding sites determine ion selectivity. Unlike the Na+,K+-ATPase, however, protonation of an ion-binding glutamate residue (E936) confers Na+ sensitivity.

Drastic differences between the release kinetics of two highly related porphyrins in liposomal membranes: mTHPP and pTHPP

HYPOTHESIS

The release of hydrophobic compounds from liposomal membranes occurs by partitioning and is thus determined by the physicochemical properties (e.g. logP and water solubility) of the drug. We postulate that even minor structural differences, e.g. the position of the phenolic OH-group of the hydrophobic porphyrins mTHPP and pTHPP (meta vs. para substitution), distinctly affect their partitioning and release behavior from liposomes.

EXPERIMENTS

The release and redistribution of mTHPP and pTHPP from lecithin or POPC/POPG liposomes to different acceptor particles (DSPE-mPEG micelles and liposomes) was studied by asymmetrical flow field-flow fractionation to separate donor and acceptor particles. Reversed phase HPLC was applied to detect differences in partitioning. Molecular dynamics (MD) simulations were carried out to obtain molecular insight in the different behavior of the two compounds inside a lipid bilayer.

FINDINGS

Despite the minor differences in chemical structure, mTHPP is more hydrophobic and redistributes much slower to both acceptor phases than pTHPP. MD simulations indicate that compared to pTHPP, mTHPP makes stronger hydrogen bonds with the lipid head groups, is oriented more parallel to the lipid tails and is embedded slightly deeper in the membrane.

ATP-Bound State of the Uncoupling Protein 1 (UCP1) from Molecular Simulations

The uncoupling protein 1 (UCP1) dissipates the transmembrane (TM) proton gradient in the inner mitochondrial membrane (IMM) by leaking protons across the membrane and producing heat in the process. Such a nonshivering production of heat in the brown adipose tissue can combat obesity-related diseases. UCP1-associated proton leak is activated by free fatty acids and inhibited by purine nucleotides. The mechanism of proton leak and the binding sites of the activators (fatty acids) remain unknown, while the binding site of the inhibitors (nucleotides) was described recently. Using molecular dynamics simulations, we generated a conformational ensemble of UCP1. Using metadynamics-based free energy calculations, we obtained the most likely ATP-bound conformation of UCP1. Our conformational ensemble provides a molecular basis for a breadth of prior biochemical data available for UCP1. Based on the simulations, we make the following testable predictions about the mechanisms of activation of proton leak and proton leak inhibition by ATP: (1) R277 plays the dual role of stabilizing ATP at the binding site for inhibition and acting as a proton surrogate for D28 in the absence of a proton during proton transport, (2) the binding of ATP to UCP1 is mediated by residues R84, R92, R183, and S88, (3) R92 shuttles ATP from the E191-R92 gate in the intermembrane space to the nucleotide binding site and serves to increase ATP affinity, (4) ATP can inhibit proton leak by controlling the ionization states of matrix facing lysine residues such as K269 and K56, and (5) fatty acids can bind to UCP1 from the IMM either via the cavity between TM1 and TM2 or between TM5 and TM6. Our simulations set the platform for future investigations into the proton transport and inhibition mechanisms of UCP1.

The molecular basis of the antidepressant action of the magic mushroom extract, psilocin

Magic mushrooms, and their extract psilocybin, are well-known for their psychedelic properties and recreational use. Psilocin, the bio-active form of psilocybin, can potentially treat various psychiatric diseases. Psilocin putatively exerts its psychedelic effect as an agonist to the serotonin 2A receptor (5-HT2AR), which is also the receptor for the neurological hormone serotonin. The two key chemical differences between the two molecules are first, that the primary amine in serotonin is replaced with a tertiary amine in psilocin, and second, the hydroxyl group is substituted differently on the aromatic ring. Here, we find that psilocin can bind to 5-HT2AR with an affinity higher than serotonin, and provide the molecular logic behind the higher binding affinity of psilocin using extensive molecular dynamics simulations and free energy calculations. The binding free energy of psilocin is dependent upon the protonation states of the ligands, as well as that of the key residue in the binding site: Aspartate 155. We find that the tertiary amine of psilocin, and not the altered substitution of the hydroxyl group in the ring is responsible for the increased affinity of psilocin. We propose design rules for effective antidepressants based on molecular insights from our simulations.

Engineering a Membrane-binding Protein to Trimerize and Induce High Membrane Curvature

The annexins are a family of Ca²⁺-dependent peripheral membrane proteins. Several annexins are implicated in plasma membrane repair and are overexpressed in cancer cells. Annexin A4 (ANXA4) and Annexin A5 (ANXA5) form trimers that induce high curvature on a membrane surface, a phenomenon deemed to accelerate membrane repair. Despite being highly homologous to ANXA4, Annexin A3 (ANXA3) does not form trimers on the membrane surface. Using molecular dynamics (MD) simulations, we have reverse-engineered an ANXA3-mutant (ANXA3-MUT) to trimerize on the surface of the membrane and induce high curvature reminiscent of ANXA4. In addition, AFM images show that like ANXA4, the engineered protein forms crystalline arrays on a supported lipid membrane. In spite of the trimer-forming and curvature-inducing properties of the engineered ANXA3, it does not accumulate near a membrane lesion in laser-punctured cells and is unable to repair the lesion. Our investigation provides insights into the factors which drive annexin-mediated membrane repair and shows that the membrane repairing property of trimer-forming annexins also necessitates high membrane binding affinity, other than trimer formation and induction of negative membrane curvature.

Interaction of psychedelic tryptamine derivatives with a lipid bilayer

Naturally occurring psychedelics have been used for a long time as remedies or in religious ceremonies and recreational activities. Recent studies have proven the therapeutic potential of some psychedelic compounds to safely treat a wide range of diseases such as anxiety, depression, migraine, and addiction. It is hypothesized that psychedelic compounds like tryptamines can exert their effects by two possible mechanisms: binding to the transmembrane serotonin receptor and/or modifying the properties of the neuronal membrane that can alter the conformational equilibrium and desensitize receptors. The impact of three different tryptamine class compounds with a tertiary amine (dimethyltryptamine, bufotenine, and 5-MeO-DMT) in both neutral and charged forms on a model bilayer lipid membrane are studied using all-atom MD simulations. All compounds partition into the bilayer, and change membrane properties, but to different extents. We determine the tendency of compounds to partition into the membrane by free energy calculations. Neutral tryptamines partition into the bilayer almost completely. Dimethyltryptamine and 5-MeO-DMT cross the membrane spontaneously during the simulation time, but bufotenine does not, although it has the maximum effect on the structural properties of the membrane. However, protonated compounds partition partially into the bilayer and cannot pass through the middle of the membrane during the simulation time. In this way, subtle alteration of chemical structure can play a significant role in the improvement or deterioration of partitioning of these compounds into the bilayer and their passage across the membrane.

Molecular Mechanism of Hydrotropic Properties of GTP and ATP

Hydrotropes are small amphiphilic compounds that increase the aqueous solubility of hydrophobic molecules. Recent evidence suggests that adenosine triphosphate (ATP), which is the primary energy carrier in cells, also assumes hydrotropic properties to prevent the aggregation of hydrophobic proteins, but the mechanism of hydrotropy is unknown. Here, we compare the hydrotropic behavior of all four biological nucleoside triphosphates (NTPs) using molecular dynamics (MD) simulations. We launch all atom MD simulations of aqueous solutions of NTPs [ATP, guanosine triphosphate (GTP), cytidine triphosphate (CTP), and uridine triphosphate (UTP)] with pyrene, which acts both as a model hydrophobic compound and as a spectroscopic reporter for aggregation. GTP prevents pyrene aggregation effectively. Dissolution is not achieved in the presence of CTP and UTP. The higher stability of the base stacking in guanine is responsible for the higher hydrotropic efficiency of GTP. Consistent with the simulations, spectroscopic measurements also suggest that the hydrotropic activity of GTP is higher than ATP. Stacking of aromatic pyrene with the aromatic base of NTPs is a characteristic feature of this hydrotropic property. Both ATP and GTP also dissolve clusters of di- and tripeptides containing tryptophan but with equal potency. Importantly, the presence of aromatic amino acids is a necessary condition for the hydrotropic potency of ATP and GTP. Our results can have broad implications for hydrotrope design in the pharmaceutical industry, as well as the possibility of cells employing GTP as a hydrotrope to regulate the hydrophobic protein aggregation in membrane-less biological condensates.

Magic mushroom extracts in lipid membranes

The active hallucinogen of magic mushrooms, psilocin, is being repurposed to treat nicotine addiction and treatment-resistant depression. Psilocin belongs to the tryptamine class of psychedelic compounds which include the hormone serotonin. It is believed that psilocin exerts its effect by binding to the serotonin 5-HT_2A receptor. However, recent in-vivo evidence suggests that psilocin may employ a different mechanism to exert its effects. Membrane-mediated receptor desensitization of neurotransmitter receptors is one such mechanism. We compare the impact of the neutral and charged versions of psilocin and serotonin on the properties of zwitterionic and anionic lipid membranes using molecular dynamics simulations and calorimetry. Both compounds partition to the lipid interface and induce membrane thinning. The tertiary amine in psilocin, as opposed to the primary amine in serotonin, limits psilocin's impact on the membrane although more psilocin partitions into the membrane than serotonin. Calorimetry corroborates that both compounds induce a classical melting point depression like anesthetics do. Our results also lend support to a membrane-mediated receptor-binding mechanism for both psilocin and serotonin and provide physical insights into subtle chemical changes that can alter the membrane-binding of psychedelic compounds.

Structural Basis for Binding of Potassium-Competitive Acid Blockers to the Gastric Proton Pump

As specific inhibitors of the gastric proton pump, responsible for gastric acidification, K⁺-competitive acid blockers (P-CABs) have recently been utilized in the clinical treatment of gastric acid-related diseases in Asia. However, as these compounds have been developed based on phenotypic screening, their detailed binding poses are unknown. We show crystal and cryo-EM structures of the gastric proton pump in complex with four different P-CABs, tegoprazan, soraprazan, PF-03716556 and revaprazan, at resolutions reaching 2.8 Å. The structures describe molecular details of their interactions and are supported by functional analyses of mutations and molecular dynamics simulations. We reveal that revaprazan has a novel binding mode in which its tetrahydroisoquinoline moiety binds deep in the cation transport conduit. The mechanism of action of these P-CABs can now be evaluated at the molecular level, which will facilitate the rational development and improvement of currently available P-CABs to provide better treatment of acid-related gastrointestinal diseases.

An Intracellular Pathway Controlled by the N-terminus of the Pump Subunit Inhibits the Bacterial KdpFABC Ion Pump in High K+ Conditions

The heterotetrameric bacterial KdpFABC transmembrane protein complex is an ion channel-pump hybrid that consumes ATP to import K⁺ against its transmembrane chemical potential gradient in low external K⁺ environments. The KdpB ion-pump subunit of KdpFABC is a P-type ATPase, and catalyses ATP hydrolysis. Under high external K⁺ conditions, K⁺ can diffuse into the cells through passive ion channels. KdpFABC must therefore be inhibited in high K⁺ conditions to conserve cellular ATP. Inhibition is thought to occur via unusual phosphorylation of residue Ser162 of the TGES motif of the cytoplasmic A domain. It is proposed that phosphorylation most likely traps KdpB in an inactive E1-P like conformation, but the molecular mechanism of phosphorylation-mediated inhibition remains unknown. Here, we employ molecular dynamics (MD) simulations of the dephosphorylated and phosphorylated versions of KdpFABC to demonstrate that phosphorylated KdpB is trapped in a conformation where the ion-binding site is hydrated by an intracellular pathway between transmembrane helices M1 and M2 which opens in response to the rearrangement of cytoplasmic domains resulting from phosphorylation. Cytoplasmic access of water to the ion-binding site is accompanied by a remarkable loss of secondary structure of the KdpB N-terminus and disruption of a key salt bridge between Glu87 in the A domain and Arg212 in the P domain. Our results provide the molecular basis of a unique mechanism of regulation amongst P-type ATPases, and suggest that the N-terminus has a significant role to play in the conformational cycle and regulation of KdpFABC.

Annexin A4 trimers are recruited by high membrane curvatures in giant plasma membrane vesicles

The plasma membrane (PM) of eukaryotic cells consists of a crowded environment comprised of a high diversity of proteins in a complex lipid matrix. The lateral organization of membrane proteins in the PM is closely correlated with biological functions such as endocytosis, membrane budding and other processes which involve protein mediated shaping of the membrane into highly curved structures. Annexin A4 (ANXA4) is a prominent player in a number of biological functions including PM repair. Its binding to membranes is activated by Ca2+ influx and it is therefore rapidly recruited to the cell surface near rupture sites where Ca2+ influx takes place. However, the free edges near rupture sites can easily bend into complex curvatures and hence may accelerate recruitment of curvature sensing proteins to facilitate rapid membrane repair. To analyze the curvature sensing behavior of curvature inducing proteins in crowded membranes, we quantifify the affinity of ANXA4 monomers and trimers for high membrane curvatures by extracting membrane nanotubes from giant PM vesicles (GPMVs). ANXA4 is found to be a sensor of negative membrane curvatures. Multiscale simulations, in which we extract molecular information from atomistic scale simulations as input to our macroscopic scale simulations, furthermore predicted that ANXA4 trimers generate membrane curvature upon binding and have an affinity for highly curved membrane regions only within a well defined membrane curvature window. Our results indicate that curvature sensing and mobility of ANXA4 depend on the trimer structure of ANXA4 which could provide new biophysical insight into the role of ANXA4 in membrane repair and other biological processes.

Long chain sphingomyelin depletes cholesterol from the cytoplasmic leaflet in asymmetric lipid membranes

The transbilayer distribution of cholesterol (CHL) in complex asymmetric lipid membranes remains controversial, with contrasting investigations suggesting that there is more CHL either in the exoplasmic, outer leaflet (OL) or the cytoplasmic, inner leaflet (IL) depending on cell type or model, membrane composition, and method of investigation. Here, we launch systematic coarse-grained molecular dynamics simulations to investigate the impact of the sphingomyelin (SM) acyl chain length upon CHL distribution in asymmetric lipid membrane mixtures which account for the variation of the most abundant headgroups and acyl chain unsaturation in the two membrane leaflets. We find that there is always more CHL in the OL, but longer chain SM depletes more CHL from the IL than short chain SM in simple membrane mixtures containing SM and 16 : 0, 18 : 1 phospholipids. The difference between longer and shorter chain SM is neutralised in a more complex asymmetric membrane, where there are more saturated tails in the outer leaflet. We propose that interdigitation of long-chain SM into the opposing IL pushes cytoplasmic CHL towards the OL, but higher chain saturation of the outer leaflet compensates for the effect of SM chain length.

Long acyl chain sphingomyelin and saturated phospholipid tails in the outer membrane leaflet deplete cholesterol from the inner leaflet in mammalian membranes.

Cholesterol binding to the sterol-sensing region of Niemann Pick C1 protein confines dynamics of its N-terminal domain

Lysosomal accumulation of cholesterol is a hallmark of Niemann Pick type C (NPC) disease caused by mutations primarily in the lysosomal membrane protein NPC1. NPC1 contains a transmembrane sterol-sensing domain (SSD), which is supposed to regulate protein activity upon cholesterol binding, but the mechanisms underlying this process are poorly understood. Using atomistic simulations, we show that in the absence of cholesterol in the SSD, the luminal domains of NPC1 are highly dynamic, resulting in the disengagement of the NTD from the rest of the protein. The disengaged NPC1 adopts a flexed conformation that approaches the lipid bilayer, and could represent a conformational state primed to receive a sterol molecule from the soluble lysosomal cholesterol carrier NPC2. The binding of cholesterol to the SSD of NPC1 allosterically suppresses the conformational dynamics of the luminal domains resulting in an upright NTD conformation. The presence of an additional 20% cholesterol in the membrane has negligible impact on this process. The additional presence of an NTD-bound cholesterol suppresses the flexing of the NTD. We propose that cholesterol acts as an allosteric effector, and the modulation of NTD dynamics by the SSD-bound cholesterol constitutes an allosteric feedback mechanism in NPC1 that controls cholesterol abundance in the lysosomal membrane.

Cholesterol is absorbed from LDL particles in esterified form, and is broken down to free cholesterol in the lysosomes of cells, from where cholesterol must be transported to other cellular compartments such as the plasma membrane. The Niemann Pick type C (NPC) diseases arise from deficient cholesterol transport and result from mutations in the cholesterol transport protein NPC1. Using computer simulations, we show that cholesterol, when bound to one part of NPC1, can control the structural transitions of an 8-nm distant, different part of NPC1 protein called the N-terminal domain (NTD). Such long-range control of protein conformations (allostery), controls a wide range of cellular functions mediated by proteins. Fundamental molecular insights into the function of the NPC1 protein can potentially lead to better pharmaceutical interventions for the NPC diseases.

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.

Clearance of activity-evoked K+ transients and associated glia cell swelling occur independently of AQP4: A study with an isoform-selective AQP4 inhibitor

The mammalian brain consists of 80% water, which is continuously shifted between different compartments and cellular structures by mechanisms that are, to a large extent, unresolved. Aquaporin 4 (AQP4) is abundantly expressed in glia and ependymal cells of the mammalian brain and has been proposed to act as a gatekeeper for brain water dynamics, predominantly based on studies utilizing AQP4-deficient mice. However, these mice have a range of secondary effects due to the gene deletion. An efficient and selective AQP4 inhibitor has thus been sorely needed to validate the results obtained in the AQP4^-/- mice to quantify the contribution of AQP4 to brain fluid dynamics. In AQP4-expressing Xenopus laevis oocytes monitored by a high-resolution volume recording system, we here demonstrate that the compound TGN-020 is such a selective AQP4 inhibitor. TGN-020 targets the tested species of AQP4 with an IC₅₀ of ~3.5 μM, but displays no inhibitory effect on the other AQPs (AQP1-AQP9). With this tool, we employed rat hippocampal slices and ion-sensitive microelectrodes to determine the role of AQP4 in glia cell swelling following neuronal activity. TGN-020-mediated inhibition of AQP4 did not prevent stimulus-induced extracellular space shrinkage, nor did it slow clearance of the activity-evoked K⁺ transient. These data, obtained with a verified isoform-selective AQP4 inhibitor, indicate that AQP4 is not required for the astrocytic contribution to the K⁺ clearance or the associated extracellular space shrinkage.

Interdisciplinary Synergy to Reveal Mechanisms of Annexin-Mediated Plasma Membrane Shaping and Repair

The plasma membrane surrounds every single cell and essentially shapes cell life by separating the interior from the external environment. Thus, maintenance of cell membrane integrity is essential to prevent death caused by disruption of the plasma membrane. To counteract plasma membrane injuries, eukaryotic cells have developed efficient repair tools that depend on Ca2+- and phospholipid-binding annexin proteins. Upon membrane damage, annexin family members are activated by a Ca2+ influx, enabling them to quickly bind at the damaged membrane and facilitate wound healing. Our recent studies, based on interdisciplinary research synergy across molecular cell biology, experimental membrane physics, and computational simulations show that annexins have additional biophysical functions in the repair response besides enabling membrane fusion. Annexins possess different membrane-shaping properties, allowing for a tailored response that involves rapid bending, constriction, and fusion of membrane edges for resealing. Moreover, some annexins have high affinity for highly curved membranes that appear at free edges near rupture sites, a property that might accelerate their recruitment for rapid repair. Here, we discuss the mechanisms of annexin-mediated membrane shaping and curvature sensing in the light of our interdisciplinary approach to study plasma membrane repair.

Thermodynamic Investigation of the Mechanism of Heat Production During Membrane Depolarization

When an action potential passes through a neuron, heat is first produced and then reabsorbed by the neuronal membrane, resulting in a small measurable temperature spike. Here, we describe the thermodynamics and molecular features of the heat production using a coarse-grained molecular dynamics approach. We study a simple unicomponent lipid bilayer membrane surrounded by physiological salt solution with and without an external electric field, which represents an imbalanced charge across the membrane. We show that the temperature increases significantly upon removal of the electric field under constant pressure conditions. The potential energy converted to heat is initially stored mainly in the imbalanced ion distribution across the membrane and the elastic energy of the membrane has only a minor role to play. We demonstrate that the mechanism of heat production involves interaction between ions as well as lipid headgroup dipoles while the interactions between polar water molecules and lipid headgroup dipoles absorbs a considerable portion of such produced heat upon removal of the electric field. Our data provide novel thermodynamic insights into the molecular processes governing membrane reorganization upon discharging of lipid membranes and insight into energy metabolism in nerves.

Interaction of the mononucleotide UMP with a fluid phospholipid bilayer

Interaction between mononucleotides and lipid membranes is believed to have played an important role in the origin of life on Earth. Studies on mononucleotide-lipid systems hitherto have focused on the influence of the lipid environment on the organization of the mononucleotide molecules, and the effect of the latter on the confining medium has not been investigated in detail. We have probed the interaction of the mononucleotide, uridine 5'-monophosphate (UMP), and its disodium salt (UMPDSS) with fluid dimyristoylphosphatidylcholine (DMPC) membranes, using small-angle X-ray scattering (SAXS), cryogenic scanning electron microscopy (cryo-SEM) and computer simulations. UMP adsorbs and charges the lipid membrane, resulting in the formation of unilamellar vesicles in dilute solutions. Adsorption of UMP reduces the bilayer thickness of DMPC. UMPDSS has a much weaker effect on interbilayer interactions. These observations are in very good agreement with the results of an all-atom molecular dynamics simulation of these systems. In the presence of counterions, such as Na⁺, UMP forms small aggregates in water, which bind to the bilayer without significantly perturbing it. The phosphate moiety in the lipid headgroup is found to bind to the hydrogens from the sugar ring of UMP, while the choline group tends to bind to the two oxygens from the nucleotide base. These studies provide important insights into lipid-nucleotide interactions and the effect of the nucleotide on lipid membranes.

A single K+-binding site in the crystal structure of the gastric proton pump

The gastric proton pump (H⁺,K⁺-ATPase), a P-type ATPase responsible for gastric acidification, mediates electro-neutral exchange of H⁺ and K⁺ coupled with ATP hydrolysis, but with an as yet undetermined transport stoichiometry. Here we show crystal structures at a resolution of 2.5 Å of the pump in the E2-P transition state, in which the counter-transporting cation is occluded. We found a single K⁺ bound to the cation-binding site of the H⁺,K⁺-ATPase, indicating an exchange of 1H⁺/1K⁺ per hydrolysis of one ATP molecule. This fulfills the energy requirement for the generation of a six pH unit gradient across the membrane. The structural basis of K⁺ recognition is resolved and supported by molecular dynamics simulations, establishing how the H⁺,K⁺-ATPase overcomes the energetic challenge to generate an H⁺ gradient of more than a million-fold-one of the highest cation gradients known in mammalian tissue-across the membrane.

Evidence for ATP Interaction with Phosphatidylcholine Bilayers

ATP is a fundamental intracellular molecule and is thought to diffuse freely throughout the cytosol. Evidence obtained from nucleotide-sensing sarcolemmal ion channels and red blood cells, however, suggest that ATP is compartmentalized or buffered, especially beneath the sarcolemma, but no definitive mechanism for restricted diffusion or potential buffering system has been postulated. In this study, we provide evidence from alterations to membrane dipole potential, membrane conductance, changes in enthalpy of phospholipid phase transition, and from free energy calculations that ATP associates with phospholipid bilayers. Furthermore, all-atom molecular dynamics simulations show that ATP can form aggregates in the aqueous phase at high concentrations. ATP interaction with membranes provides a new model to understand the diffusion of ATP through the cell. Coupled with previous reports of diffusion restriction in the subsarcolemmal space, these findings support the existence of compartmentalized or buffered pools of ATP.

K+ binding and proton redistribution in the E2P state of the H+, K+-ATPase

The H+, K+-ATPase (HKA) uses ATP to pump protons into the gastric lumen against a million-fold proton concentration gradient while counter-transporting K+ from the lumen. The mechanism of release of a proton into a highly acidic stomach environment, and the subsequent binding of a K+ ion necessitates a network of protonable residues and dynamically changing protonation states in the cation binding pocket dominated by five acidic amino acid residues E343, E795, E820, D824, and D942. We perform molecular dynamics simulations of spontaneous K+ binding to all possible protonation combinations of the acidic amino acids and carry out free energy calculations to determine the optimal protonation state of the luminal-open E2P state of the pump which is ready to bind luminal K+. A dynamic pKa correlation analysis reveals the likelihood of proton transfer events within the cation binding pocket. In agreement with in-vitro measurements, we find that E795 is likely to be protonated, and that E820 is at the center of the proton transfer network in the luminal-open E2P state. The acidic residues D942 and D824 are likely to remain protonated, and the proton redistribution occurs predominantly amongst the glutamate residues exposed to the lumen. The analysis also shows that a lower number of K+ ions bind at lower pH, modeled by a higher number of protons in the cation binding pocket, in agreement with the 'transport stoichiometry variation' hypothesis.

Different footprints of the Zika and dengue surface proteins on viral membranes

The flavivirus Zika virus (ZV) became an international emergency within two years of its outbreak in the Americas. Dengue virus (DENV), which is also a flavivirus, causes significant clinical harm in equatorial regions. A common feature amongst flaviviruses like ZV and DENV is an icosahedral shell of exactly 180 copies of the envelope (E) and membrane (M) proteins anchored in a lipid membrane, which engulfs the viral RNA and capsid proteins. Host recognition by both ZV and DENV is linked to the presence of phosphatidylserine (PS) and phosphatidylethanolamine (PE) lipids in the viral lipidome. Glycosylation of Asn residues on the Zika E protein may be linked to ZV induced neuropathies. We carry out coarse grained molecular dynamics simulations of the E3M3 hexamer embedded in the ZV and DENV lipidomes, and we show that the proteins have a significantly different lipid footprint in the viral lipidome. PE lipids in DENV and PS lipids in ZV enrich near the protein hexamer. We attribute the difference to a higher number of cationic amino acids in the ZV M protein. We also show that the three glycosylation sites on ZV, but not on DENV, are conformationally variant. Our data shed new light on the lipid interactions, and thus the host recognition mechanisms of the two viruses, which may be molecular determinants of the neuropathies caused by the ZV.

Faster Simulations with a 5 fs Time Step for Lipids in the CHARMM Force Field

The performance of all-atom molecular dynamics simulations is limited by an integration time step of 2 fs, which is needed to resolve the fastest degrees of freedom in the system, namely, the vibration of bonds and angles involving hydrogen atoms. The virtual interaction sites (VIS) method replaces hydrogen atoms by massless virtual interaction sites to eliminate these degrees of freedom while keeping intact nonbonded interactions and the explicit treatment of hydrogen atoms. We have modified the existing VIS algorithm for most lipids in the popular CHARMM36 force field by increasing the hydrogen atom masses at regular intervals in the lipid acyl chains and obtained lipid properties and pore formation free energies in very good agreement with those calculated in simulations without VIS. Our modified VIS scheme enables a 5 fs time step resulting in a significant performance gain for all-atom simulations of membranes. The method has the potential to make longer time and length scales accessible in all-atom simulations of membrane-protein complexes.

Lipid Configurations from Molecular Dynamics Simulations

The extent to which current force fields faithfully reproduce conformational properties of lipids in bilayer membranes, and whether these reflect the structural principles established for phospholipids in bilayer crystals, are central to biomembrane simulations. We determine the distribution of dihedral angles in palmitoyl-oleoyl phosphatidylcholine from molecular dynamics simulations of hydrated fluid bilayer membranes. We compare results from the widely used lipid force field of Berger et al. with those from the most recent C36 release of the CHARMM force field for lipids. Only the CHARMM force field produces the chain inequivalence with sn-1 as leading chain that is characteristic of glycerolipid packing in fluid bilayers. The exposure and high partial charge of the backbone carbonyls in Berger lipids leads to artifactual binding of Na+ ions reported in the literature. Both force fields predict coupled, near-symmetrical distributions of headgroup dihedral angles, which is compatible with models of interconverting mirror-image conformations used originally to interpret NMR order parameters. The Berger force field produces rotamer populations that correspond to the headgroup conformation found in a phosphatidylcholine lipid bilayer crystal, whereas CHARMM36 rotamer populations are closer to the more relaxed crystal conformations of phosphatidylethanolamine and glycerophosphocholine. CHARMM36 alone predicts the correct relative signs of the time-average headgroup order parameters, and reasonably reproduces the full range of NMR data from the phosphate diester to the choline methyls. There is strong motivation to seek further experimental criteria for verifying predicted conformational distributions in the choline headgroup, including the 31P chemical shift anisotropy and 14N and CD3 NMR quadrupole splittings.

Correction to: The CAPOS mutation in ATP1A3 alters Na/K-ATPase function and results in auditory neuropathy which has implications for management

The following information was inadvertently omitted in the original publication.

Membrane Tubulation in Lipid Vesicles Triggered by the Local Application of Calcium Ions

Experimental and theoretical studies on ion-lipid interactions predict that binding of calcium ions to cell membranes leads to macroscopic mechanical effects and membrane remodeling. Herein, we provide experimental evidence that a point source of Ca²⁺ acting upon a negatively charged membrane generates spontaneous curvature and triggers the formation of tubular protrusions that point away from the ion source. This behavior is rationalized by strong binding of the divalent cations to the surface of the charged bilayer, which effectively neutralizes the surface charge density of outer leaflet of the bilayer. The mismatch in the surface charge density of the two leaflets leads to nonzero spontaneous curvature. We probe this mismatch through the use of molecular dynamics simulations and validate that calcium ion binding to a lipid membrane is sufficient to generate inward spontaneous curvature, bending the membrane. Additionally, we demonstrate that the formed tubular protrusions can be translated along the vesicle surface in a controlled manner by repositioning the site of localized Ca²⁺ exposure. The findings demonstrate lipid membrane remodeling in response to local chemical gradients and offer potential insights into the cell membrane behavior under conditions of varying calcium ion concentrations.

Electrostatic Stabilization Plays a Central Role in Autoinhibitory Regulation of the Na+,K+-ATPase

The Na⁺,K⁺-ATPase is present in the plasma membrane of all animal cells. It plays a crucial role in maintaining the Na⁺ and K⁺ electrochemical potential gradients across the membrane, which are essential in numerous physiological processes, e.g., nerve, muscle, and kidney function. Its cellular activity must, therefore, be under tight metabolic control. Consideration of eosin fluorescence and stopped-flow kinetic data indicates that the enzyme's E2 conformation is stabilized by electrostatic interactions, most likely between the N-terminus of the protein's catalytic α-subunit and the adjacent membrane. The electrostatic interactions can be screened by increasing ionic strength, leading to a more evenly balanced equilibrium between the E1 and E2 conformations. This represents an ideal situation for effective regulation of the Na⁺,K⁺-ATPase's enzymatic activity, because protein modifications, which perturb this equilibrium in either direction, can then easily lead to activation or inhibition. The effect of ionic strength on the E1:E2 distribution and the enzyme's kinetics can be mathematically described by the Gouy-Chapman theory of the electrical double layer. Weakening of the electrostatic interactions and a shift toward E1 causes a significant increase in the rate of phosphorylation of the enzyme by ATP. Electrostatic stabilization of the Na⁺,K⁺-ATPase's E2 conformation, thus, could play an important role in regulating the enzyme's physiological catalytic turnover.

Glutamate Water Gates in the Ion Binding Pocket of Na+ Bound Na+, K+-ATPase

The dynamically changing protonation states of the six acidic amino acid residues in the ion binding pocket of the Na+, K+ -ATPase (NKA) during the ion transport cycle are proposed to drive ion binding, release and possibly determine Na+ or K+ selectivity. We use molecular dynamics (MD) and density functional theory (DFT) simulations to determine the protonation scheme of the Na+ bound conformation of NKA. MD simulations of all possible protonation schemes show that the bound Na+ ions are most stably bound when three or four protons reside in the binding sites, and that Glu954 in site III is always protonated. Glutamic acid residues in the three binding sites act as water gates, and their deprotonation triggers water entry to the binding sites. From DFT calculations of Na+ binding energies, we conclude that three protons in the binding site are needed to effectively bind Na+ from water and four are needed to release them in the next step. Protonation of Asp926 in site III will induce Na+ release, and Glu327, Glu954 and Glu779 are all likely to be protonated in the Na+ bound occluded conformation. Our data provides key insights into the role of protons in the Na+ binding and release mechanism of NKA.

Membrane invagination induced by Shiga toxin B-subunit: from molecular structure to tube formation

The bacterial Shiga toxin is composed of an enzymatically active A-subunit, and a receptor-binding homopentameric B-subunit (STxB) that mediates intracellular toxin trafficking. Upon STxB-mediated binding to the glycolipid globotriaosylceramide (Gb3) at the plasma membrane of target cells, Shiga toxin is internalized by clathrin-dependent and independent endocytosis. The formation of tubular membrane invaginations is an essential step in the clathrin-independent STxB uptake process. However, the mechanism by which STxB induces these invaginations has remained unclear. Using a combination of all-atom molecular dynamics and Monte Carlo simulations we show that the molecular architecture of STxB enables the following sequence of events: the Gb3 binding sites on STxB are arranged such that tight avidity-based binding results in a small increment of local curvature. Membrane-mediated clustering of several toxin molecules then creates a tubular membrane invagination that drives toxin entry into the cell. This mechanism requires: (1) a precise molecular architecture of the STxB binding sites; (2) a fluid bilayer in order for the tubular invagination to form. Although, STxB binding to the membrane requires specific interactions with Gb3 lipids, our study points to a generic molecular design principle for clathrin-independent endocytosis of nanoparticles.

An overview of molecular dynamics simulations of oxidized lipid systems, with a comparison of ELBA and MARTINI force fields for coarse grained lipid simulations

Biological membranes and model lipid systems containing high amounts of unsaturated lipids and sterols are subject to chemical and/or photo-induced lipid oxidation, which leads to the creation of exotic oxidized lipid products (OxPLs). OxPLs are known to have significant physiological impact in cellular systems and also affect physical properties of both biological and model lipid bilayers. In this paper we (i) provide a perspective on the existing literature on simulations of lipid bilayer systems containing oxidized lipid species as well as the main related experimental results, (ii) describe our new data of all-atom and coarse-grained simulations of hydroperoxidized lipid monolayer and bilayer systems and (iii) provide a comparison of the MARTINI and ELBA coarse grained force fields for lipid bilayer systems. We show that the better electrostatic treatment of interactions in ELBA is able to resolve previous conflicts between experiments and simulations. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Perillyl alcohol: Dynamic interactions with the lipid bilayer and implications for long-term inhalational chemotherapy for gliomas

Background:

Gliomas display a high degree of intratumor heterogeneity, including changes in physiological parameters and lipid composition of the plasma membrane, which may contribute to the development of drug resistance. Biophysical interactions between therapeutic agents and the lipid components at the outer plasma membrane interface are critical for effective drug uptake. Amphipathic molecules such as perillyl alcohol (POH) have a high partition coefficient and generally lead to altered lipid acyl tail dynamics near the lipid-water interface, impacting the lipid bilayer structure and transport dynamics. We therefore hypothesized that glioma cells may display enhanced sensitivity to POH-induced apoptosis due to plasma membrane alterations, while in non-transformed cells, POH may be expelled through thermal agitation.

Methods:

Interactions between POH and the plasma membrane was studied using molecular dynamics simulations. In this phase I/II trial, we set up to evaluate the clinical effectiveness of long-term (up to 5 years) daily intranasal administration of POH in a cohort of 19 patients with low-grade glioma (LGG). Importantly, in a series of clinical studies previously published by our group, we have successfully established that intranasal delivery of POH to patients with malignant gliomas is a viable and effective therapeutic strategy.

Results:

POH altered the plasma membrane potential of the lipid bilayer of gliomas and prolonged intranasal administration of POH in a cohort of patients with LGG halted disease progression with virtually no toxicity.

Conclusion:

Altogether, the results suggest that POH-induced alterations of the plasma membrane might be contributing to its therapeutic efficacy in preventing LGG progression.

Membrane accessibility of glutathione

Regulation of the ion pumping activity of the Na+,K+-ATPase is crucial to the survival of animal cells. Recent evidence has suggested that the activity of the enzyme could be controlled by glutathionylation of cysteine residue 45 of the β-subunit. Crystal structures so far available indicate that this cysteine is in a transmembrane domain of the protein. Here we have analysed via fluorescence and NMR spectroscopy as well as molecular dynamics simulations whether glutathione is able to penetrate into the interior of a lipid membrane. No evidence for any penetration of glutathione into the membrane was found. Therefore, the most likely mechanism whereby the cysteine residue could become glutathionylated is via a loosening of the α-β subunit association, creating a hydrophilic passageway between them to allow access of glutathione to the cysteine residue. By such a mechanism, glutathionylation of the protein would be expected to anchor the modified cysteine residue in a hydrophilic environment, inhibiting further motion of the β-subunit during the enzyme's catalytic cycle and suppressing enzymatic activity, as has been experimentally observed. The results obtained, therefore, suggest a possible structural mechanism of how the Na+,K+-ATPase could be regulated by glutathione.

The N Terminus of Sarcolipin Plays an Important Role in Uncoupling Sarco-endoplasmic Reticulum Ca2+-ATPase (SERCA) ATP Hydrolysis from Ca2+ Transport

The sarcoendoplasmic reticulum Ca(2+)-ATPase (SERCA) is responsible for intracellular Ca(2+) homeostasis. SERCA activity in muscle can be regulated by phospholamban (PLB), an affinity modulator, and sarcolipin (SLN), an uncoupler. Although PLB gets dislodged from Ca(2+)-bound SERCA, SLN continues to bind SERCA throughout its kinetic cycle and promotes uncoupling of Ca(2+) transport from ATP hydrolysis. To determine the structural regions of SLN that mediate uncoupling of SERCA, we employed mutagenesis and generated chimeras of PLB and SLN. In this study we demonstrate that deletion of SLN N-terminal residues (2)ERSTQ leads to loss of the uncoupling function even though the truncated peptide can target and constitutively bind SERCA. Furthermore, molecular dynamics simulations of SLN and SERCA interaction showed a rearrangement of SERCA residues that is altered when the SLN N terminus is deleted. Interestingly, transfer of the PLB cytosolic domain to the SLN transmembrane (TM) and luminal tail causes the chimeric protein to lose SLN-like function. Further introduction of the PLB TM region into this chimera resulted in conversion to full PLB-like function. We also found that swapping PLB N and C termini with those from SLN caused the resulting chimera to acquire SLN-like function. Swapping the C terminus alone was not sufficient for this conversion. These results suggest that domains can be switched between SLN and PLB without losing the ability to regulate SERCA activity; however, the resulting chimeras acquire functions different from the parent molecules. Importantly, our studies highlight that the N termini of SLN and PLB influence their respective unique functions.

Selective chemical binding enhances cesium tolerance in plants through inhibition of cesium uptake

High concentrations of cesium (Cs⁺) inhibit plant growth but the detailed mechanisms of Cs⁺ uptake, transport and response in plants are not well known. In order to identify small molecules with a capacity to enhance plant tolerance to Cs⁺, chemical library screening was performed using Arabidopsis. Of 10,000 chemicals tested, five compounds were confirmed as Cs⁺ tolerance enhancers. Further investigation and quantum mechanical modelling revealed that one of these compounds reduced Cs⁺ concentrations in plants and that the imidazole moiety of this compound bound specifically to Cs⁺. Analysis of the analogous compounds indicated that the structure of the identified compound is important for the effect to be conferred. Taken together, Cs⁺ tolerance enhancer isolated here renders plants tolerant to Cs⁺ by inhibiting Cs⁺ entry into roots via specific binding to the ion thus, for instance, providing a basis for phytostabilisation of radiocesium-contaminated farmland.

The effects of globotriaosylceramide tail saturation level on bilayer phases

Globotriaosylceramide (Gb3) is a glycosphingolipid present in the plasma membrane that is the natural receptor of the bacterial Shiga toxin. The unsaturation level of Gb3 acyl chains has a drastic impact on lipid bilayer properties and phase behaviour, and on many Gb3-related cellular processes. For example: the Shiga toxin B subunit forms tubular invaginations in the presence of Gb3 with an unsaturated acyl chain (U-Gb3), while in the presence of Gb3 with a saturated acyl chain (S-Gb3) such invagination does not occur. We have used all-atom molecular dynamics simulations to investigate the effects of the Gb3 concentration and its acyl chain saturation on the phase behaviour of a mixed bilayer of dioleoylphosphatidylcholine and Gb3. The simulation results show that: (1) the Gb3 acyl chains (longer tails) from one leaflet interdigitate into the opposing leaflet and lead to significant bilayer rigidification and immobilisation of the lipid tails. S-Gb3 can form a highly ordered, relatively immobile phase which is resistant to bending while these changes for U-Gb3 are not significant. (2) At low concentrations of Gb3, U-Gb3 and S-Gb3 have a similar impact on the bilayer reminiscent of the effect of sphingomyelin lipids and (3) At higher Gb3 concentrations, U-Gb3 mixes better with dioleoylphosphatidylcholine than S-Gb3. Our simulations also provide the first molecular level structural model of Gb3 in membranes.

K+ congeners that do not compromise Na+ activation of the Na+,K+-ATPase: hydration of the ion binding cavity likely controls ion selectivity

The Na(+),K(+)-ATPase is essential for ionic homeostasis in animal cells. The dephosphoenzyme contains Na(+) selective inward facing sites, whereas the phosphoenzyme contains K(+) selective outward facing sites. Under normal physiological conditions, K(+) inhibits cytoplasmic Na(+) activation of the enzyme. Acetamidinium (Acet(+)) and formamidinium (Form(+)) have been shown to permeate the pump through the outward facing sites. Here, we show that these cations, unlike K(+), are unable to enter the inward facing sites in the dephosphorylated enzyme. Consistently, the organic cations exhibited little to no antagonism to cytoplasmic Na(+) activation. Na(+),K(+)-ATPase structures revealed a previously undescribed rotamer transition of the hydroxymethyl side chain of the absolutely conserved Thr(772) of the α-subunit. The side chain contributes its hydroxyl to Na(+) in site I in the E1 form and rotates to contribute its methyl group toward K(+) in the E2 form. Molecular dynamics simulations to the E1·AlF4 (-)·ADP·3Na(+) structure indicated that 1) bound organic cations differentially distorted the ion binding sites, 2) the hydroxymethyl of Thr(772) rotates to stabilize bound Form(+) through water molecules, and 3) the rotamer transition is mediated by water traffic into the ion binding cavity. Accordingly, dehydration induced by osmotic stress enhanced the interaction of the congeners with the outward facing sites and profoundly modified the organization of membrane domains of the α-subunit. These results assign a catalytic role for water in pump function, and shed light on a backbone-independent but a conformation-dependent switch between H-bond and dispersion contact as part of the catalytic mechanism of the Na(+),K(+)-ATPase.