Membrane pyrophosphatases from Thermotoga maritima and Vigna radiata suggest a conserved coupling mechanism

Rapid kinetics of H+ transport by membrane pyrophosphatase: Evidence for a "direct-coupling" mechanism

Stress resistance-conferring membrane pyrophosphatase (mPPase) found in microbes and plants couples pyrophosphate hydrolysis with H+ transport out of the cytoplasm. There are two opposing views on the energy-coupling mechanism in this transporter: the pumping is associated with either pyrophosphate binding to mPPase or the hydrolysis step. We used our recently developed stopped-flow pyranine assay to measure H+ transport into mPPase-containing inverted membrane vesicles on the timescale of a single turnover. The vesicles were prepared from Escherichia coli overproducing the H+-translocating mPPase of Desulfitobacterium hafniense. Pyrophosphate induced linear accumulation of H+ in the vesicles, without evident lag or burst. In contrast, the binding of three nonhydrolyzable pyrophosphate analogs essentially induced no H+ accumulation. These findings are inconsistent with the "pumping-before-hydrolysis" model of mPPase functioning and support the alternative model positing the hydrolysis reaction as the source of the transported H+ ions. mPPase is thus a first "directly-coupled" proton pump.

Enabling structural biological electron paramagnetic resonance spectroscopy in membrane proteins through spin labelling

Pulsed dipolar electron paramagnetic resonance spectroscopy (PDS), combined with site-directed spin-labelling, represents a powerful tool for the investigation of biomacromolecules, emerging as a keystone approach in structural biology. Increasingly, PDS is applied to study highly complex integral membrane protein systems, such as mechanosensitive ion channels, transporters, G-protein coupled receptors, ion pumps, and outer membrane proteins elucidating their dynamics and revealing conformational ensembles. Indeed, PDS offers a platform to study intermediate or lowly-populated states that are otherwise invisible to other modern methods, such as X-ray crystallography, cryo-EM, and hydrogen-deuterium exchange-mass spectrometry. Importantly, advances in spin labelling strategies welcome a new era of membrane protein investigation under near-native or in-cell conditions. Here, we review recent integral membrane protein PDS applications, and highlight well-suited, emerging spin labelling strategies that show promise for future studies.

Na+ Translocation Dominates over H+-Translocation in the Membrane Pyrophosphatase with Dual Transport Specificity

Cation-pumping membrane pyrophosphatases (mPPases; EC 7.1.3.1) vary in their transport specificity from obligatory H+ transporters found in all kingdoms of life, to Na+/H+-co-transporters found in many prokaryotes. The available data suggest a unique "direct-coupling" mechanism of H+ transport, in which the transported proton is generated from nucleophilic water molecule. Na+ transport is best rationalized by assuming that the water-borne proton propels a prebound Na+ ion through the ion conductance channel ("billiard" mechanism). However, the "billiard" mechanism, in its simple form, is not applicable to the mPPases that simultaneously transport Na+ and H+ without evident competition between the cations (Na+,H+-PPases). In this study, we used a pyranine-based fluorescent assay to explore the relationship between the cation transport reactions catalyzed by recombinant Bacteroides vulgatus Na+,H+-PPase in membrane vesicles. Under appropriately chosen conditions, including the addition of an H+ ionophore to convert Na+ influx into equivalent H+ efflux, the pyranine signal measures either H+ or Na+ translocation. Using a stopped-flow version of this assay, we demonstrate that H+ and Na+ are transported by Na+,H+-PPase in a ratio of approximately 1:8, which is independent of Na+ concentration. These findings were rationalized using an "extended billiard" model, whose most likely variant predicts the kinetic limitation of Na+ delivery to the pump-loading site.

H+-translocating pyrophosphatases in protozoan parasites

Integral membrane pyrophosphatases (mPPases) hydrolyze pyrophosphate. This enzymatic mechanism is coupled with the pumping of H + and/or Na + across membranes, which can be either K + -dependent or K + -independent. Inorganic proton-translocating pyrophosphatases (H + -PPases) can transport protons across cell membranes and are reported in various organisms such as plants, bacteria, and protozoan parasites. The evolutionary implications of these enzymes are of great interest for proposing approaches related to the treatment of parasitic of phytopathogenic diseases. This work presents a literature review on pyrophosphate, pyrophosphatases, their inhibitors and emphasizes H + -PPases found in various medically significant protozoan parasites such as Toxoplasma gondii, the causative agent of toxoplasmosis, and Plasmodium falciparum, the causative agent of malaria, as well as protozoan species that primarily affect animals, such as Eimeria maxima and Besnoitia besnoiti.

The inorganic pyrophosphatases of microorganisms: a structural and functional review

Pyrophosphatases (PPases) are enzymes that catalyze the hydrolysis of pyrophosphate (PPi), a byproduct of the synthesis and degradation of diverse biomolecules. The accumulation of PPi in the cell can result in cell death. Although the substrate is the same, there are variations in the catalysis and features of these enzymes. Two enzyme forms have been identified in bacteria: cytoplasmic or soluble pyrophosphatases and membrane-bound pyrophosphatases, which play major roles in cell bioenergetics. In eukaryotic cells, cytoplasmic enzymes are the predominant form of PPases (c-PPases), while membrane enzymes (m-PPases) are found only in protists and plants. The study of bacterial cytoplasmic and membrane-bound pyrophosphatases has slowed in recent years. These enzymes are central to cell metabolism and physiology since phospholipid and nucleic acid synthesis release important amounts of PPi that must be removed to allow biosynthesis to continue. In this review, two aims were pursued: first, to provide insight into the structural features of PPases known to date and that are well characterized, and to provide examples of enzymes with novel features. Second, the scientific community should continue studying these enzymes because they have many biotechnological applications. Additionally, in this review, we provide evidence that there are m-PPases present in fungi; to date, no examples have been characterized. Therefore, the diversity of PPase enzymes is still a fruitful field of research. Additionally, we focused on the roles of H+/Na+ pumps and m-PPases in cell bioenergetics. Finally, we provide some examples of the applications of these enzymes in molecular biology and biotechnology, especially in plants. This review is valuable for professionals in the biochemistry field of protein structure-function relationships and experts in other fields, such as chemistry, nanotechnology, and plant sciences.

Structural and biochemical characterization of active sites mutant in human inorganic pyrophosphatase

Inorganic pyrophosphatases (PPases) are enzymes that catalyze the conversion of inorganic pyrophosphate (PPi) into phosphate (Pi). Human inorganic pyrophosphatase 1 (Hu-PPase) exhibits high expression levels in a variety of tumors and plays roles in cell proliferation, apoptosis, invasion and metastasis, making it a promising prognostic biomarker and a target for cancer therapy. Despite its widespread presence, the catalytic mechanism of Hu-PPase in humans remains inadequately understood. The signature motif amino acid sequence (DXDPXD) within the active sites of PPases is preserved across different species. In this research, an enzymatic activity assay revealed that mutations led to a notable reduction in enzymatic function, although the impact of the four amino acids on the activity of the pocket varied. To investigate the influence of these residues on the substrate binding and enzymatic function of PPase, the crystal structure of the Hu-PPase-ED quadruple mutant (D116A/D118A/P119A/D121A) was determined at 1.69 Å resolution. The resulting structure maintained a barrel-like shape similar to that of the wild-type, albeit lacking Mg2+ ions. Molecular docking analysis demonstrated a decreased ability of Hu-PPase-ED to bind to PPi. Further, molecular dynamics simulation analysis indicated that the mutation rendered the loop of Mg2+ ion-binding residues less stable. Therefore, the effect on enzyme activity did not result from a change in the gross protein structure but rather from a mutation that abolished the Mg2+-coordinating groups, thereby eliminating Mg2+ binding and leading to the loss of enzyme activity.

Exploration of the Catalytic Cycle Dynamics of Vigna Radiata H+-Translocating Pyrophosphatases Through Hydrogen-Deuterium Exchange Mass Spectrometry

Vigna radiata H+-translocating pyrophosphatases (VrH+-PPases, EC 3.6.1.1) are present in various endomembranes of plants, bacteria, archaea, and certain protozoa. They transport H+ into the lumen by hydrolyzing pyrophosphate, which is a by-product of many essential anabolic reactions. Although the crystal structure of H+-PPases has been elucidated, the H+ translocation mechanism of H+-PPases in the solution state remains unclear. In this study, we used hydrogen-deuterium exchange (HDX) coupled with mass spectrometry (MS) to investigate the dynamics of H+-PPases between the previously proposed R state (resting state, Apo form), I state (intermediate state, bound to a substrate analog), and T state (transient state, bound to inorganic phosphate). When hydrogen was replaced by proteins in deuterium oxide solution, the backbone hydrogen atoms, which were exchanged with deuterium, were identified through MS. Accordingly, we used deuterium uptake to examine the structural dynamics and conformational changes of H+-PPases in solution. In the highly conserved substrate binding and proton exit regions, HDX-MS revealed the existence of a compact conformation with deuterium exchange when H+-PPases were bound with a substrate analog and product. Thus, a novel working model was developed to elucidate the in situ catalytic mechanism of pyrophosphate hydrolysis and proton transport. In this model, a proton is released in the I state, and the TM5 inner wall serves as a proton piston.

A self-sustaining serpentinization mega-engine feeds the fougerite nanoengines implicated in the emergence of guided metabolism

The demonstration by Ivan Barnes et al. that the serpentinization of fresh Alpine-type ultramafic rocks results in the exhalation of hot alkaline fluids is foundational to the submarine alkaline vent theory (AVT) for life's emergence to its 'improbable' thermodynamic state. In AVT, such alkaline fluids ≤ 150°C, bearing H2 > CH4 > HS--generated and driven convectively by a serpentinizing exothermic mega-engine operating in the ultramafic crust-exhale into the iron-rich, CO2> > > NO3--bearing Hadean ocean to result in hydrothermal precipitate mounds comprising macromolecular ferroferric-carbonate oxyhydroxide and minor sulfide. As the nanocrystalline minerals fougerite/green rust and mackinawite (FeS), they compose the spontaneously precipitated inorganic membranes that keep the highly contrasting solutions apart, thereby maintaining redox and pH disequilibria. They do so in the form of fine chimneys and chemical gardens. The same disequilibria drive the reduction of CO2 to HCOO- or CO, and the oxidation of CH4 to a methyl group-the two products reacting to form acetate in a sequence antedating the 'energy-producing' acetyl coenzyme-A pathway. Fougerite is a 2D-layered mineral in which the hydrous interlayers themselves harbor 2D solutions, in effect constricted to ~ 1D by preferentially directed electron hopping/tunneling, and proton Gröthuss 'bucket-brigading' when subject to charge. As a redox-driven nanoengine or peristaltic pump, fougerite forces the ordered reduction of nitrate to ammonium, the amination of pyruvate and oxalate to alanine and glycine, and their condensation to short peptides. In turn, these peptides have the flexibility to sequester the founding inorganic iron oxyhydroxide, sulfide, and pyrophosphate clusters, to produce metal- and phosphate-dosed organic films and cells. As the feed to the hydrothermal mound fails, the only equivalent sustenance on offer to the first autotrophs is the still mildly serpentinizing upper crust beneath. While the conditions here are very much less bountiful, they do offer the similar feed and disequilibria the survivors are accustomed to. Sometime during this transition, a replicating non-ribosomal guidance system is discovered to provide the rules to take on the incrementally changing surroundings. The details of how these replicating apparatuses emerged are the hard problem, but by doing so the progenote archaea and bacteria could begin to colonize what would become the deep biosphere. Indeed, that the anaerobic nitrate-respiring methanotrophic archaea and the deep-branching Acetothermia presently comprise a portion of that microbiome occupying serpentinizing rocks offers circumstantial support for this notion. However, the inescapable, if jarring conclusion is drawn that, absent fougerite/green rust, there would be no structured channelway to life.

May the proton motive force be with you: A plant transporter review

As our ecosystems experience challenges associated with climate change, an improved understanding of the fundamental biochemical processes governing plant physiology is needed. Strikingly, current structural information on plant membrane transporters is severely limited compared to other kingdoms of life, with only 18 unique structures in total. To advance future breakthroughs and insight in plant cell molecular biology, structural knowledge of membrane transporters is indispensable. This review summarizes the current status of structural knowledge in the plant membrane transporter field. Plants utilize the proton motive force (PMF) to drive secondary active transport. We discuss the PMF, how it relates to secondary active transport and provide a classification of PMF driven secondary active transport, discussing recently published structures of symporters, antiporters, and uniporters from plants.

Spark of Life: Role of Electrotrophy in the Emergence of Life

The emergence of life has been a subject of intensive research for decades. Different approaches and different environmental "cradles" have been studied, from space to the deep sea. Since the recent discovery of a natural electrical current through deep-sea hydrothermal vents, a new energy source is considered for the transition from inorganic to organic. This energy source (electron donor) is used by modern microorganisms via a new trophic type, called electrotrophy. In this review, we draw a parallel between this metabolism and a new theory for the emergence of life based on this electrical electron flow. Each step of the creation of life is revised in the new light of this prebiotic electrochemical context, going from the evaluation of similar electrical current during the Hadean, the CO₂ electroreduction into a prebiotic primordial soup, the production of proto-membranes, the energetic system inspired of the nitrate reduction, the proton gradient, and the transition to a planktonic proto-cell. Finally, this theory is compared to the two other theories in hydrothermal context to assess its relevance and overcome the limitations of each. Many critical factors that were limiting each theory can be overcome given the effect of electrochemical reactions and the environmental changes produced.

mPPases create a conserved anionic membrane fingerprint as identified via multi-scale simulations

Membrane-integral pyrophosphatases (mPPases) are membrane-bound enzymes responsible for hydrolysing inorganic pyrophosphate and translocating a cation across the membrane. Their function is essential for the infectivity of clinically relevant protozoan parasites and plant maturation. Recent developments have indicated that their mechanism is more complicated than previously thought and that the membrane environment may be important for their function. In this work, we use multiscale molecular dynamics simulations to demonstrate for the first time that mPPases form specific anionic lipid interactions at 4 sites at the distal and interfacial regions of the protein. These interactions are conserved in simulations of the mPPases from Thermotoga maritima, Vigna radiata and Clostridium leptum and characterised by interactions with positive residues on helices 1, 2, 3 and 4 for the distal site, or 9, 10, 13 and 14 for the interfacial site. Due to the importance of these helices in protein stability and function, these lipid interactions may play a crucial role in the mPPase mechanism and enable future structural and functional studies.

The Mechanism of Energy Coupling in H+/Na+-Pumping Membrane Pyrophosphatase-Possibilities and Probabilities

Membrane pyrophosphatases (mPPases) found in plant vacuoles and some prokaryotes and protists are ancient cation pumps that couple pyrophosphate hydrolysis with the H+ and/or Na+ transport out of the cytoplasm. Because this function is reversible, mPPases play a role in maintaining the level of cytoplasmic pyrophosphate, a known regulator of numerous metabolic reactions. mPPases arouse interest because they are among the simplest membrane transporters and have no homologs among known ion pumps. Detailed phylogenetic studies have revealed various subtypes of mPPases and suggested their roles in the evolution of the "sodium" and "proton" bioenergetics. This treatise focuses on the mechanistic aspects of the transport reaction, namely, the coupling step, the role of the chemically produced proton, subunit cooperation, and the relationship between the proton and sodium ion transport. The available data identify H+-PPases as the first non-oxidoreductase pump with a "direct-coupling" mechanism, i.e., the transported proton is produced in the coupled chemical reaction. They also support a "billiard" hypothesis, which unifies the H+ and Na+ transport mechanisms in mPPase and, probably, other transporters.

A Lumenal Loop Associated with Catalytic Asymmetry in Plant Vacuolar H+-Translocating Pyrophosphatase

Membrane-integral inorganic pyrophosphatases (mPPases) couple pyrophosphate hydrolysis with H⁺ and Na⁺ pumping in plants and microbes. mPPases are homodimeric transporters with two catalytic sites facing the cytoplasm and demonstrating highly different substrate-binding affinities and activities. The structural aspects of the functional asymmetry are still poorly understood because the structure of the physiologically relevant dimer form with only one active site occupied by the substrate is unknown. We addressed this issue by molecular dynamics (MD) simulations of the H⁺-transporting mPPase of Vigna radiata, starting from its crystal structure containing a close substrate analog (imidodiphosphate, IDP) in both active sites. The MD simulations revealed pre-existing subunit asymmetry, which increased upon IDP binding to one subunit and persisted in the fully occupied dimer. The most significant asymmetrical change caused by IDP binding is a ‘rigid body’-like displacement of the lumenal loop connecting α-helices 2 and 3 in the partner subunit and opening its exit channel for water. This highly conserved 14–19-residue loop is found only in plant vacuolar mPPases and may have a regulatory function, such as pH sensing in the vacuole. Our data define the structural link between the loop and active sites and are consistent with the published structural and functional data.

Exploration of Pyrazolo[1,5-a]pyrimidines as Membrane-Bound Pyrophosphatase Inhibitors

Inhibition of membrane-bound pyrophosphatase (mPPase) with small molecules offer a new approach in the fight against pathogenic protozoan parasites. mPPases are absent in humans, but essential for many protists as they couple pyrophosphate hydrolysis to the active transport of protons or sodium ions across acidocalcisomal membranes. So far, only few nonphosphorus inhibitors have been reported. Here, we explore the chemical space around previous hits using a combination of screening and synthetic medicinal chemistry, identifying compounds with low micromolar inhibitory activities in the Thermotoga maritima mPPase test system. We furthermore provide early structure-activity relationships around a new scaffold having a pyrazolo[1,5-a]pyrimidine core. The most promising pyrazolo[1,5-a]pyrimidine congener was further investigated and found to inhibit Plasmodium falciparum mPPase in membranes as well as the growth of P. falciparum in an ex vivo survival assay.

Catalytic Asymmetry in Homodimeric H+-Pumping Membrane Pyrophosphatase Demonstrated by Non-Hydrolyzable Pyrophosphate Analogs

Membrane-bound inorganic pyrophosphatase (mPPase) resembles the F-ATPase in catalyzing polyphosphate-energized H⁺ and Na⁺ transport across lipid membranes, but differs structurally and mechanistically. Homodimeric mPPase likely uses a “direct coupling” mechanism, in which the proton generated from the water nucleophile at the entrance to the ion conductance channel is transported across the membrane or triggers Na⁺ transport. The structural aspects of this mechanism, including subunit cooperation, are still poorly understood. Using a refined enzyme assay, we examined the inhibition of K⁺-dependent H⁺-transporting mPPase from Desulfitobacterium hafniensee by three non-hydrolyzable PP_i analogs (imidodiphosphate and C-substituted bisphosphonates). The kinetic data demonstrated negative cooperativity in inhibitor binding to two active sites, and reduced active site performance when the inhibitor or substrate occupied the other active site. The nonequivalence of active sites in PP_i hydrolysis in terms of the Michaelis constant vanished at a low (0.1 mM) concentration of Mg²⁺ (essential cofactor). The replacement of K⁺, the second metal cofactor, by Na⁺ increased the substrate and inhibitor binding cooperativity. The detergent-solubilized form of mPPase exhibited similar active site nonequivalence in PP_i hydrolysis. Our findings support the notion that the mPPase mechanism combines Mitchell’s direct coupling with conformational coupling to catalyze cation transport across the membrane.

Molecular characterization and transcriptional regulation of two types of H+-pyrophosphatases in the scuticociliate parasite Philasterides dicentrarchi

Proton-translocating inorganic pyrophosphatases (H⁺-PPases) are an ancient family of membrane bound enzymes that couple pyrophosphate (PPi) hydrolysis to H⁺ translocation across membranes. In this study, we conducted a molecular characterization of two isoenzymes (PdVP1 and PdVP2) located in respectively the alveolar sacs and in the membranes of the intracellular vacuoles of a scuticociliate parasite (Philasterides dicentrarchi) of farmed turbot. We analyzed the genetic expression of the isoenzymes after administration of antiparasitic drugs and after infection in the host. PdVP1 and PdVP2 are encoded by two genes of 2485 and 3069 bp, which respectively contain 3 and 11 exons and express proteins of 746 and 810 aa of molecular mass 78.9 and 87.6 kDa. Topological predictions from isoenzyme sequences indicate the formation of thirteen transmembrane regions (TMRs) for PdVP1 and seventeen TMRs for PdVP2. Protein structure modelling indicated that both isoenzymes are homodimeric, with three Mg²⁺ binding sites and an additional K⁺ binding site in PdVP2. The levels of identity and similarity between the isoenzyme sequences are respectively 33.5 and 51.2%. The molecular weights of the native proteins are 158 kDa (PdVP1) and 178 kDa (PdVP2). The isoenzyme sequences are derived from paralogous genes that form a monophyletic grouping with other ciliate species. Genetic expression of the isoenzymes is closely related to the acidification of alveolar sacs (PdVP1) and intracellular vacuoles (PdVP2): antiparasitic drugs inhibit transcription, while infection increases transcription of both isoenzymes. The study findings show that P. dicentrarchi possesses two isoenzymes with H⁺-PPase activity which are located in acidophilic cell compartment membranes and which are activated during infection in the host and are sensitive to antiparasitic drugs. The findings open the way to using molecular modelling to design drugs for the treatment of scuticociliatosis.

Mechanisms of Energy Transduction by Charge Translocating Membrane Proteins

Life relies on the constant exchange of different forms of energy, i.e., on energy transduction. Therefore, organisms have evolved in a way to be able to harvest the energy made available by external sources (such as light or chemical compounds) and convert these into biological useable energy forms, such as the transmembrane difference of electrochemical potential (Δμ̃). Membrane proteins contribute to the establishment of Δμ̃ by coupling exergonic catalytic reactions to the translocation of charges (electrons/ions) across the membrane. Irrespectively of the energy source and consequent type of reaction, all charge-translocating proteins follow two molecular coupling mechanisms: direct- or indirect-coupling, depending on whether the translocated charge is involved in the driving reaction. In this review, we explore these two coupling mechanisms by thoroughly examining the different types of charge-translocating membrane proteins. For each protein, we analyze the respective reaction thermodynamics, electron transfer/catalytic processes, charge-translocating pathways, and ion/substrate stoichiometries.

The H+-Translocating Inorganic Pyrophosphatase From Arabidopsis thaliana Is More Sensitive to Sodium Than Its Na+-Translocating Counterpart From Methanosarcina mazei

Overexpression of membrane-bound K+-dependent H+-translocating inorganic pyrophosphatases (H+-PPases) from higher plants has been widely used to alleviate the sensitivity toward NaCl in these organisms, a strategy that had been previously tested in Saccharomyces cerevisiae. On the other hand, H+-PPases have been reported to functionally complement the yeast cytosolic soluble pyrophosphatase (IPP1). Here, the efficiency of the K+-dependent Na+-PPase from the archaeon Methanosarcina mazei (MVP) to functionally complement IPP1 has been compared to that of its H+-pumping counterpart from Arabidopsis thaliana (AVP1). Both membrane-bound integral PPases (mPPases) supported yeast growth equally well under normal conditions, however, cells expressing MVP grew significantly better than those expressing AVP1 under salt stress. The subcellular distribution of the heterologously-expressed mPPases was crucial in order to observe the phenotypes associated with the complementation. In vitro studies showed that the PPase activity of MVP was less sensitive to Na+ than that of AVP1. Consistently, when yeast cells expressing MVP were grown in the presence of NaCl only a marginal increase in their internal PPi levels was observed with respect to control cells. By contrast, yeast cells that expressed AVP1 had significantly higher levels of this metabolite under the same conditions. The H+-pumping activity of AVP1 was also markedly inhibited by Na+. Our results suggest that mPPases primarily act by hydrolysing the PPi generated in the cytosol when expressed in yeast, and that AVP1 is more susceptible to Na+ inhibition than MVP both in vivo and in vitro. Based on this experimental evidence, we propose Na+-PPases as biotechnological tools to generate salt-tolerant plants.

Discovery of Membrane-Bound Pyrophosphatase Inhibitors Derived from an Isoxazole Fragment

Membrane-bound pyrophosphatases (mPPases) regulate energy homeostasis in pathogenic protozoan parasites and lack human homologues, which makes them promising targets in e.g. malaria. Yet only few nonphosphorus inhibitors have been reported so far. Here, we explore an isoxazole fragment hit, leading to the discovery of small mPPase inhibitors with 6–10 μM IC₅₀ values in the Thermotoga maritima test system. Promisingly, the compounds retained activity against Plasmodium falciparum mPPase in membranes and inhibited parasite growth.

Functional Characterization of SLC Transporters Using Solid Supported Membranes

Here, we present a protocol for the functional characterization of the H⁺-coupled human peptide transporter PepT1 and sufficient notes to transfer the protocol to the Na⁺-coupled sugar transporter SGLT1, the organic cation transporter OCT2, the Na⁺/Ca²⁺ exchanger NCX, and the neuronal glutamate transporter EAAT3.The assay was developed for the commercially available SURFE²R N1 instrument (Nanion Technologies GmbH) which applies solid supported membrane (SSM)-based electrophysiology. This technique is widely used for the functional characterization of membrane transporters with more than 100 different transporters characterized so far. The technique is cost-effective, easy to use, and capable of high-throughput measurements.SSM-based electrophysiology utilizes SSM-coated gold sensors to physically adsorb membrane vesicles containing the protein of interest. A fast solution exchange provides the substrate and activates transport. For the measurement of PepT1 activity, we applied a peptide concentration jump to activate H⁺/peptide symport. Proton influx charges the sensor. A capacitive current is measured reflecting the transport activity of PepT1 . Multiple measurements on the same sensor allow for comparison of transport activity under different conditions. Here, we determine EC₅₀ for PepT1-mediated glycylglycine transport and perform an inhibition experiment using the specific peptide inhibitor Lys[Z(NO₂)]-Val.

On the beneficent thickness of water

In the 1930s, Lars Onsager published his famous 'reciprocal relations' describing free energy conversion processes. Importantly, these relations were derived on the assumption that the fluxes of the processes involved in the conversion were proportional to the forces (free energy gradients) driving them. For chemical reactions, however, this condition holds only for systems operating close to equilibrium-indeed very close; nominally requiring driving forces to be smaller than k B T. Fairly soon thereafter, however, it was quite inexplicably observed that in at least some biological conversions both the reciprocal relations and linear flux-force dependency appeared to be obeyed no matter how far from equilibrium the system was being driven. No successful explanation of how this 'paradoxical' behaviour could occur has emerged and it has remained a mystery. We here argue, however, that this anomalous behaviour is simply a gift of water, of its viscosity in particular; a gift, moreover, without which life almost certainly could not have emerged. And a gift whose appreciation we primarily owe to recent work by Prof. R. Dean Astumian who, as providence has kindly seen to it, was led to the relevant insights by the later work of Onsager himself.

The Function of Membrane Integral Pyrophosphatases From Whole Organism to Single Molecule

Membrane integral pyrophosphatases (mPPases) are responsible for the hydrolysis of pyrophosphate. This enzymatic mechanism is coupled to the pumping of H+ or Na+ across membranes in a process that can be K+ dependent or independent. Understanding the movements and dynamics throughout the mPPase catalytic cycle is important, as this knowledge is essential for improving or impeding protein function. mPPases have been shown to play a crucial role in plant maturation and abiotic stress tolerance, and so have the potential to be engineered to improve plant survival, with implications for global food security. mPPases are also selectively toxic drug targets, which could be pharmacologically modulated to reduce the virulence of common human pathogens. The last few years have seen the publication of many new insights into the function and structure of mPPases. In particular, there is a new body of evidence that the catalytic cycle is more complex than originally proposed. There are structural and functional data supporting a mechanism involving half-of-the-sites reactivity, inter-subunit communication, and exit channel motions. A more advanced and in-depth understanding of mPPases has begun to be uncovered, leaving the field of research with multiple interesting avenues for further exploration and investigation.

Asymmetry in catalysis by Thermotoga maritima membrane-bound pyrophosphatase demonstrated by a nonphosphorus allosteric inhibitor

Membrane-bound pyrophosphatases are homodimeric integral membrane proteins that hydrolyze pyrophosphate into orthophosphates, coupled to the active transport of protons or sodium ions across membranes. They are important in the life cycle of bacteria, archaea, plants, and parasitic protists, but no homologous proteins exist in vertebrates, making them a promising drug target. Here, we report the first nonphosphorus allosteric inhibitor of the thermophilic bacterium Thermotoga maritima membrane-bound pyrophosphatase and its bound structure together with the substrate analog imidodiphosphate. The unit cell contains two protein homodimers, each binding a single inhibitor dimer near the exit channel, creating a hydrophobic clamp that inhibits the movement of β-strand 1-2 during pumping, and thus prevents the hydrophobic gate from opening. This asymmetry of inhibitor binding with respect to each homodimer provides the first clear structural demonstration of asymmetry in the catalytic cycle of membrane-bound pyrophosphatases.

Roles of the Hydrophobic Gate and Exit Channel in Vigna radiata Pyrophosphatase Ion Translocation

Membrane-embedded pyrophosphatase (M-PPase) hydrolyzes pyrophosphate to drive ion (H⁺ and/or Na⁺) translocation. We determined crystal structures and functions of Vigna radiata M-PPase (VrH⁺-PPase), the VrH⁺-PPase-2P_i complex and mutants at hydrophobic gate (residue L555) and exit channel (residues T228 and E225). Ion pore diameters along the translocation pathway of three VrH⁺-PPases complexes (P_i-, 2P_i- and imidodiphosphate-bound states) present a unique wave-like profile, with different pore diameters at the hydrophobic gate and exit channel, indicating that the ligands induced pore size alterations. The 2P_i-bound state with the largest pore diameter might mimic the hydrophobic gate open. In mutant structures, ordered waters detected at the hydrophobic gate among VrH⁺-PPase imply the possibility of solvation, and numerous waters at the exit channel might signify an open channel. A salt-bridge, E225-R562 is at the way out of the exit channel of VrH⁺-PPase; E225A mutant makes the interaction eliminated and reveals a decreased pumping ability. E225-R562 might act as a latch to regulate proton release. A water wire from the ion gate (R-D-K-E) through the hydrophobic gate and into the exit channel may reflect the path of proton transfer.

The flip side of the Arabidopsis type I proton-pumping pyrophosphatase (AVP1): Using a transmembrane H+ gradient to synthesize pyrophosphate

Energy partitioning and plant growth are mediated in part by a type I H⁺-pumping pyrophosphatase (H⁺-PPase). A canonical role for this transporter has been demonstrated at the tonoplast where it serves a job-sharing role with V-ATPase in vacuolar acidification. Here, we investigated whether the plant H⁺-PPase from Arabidopsis also functions in "reverse mode" to synthesize PP_i using the transmembrane H⁺ gradient. Using patch-clamp recordings on Arabidopsis vacuoles, we observed inward currents upon P_i application on the cytosolic side. These currents were strongly reduced in vacuoles from two independent H⁺-PPase mutant lines (vhp1-1 and fugu5-1) lacking the classical PP_i-induced outward currents related to H⁺ pumping, whereas they were significantly larger in vacuoles with engineered heightened expression of the H⁺-PPase. Current amplitudes related to reverse-mode H⁺ transport depended on the membrane potential, cytosolic P_i concentration, and magnitude of the pH gradient across the tonoplast. Of note, experiments on vacuolar membrane-enriched vesicles isolated from yeast expressing the Arabidopsis H⁺-PPase (AVP1) demonstrated P_i-dependent PP_i synthase activity in the presence of a pH gradient. Our work establishes that a plant H⁺-PPase can operate as a PP_i synthase beyond its canonical role in vacuolar acidification and cytosolic PP_i scavenging. We propose that the PP_i synthase activity of H⁺-PPase contributes to a cascade of events that energize plant growth.

Tuning Vector Stability and Integration Frequency Elevates Functional GPCR Production and Homogeneity in Saccharomyces cerevisiae

Membrane proteins play a valuable role in biotechnology, yet the difficulty of producing high yields of functional membrane protein limits their use in synthetic biology. The practical application of G protein-coupled receptors in whole cell biosensors, for example, is restricted to those that are functionally produced at the cell surface in the chosen host, limiting the range of detectable molecules. Here, we present a facile approach to significantly improve the yield and homogeneity of functional membrane proteins in Saccharomyces cerevisiae by altering only the choice of expression vector. Expression of a model GPCR, the human adenosine A₂a receptor, from commonly used centromeric and episomal vectors leads to low yields and cellular heterogeneity due to plasmid loss in 20-90% of the cell population. In contrast, homogeneous production of GPCR is attained using a multisite integrating vector or a novel, modified high copy vector that does not require genomic integration or addition of any selection agents. Finally, we introduce a FACS-based screen, which enables rapid isolation of cells with 4- to 15-fold increases in gene dosage and up to a 9-fold increase in functional protein yield without loss of homogeneity compared to a strain isolated through conventional, low-throughput methods. These results can be extended to improve the cellular homogeneity and yield of other membrane proteins, expanding the repertoire of useful receptors for synthetic biology applications.

Role of the potassium/lysine cationic center in catalysis and functional asymmetry in membrane-bound pyrophosphatases

Membrane-bound pyrophosphatases (mPPases), which couple pyrophosphate hydrolysis to transmembrane transport of H⁺ and/or Na⁺ ions, are divided into K⁺,Na⁺-independent, Na⁺-regulated, and K⁺-dependent families. The first two families include H⁺-transporting mPPases (H⁺-PPases), whereas the last family comprises one Na⁺-transporting, two Na⁺- and H⁺-transporting subfamilies (Na⁺-PPases and Na⁺,H⁺-PPases, respectively), and three H⁺-transporting subfamilies. Earlier studies of the few available model mPPases suggested that K⁺ binds to a site located adjacent to the pyrophosphate-binding site, but is substituted by the ε-amino group of an evolutionarily acquired lysine residue in the K⁺-independent mPPases. Here, we performed a systematic analysis of the K⁺/Lys cationic center across all mPPase subfamilies. An Ala → Lys replacement in K⁺-dependent mPPases abolished the K⁺ dependence of hydrolysis and transport activities and decreased these activities close to the level (4-7%) observed for wild-type enzymes in the absence of monovalent cations. In contrast, a Lys → Ala replacement in K⁺,Na⁺-independent mPPases conferred partial K⁺ dependence on the enzyme by unmasking an otherwise conserved K⁺-binding site. Na⁺ could partially replace K⁺ as an activator of K⁺-dependent mPPases and the Lys → Ala variants of K⁺,Na⁺-independent mPPases. Finally, we found that all mPPases were inhibited by excess substrate, suggesting strong negative co-operativity of active site functioning in these homodimeric enzymes; moreover, the K⁺/Lys center was identified as part of the mechanism underlying this effect. These findings suggest that the mPPase homodimer possesses an asymmetry of active site performance that may be an ancient prototype of the rotational binding-change mechanism of F-type ATPases.

Defining Dynamics of Membrane-Bound Pyrophosphatases by Experimental and Computational Single-Molecule FRET

Membrane-bound pyrophosphatases couple the hydrolysis of inorganic pyrophosphate to the pumping of ions (sodium or protons) across a membrane in order to generate an electrochemical gradient. This class of membrane protein is widely conserved across plants, fungi, archaea, and bacteria, but absent in multicellular animals, making them a viable target for drug design against protozoan parasites such as Plasmodium falciparum. An excellent understanding of many of the catalytic states throughout the enzymatic cycle has already been afforded by crystallography. However, the dynamics and kinetics of the catalytic cycle between these static snapshots remain to be elucidated. Here, we employ single-molecule Förster resonance energy transfer (FRET) measurements to determine the dynamic range and frequency of conformations available to the enzyme in a lipid bilayer during the catalytic cycle. First, we explore issues related to the introduction of fluorescent dyes by cysteine mutagenesis; we discuss the importance of residue selection for dye attachment, and the balance between mutating areas of the protein that will provide useful dynamics while not altering highly conserved residues that could disrupt protein function. To complement and guide the experiments, we used all-atom molecular dynamics simulations and computational methods to estimate FRET efficiency distributions for dye pairs at different sites in different protein conformational states. We present preliminary single-molecule FRET data that points to insights about the binding modes of different membrane-bound pyrophosphatase substrates and inhibitors.

Insights into the mechanism of membrane pyrophosphatases by combining experiment and computer simulation

Membrane-integral pyrophosphatases (mPPases) couple the hydrolysis of pyrophosphate (PPi) to the pumping of Na+, H+, or both these ions across a membrane. Recently solved structures of the Na+-pumping Thermotoga maritima mPPase (TmPPase) and H+-pumping Vigna radiata mPPase revealed the basis of ion selectivity between these enzymes and provided evidence for the mechanisms of substrate hydrolysis and ion-pumping. Our atomistic molecular dynamics (MD) simulations of TmPPase demonstrate that loop 5-6 is mobile in the absence of the substrate or substrate-analogue bound to the active site, explaining the lack of electron density for this loop in resting state structures. Furthermore, creating an apo model of TmPPase by removing ligands from the TmPPase:IDP:Na structure in MD simulations resulted in increased dynamics in loop 5-6, which results in this loop moving to uncover the active site, suggesting that interactions between loop 5-6 and the imidodiphosphate and its associated Mg2+ are important for holding a loop-closed conformation. We also provide further evidence for the transport-before-hydrolysis mechanism by showing that the non-hydrolyzable substrate analogue, methylene diphosphonate, induces low levels of proton pumping by VrPPase.