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Cells can generate and conserve metabolic energy with complex pathways, like oxidative phosphorylation, or simpler pathways, like deamination of amino acids, oxidation of carboxylic acids or using light. This thesis focusses on deamination of arginine to create adenosine triphosphate (ATP) inside synthetic vesicles, which is carried out by the arginine deiminase pathway in cells. This pathway consists of three cytosolic proteins (arginine deiminase, ornithine transcarbamoylase and carbamate kinase) and one arginine/ornithine antiporter.First, these proteins were purified from Lactococcus lactis and characterized, to determine their kinetic constants and the effect of varying conditions on their activity. The enzymes perform their reactions as expected and work relatively well in the tested conditions. The antiporter is relatively slow and possibly the rate-determining reaction of the pathway.Next, the pathway was reconstituted in synthetic vesicles, shifting the focus from the individual proteins to measuring properties of the entire pathway. These properties were measured by studying internal pH, metabolic energy levels and concentrations of arginine, ornithine and citrulline. Without an ATP-consuming process, arginine is found to be hydrolyzed into citrulline, which is a wasteful side reaction that hinders the conservation of metabolic energy. Finally, the ATP-driven transporter OpuA was added to the vesicles, which imports the compatible solute glycine betaine for volume regulation in osmotically stressed vesicles. The addition of OpuA does not only help as an ATP-consuming process, but the presence of glycine betaine protects the proteins from osmotic stress. This leads to higher metabolic energy levels and thus metabolic energy conservation.

In cells, the breakdown of arginine to ornithine and ammonium ion plus carbon dioxide is coupled to the generation of metabolic energy in the form of ATP. The arginine breakdown pathway is minimally composed of arginine deiminase, ornithine transcarbamoylase, carbamate kinase, and an arginine/ornithine antiporter; ammonia and carbon dioxide most likely diffuse passively across the membrane. The genes for the enzymes and transporter have been cloned and expressed, and the proteins have been purified from Lactococcus lactis IL1403 and incorporated into lipid vesicles for sustained production of ATP. Here, we study the kinetic parameters and biochemical properties of the individual enzymes and the antiporter, and we determine how the physicochemical conditions, effector composition, and effector concentration affect the enzymes. We report the K M and V MAX values for catalysis and the native oligomeric state of all proteins, and we measured the effect of pathway intermediates, pH, temperature, freeze–thaw cycles, and salts on the activity of the cytosolic enzymes. We also present data on the protein‐to‐lipid ratio and lipid composition dependence of the antiporter., In cells, the breakdown of arginine to ornithine and ammonium ion plus carbon dioxide is coupled to the generation of ATP. The proteins involved in arginine breakdown and ATP production include the following: arginine deiminase, ornithine transcarbamoylase, carbamate kinase, and an arginine/ornithine antiporter. We report the kinetic parameters of all the reactions, the native oligomeric state of the proteins, and the effect of pathway intermediates, pH, temperature, freeze–thaw cycles, salts, and lipids on the enzymes.

One of the grand challenges in chemistry is the construction of functional out-of-equilibrium networks, which are typical of living cells. Building such a system from molecular components requires control over the formation and degradation of the interacting chemicals and homeostasis of the internal physical-chemical conditions. The provision and consumption of ATP lies at the heart of this challenge. Here we report the in vitro construction of a pathway in vesicles for sustained ATP production that is maintained away from equilibrium by control of energy dissipation. We maintain a constant level of ATP with varying load on the system. The pathway enables us to control the transmembrane fluxes of osmolytes and to demonstrate basic physicochemical homeostasis. Our work demonstrates metabolic energy conservation and cell volume regulatory mechanisms in a cell-like system at a level of complexity minimally needed for life., Functional out-of-equilibrium networks are typical of living cells. Here the authors report the construction of a sustained ATP production system in vesicles with controlled energy dissipation and physicochemical homeostasis.

We are aiming for a blue print for synthesizing (moderately complex) subcellular systems from molecular components and ultimately for constructing life. However, without comprehensive instructions and design principles, we rely on simple reaction routes to operate the essential functions of life. The first forms of synthetic life will not make every building block for polymers de novo according to complex pathways, rather they will be fed with amino acids, fatty acids and nucleotides. Controlled energy supply is crucial for any synthetic cell, no matter how complex. Herein, we describe the simplest pathways for the efficient generation of ATP and electrochemical ion gradients. We have estimated the demand for ATP by polymer synthesis and maintenance processes in small cell-like systems, and we describe circuits to control the need for ATP. We also present fluorescence-based sensors for pH, ionic strength, excluded volume, ATP/ADP, and viscosity, which allow the major physicochemical conditions inside cells to be monitored and tuned.

Niosomes are used in studies for drug delivery or gene transfer. However, their physical properties and features relative to liposomes are not well documented. To characterize and more rationally optimize niosome formulations, the properties of these vesicle systems are compared to those of liposomes composed of phosphatidylcholine and phosphatidylethanolamine lipids plus cholesterol. Niosomes are highly stable and only slightly more leaky than liposomes as assayed by calcein leakage; the permeability for ions (KCl) is higher than that of liposomes. Contrary to liposomes, the size of niosomes decreases substantially upon freezing in liquid nitrogen and subsequent thawing, as shown by cryo-EM and dynamic light scattering. The packing of niosomal membranes was determined by laurdan fluorescence and is slightly lower than that of liposomes. We did not succeed in the functional reconstitution of the L-arginine/L-ornithine antiporter ArcD2 in niosomes, which we attribute to the non-ionic nature of the surfactants. The antimicrobial peptides alamethicin and melittin act similarly on niosomes and liposomes composed of unsaturated components, whereas both niosomes and liposomes are unaffected when saturated amphiphiles are used. In conclusion, in terms of stability and permeability for drug-size molecules niosomes are comparable to liposomes and they may offer an excellent, inexpensive alternative for delivery purposes.

The import of basic amino acids in Saccharomyces cerevisiae has been reported to be unidirectional, which is not typical of how secondary transporters work. Since studies of energy coupling and transport kinetics are complicated in vivo, we purified the major lysine transporter (Lyp1) of yeast and reconstituted the protein into lipid vesicles. We show that the Michaelis constant (KM) of transport from out-to-in is well in the millimolar range and at least 3 to 4-orders of magnitude higher than that of transport in the opposite direction, disfavoring the efflux of solute via Lyp1. We also find that at low values of the proton motive force, the transport by Lyp1 is comparatively slow. We benchmarked the properties of eukaryotic Lyp1 to that of the prokaryotic homologue LysP and find that LysP has a similar KM for transport from in-to-out and out-to-in, consistent with rapid influx and efflux. We thus explain the previously described unidirectional nature of lysine transport in S. cerevisiae by the extraordinary kinetics of Lyp1 and provide a mechanism and rationale for previous observations. The high asymmetry in transport together with secondary storage in the vacuole allow the cell to accumulate basic amino acids to very high levels.