ATP synthesis by purified ATP-synthase from beef heart mitochondria after coreconstitution with bacteriorhodopsin

Artificial organelles for sustainable chemical energy conversion and production in artificial cells: Artificial mitochondrion and chloroplasts

Sustainable energy conversion modules are the main challenges for building complex reaction cascades in artificial cells. Recent advances in biotechnology have enabled this sustainable energy supply, especially the adenosine triphosphate (ATP), by mimicking the organelles, which are the core structures for energy conversion in living cells. Three components are mainly shared by the artificial organelles: the membrane compartment separating the inner and outer parts, membrane proteins for proton translocation, and the molecular rotary machine for ATP synthesis. Depending on the initiation factors, they are further categorized into artificial mitochondrion and artificial chloroplasts, which use chemical nutrients for oxidative phosphorylation and light for photosynthesis, respectively. In this review, we summarize the essential components needed for artificial organelles and then review the recent progress on two different artificial organelles. Recent strategies, purified and identified proteins, and working principles are discussed. With more study on the artificial mitochondrion and artificial chloroplasts, they are expected to be very powerful tools, allowing us to achieve complex cascading reactions in artificial cells, like the ones that happen in real cells.

Modularize and Unite: Toward Creating a Functional Artificial Cell

An artificial cell is a simplified model of a living system, bringing breakthroughs into both basic life science and applied research. The bottom-up strategy instructs the construction of an artificial cell from nonliving materials, which could be complicated and interdisciplinary considering the inherent complexity of living cells. Although significant progress has been achieved in the past 2 decades, the area is still facing some problems, such as poor compatibility with complex bio-systems, instability, and low standardization of the construction method. In this review, we propose creating artificial cells through the integration of different functional modules. Furthermore, we divide the function requirements of an artificial cell into four essential parts (metabolism, energy supplement, proliferation, and communication) and discuss the present researches. Then we propose that the compartment and the reestablishment of the communication system would be essential for the reasonable integration of functional modules. Although enormous challenges remain, the modular construction would facilitate the simplification and standardization of an artificial cell toward a natural living system. This function-based strategy would also broaden the application of artificial cells and represent the steps of imitating and surpassing nature.

Light-Powered Reactivation of Flagella and Contraction of Microtubule Networks: Toward Building an Artificial Cell

Artificial systems capable of self-sustained movement with self-sufficient energy are of high interest with respect to the development of many challenging applications, including medical treatments, but also technical applications. The bottom-up assembly of such systems in the context of synthetic biology is still a challenging task. In this work, we demonstrate the biocompatibility and efficiency of an artificial light-driven energy module and a motility functional unit by integrating light-switchable photosynthetic vesicles with demembranated flagella. The flagellar propulsion is coupled to the beating frequency, and dynamic ATP synthesis in response to illumination allows us to control beating frequency of flagella in a light-dependent manner. In addition, we verified the functionality of light-powered synthetic vesicles in in vitro motility assays by encapsulating microtubules assembled with force-generating kinesin-1 motors and the energy module to investigate the dynamics of a contractile filamentous network in cell-like compartments by optical stimulation. Integration of this photosynthetic system with various biological building blocks such as cytoskeletal filaments and molecular motors may contribute to the bottom-up synthesis of artificial cells that are able to undergo motor-driven morphological deformations and exhibit directional motion in a light-controllable fashion.

Interface Engineering in Multiphase Systems toward Synthetic Cells and Organelles: From Soft Matter Fundamentals to Biomedical Applications

Synthetic cells have a major role in gaining insight into the complex biological processes of living cells; they also give rise to a range of emerging applications from gene delivery to enzymatic nanoreactors. Living cells rely on compartmentalization to orchestrate reaction networks for specialized and coordinated functions. Principally, the compartmentalization has been an essential engineering theme in constructing cell-mimicking systems. Here, efforts to engineer liquid-liquid interfaces of multiphase systems into membrane-bounded and membraneless compartments, which include lipid vesicles, polymer vesicles, colloidosomes, hybrids, and coacervate droplets, are summarized. Examples are provided of how these compartments are designed to imitate biological behaviors or machinery, including molecule trafficking, growth, fusion, energy conversion, intercellular communication, and adaptivity. Subsequently, the state-of-art applications of these cell-inspired synthetic compartments are discussed. Apart from being simplified and cell models for bridging the gap between nonliving matter and cellular life, synthetic compartments also are utilized as intracellular delivery vehicles for nuclei acids and nanoreactors for biochemical synthesis. Finally, key challenges and future directions for achieving the full potential of synthetic cells are highlighted.

Multistimuli Sensing Adhesion Unit for the Self-Positioning of Minimal Synthetic Cells

Cells have the ability to sense different environmental signals and position themselves accordingly in order to support their survival. Introducing analogous capabilities to the bottom-up assembled minimal synthetic cells is an important step for their autonomy. Here, a minimal synthetic cell which combines a multistimuli sensitive adhesion unit with an energy conversion module is reported, such that it can adhere to places that have the right environmental parameters for ATP production. The multistimuli sensitive adhesion unit senses light, pH, oxidative stress, and the presence of metal ions and can regulate the adhesion of synthetic cells to substrates in response to these stimuli following a chemically coded logic. The adhesion unit is composed of the light and redox responsive protein interaction of iLID and Nano and the pH sensitive and metal ion mediated binding of protein His-tags to Ni²⁺ -NTA complexes. Integration of the adhesion unit with a light to ATP conversion module into one synthetic cell allows it to adhere to places under blue light illumination, non-oxidative conditions, at neutral pH and in the presence of metal ions, which are the right conditions to synthesize ATP. Thus, the multistimuli responsive adhesion unit allows synthetic cells to self-position and execute their functions.

Light-Driven ATP Regeneration in Diblock/Grafted Hybrid Vesicles

Light-driven ATP regeneration systems combining ATP synthase and bacteriorhodopsin have been proposed as an energy supply in the field of synthetic biology. Energy is required to power biochemical reactions within artificially created reaction compartments like protocells, which are typically based on either lipid or polymer membranes. The insertion of membrane proteins into different hybrid membranes is delicate, and studies comparing these systems with liposomes are needed. Here we present a detailed study of membrane protein functionality in different hybrid compartments made of graft polymer PDMS-g-PEO and diblock copolymer PBd-PEO. Activity of more than 90 % in lipid/polymer-based hybrid vesicles could prove an excellent biocompatibility. A significant enhancement of long-term stability (80 % remaining activity after 42 days) could be demonstrated in polymer/polymer-based hybrids.

The Main (Glyco) Phospholipid (MPL) of Thermoplasma acidophilum

The main phospholipid (MPL) of Thermoplasma acidophilum DSM 1728 was isolated, purified and physico-chemically characterized by differential scanning calorimetry (DSC)/differential thermal analysis (DTA) for its thermotropic behavior, alone and in mixtures with other lipids, cholesterol, hydrophobic peptides and pore-forming ionophores. Model membranes from MPL were investigated; black lipid membrane, Langmuir-Blodgett monolayer, and liposomes. Laboratory results were compared to computer simulation. MPL forms stable and resistant liposomes with highly proton-impermeable membrane and mixes at certain degree with common bilayer-forming lipids. Monomeric bacteriorhodopsin and ATP synthase from Micrococcus luteus were co-reconstituted and light-driven ATP synthesis measured. This review reports about almost four decades of research on Thermoplasma membrane and its MPL as well as transfer of this research to Thermoplasma species recently isolated from Indonesian volcanoes.

Artificial photosynthetic cell producing energy for protein synthesis

Attempts to construct an artificial cell have widened our understanding of living organisms. Many intracellular systems have been reconstructed by assembling molecules, however the mechanism to synthesize its own constituents by self-sufficient energy has to the best of our knowledge not been developed. Here, we combine a cell-free protein synthesis system and small proteoliposomes, which consist of purified ATP synthase and bacteriorhodopsin, inside a giant unilamellar vesicle to synthesize protein by the production of ATP by light. The photo-synthesized ATP is consumed as a substrate for transcription and as an energy for translation, eventually driving the synthesis of bacteriorhodopsin or constituent proteins of ATP synthase, the original essential components of the proteoliposome. The de novo photosynthesized bacteriorhodopsin and the parts of ATP synthase integrate into the artificial photosynthetic organelle and enhance its ATP photosynthetic activity through the positive feedback of the products. Our artificial photosynthetic cell system paves the way to construct an energetically independent artificial cell.

Artificial Organelles for Energy Regeneration

One of the critical steps in sustaining life-mimicking processes in synthetic cells is energy, i.e., adenosine triphosphate (ATP) regeneration. Previous studies have shown that the simple addition of ATP or ATP regeneration systems, which do not regenerate ATP directly from ADP and P_i , have no or only limited success due to accumulation of ATP hydrolysis products. In general, ATP regeneration can be achieved by converting light or chemical energy into ATP, which may also involve redox transformations of cofactors. The present contribution provides an overview of the existing ATP regeneration strategies and the related nicotinamide adenine dinucleotide (NAD⁺ ) redox cycling, with a focus on compartmentalized systems. Special attention is being paid to those approaches where so-called artificial organelles are developed. They comprise a semipermeable membrane functionalized by biological or man-made components and employ external energy in the form of light or nutrients in order to generate a transmembrane proton gradient, which is further utilized for ATP synthesis.

Proton transport coupled ATP synthesis by the purified yeast H+ -ATP synthase in proteoliposomes

The H(+)/ATP synthase from yeast mitochondria, MF₀F₁, was purified and reconstituted into liposomes prepared from phosphatidylcholine and phosphatidic acid. Analysis by mass spectrometry revealed the presence of all subunits of the yeast enzyme with the exception of the K-subunit. The MF₀F₁ liposomes were energized by acid-base transitions (DeltapH) and a K(+)/valinomycin diffusion potential (Deltaphi). ATP synthesis was completely abolished by the addition of uncouplers as well as by the inhibitor oligomycin. The rate of ATP synthesis was optimized as a function of various parameters and reached a maximum value (turnover number) of 120s⁻¹ at a transmembrane pH difference of 3.2 units (at pH(in)=4.8 and pH(out)=8.0) and a Deltaphi of 133mV (Nernst potential). Functional studies showed that the monomeric MF₀F₁, was fully active in ATP synthesis. The turnover increased in a sigmoidal way with increasing internal and decreasing external proton concentration. The dependence of the turnover on the phosphate concentration and the dependence of K(M) on pH(out) indicated that the substrate for ATP synthesis is the monoanionic phosphate species H₂PO⁻₄.

ADP- and oligomycin-sensitive redox behavior of F0 b thiol in ATPsynthase depends on neighbored primary structure: investigations using 14-C-labeled alpha lipoic acid

Purified ATPsynthase of bovine heart mitochondria has been analyzed for its mobility and reactivity of oligomycin-sensitive sulfhydryl regions in presence of the substrate ADP and oligomycin. Labeling of thiol groups at the hydrophobic F_0 region of the ATPsynthase was increased in the enzyme initially treated with SDS, N-ethylmaleimide and dithiothreitol (modified enzyme). After dialysis or gel permeation the ATPsynthase was treated with [14C] alpha lipoic acid at a molar ratio of 35-85/1 (lipoic acid/ATPsynthase) corresponding to 4-8.6 nmol/mg protein. Under these conditions, ATPase activity of the native enzyme was significantly decreased. After preincubation with ADP, PAGE of the native, [14C] labeled enzyme revealed an increase of radioactivity at a region of 25 kDa deduced to Cys 197 of subunit b. In the modified enzyme the increase in radioactivity was found at 10 kDa. In this context, the sequence Lys-Cys-Ile around Cys 197 of subunit b suggests excessive reactivity of this thiol, as well as ready reversibility by -SH-S-S- interchange. Therefore, previously observed reaction by thiol reagents and antioxidants from outside the mitochondrion can be interpreted with Cys 197 of F0 b. It accounts for sulfhydryl unmasked by binding of ADP at F1.

Improved purification for thermophilic F1F0 ATP synthase using n-dodecyl beta-D-maltoside

Here we report a fast, simple purification for thermophilic F1F0 ATP synthase (TF1F0) that utilizes a cocktail of stabilizing reagents and the detergent n-dodecyl beta-D-maltoside to yield enzyme with an ATPase activity of 41 micromol/min/mg, 2.5-fold higher than that previously reported. ATPase activity was 80% inhibited by the F0-reactive reagent dicyclohexylcarbodiimide, indicating that F1-F0 interactions were largely intact. To measure ATP-driven proton pumping activity, purified TF1F0 was incorporated into liposomes, and the ATP-induced change in internal pH was measured using the fluorescent probe pyranine. In the presence of valinomycin, a maximum ATP-driven deltapH of 0.8 units was obtained. To measure ATP synthesis activity, TF1F0 was incorporated into liposomes with the light-dependent proton pump bacteriorhodopsin. Proteoliposomes were illuminated to generate an electrochemical gradient, after which ADP and inorganic phosphate were added to initiate ATP synthesis. A steady state ATP synthesis activity of 490 nmol/min/mg was achieved after an initial approximately 30-min lag phase.

Reconstitution of beef heart mitochondrial F0F1 in reverse phase evaporation vesicles

Beef heart mitochondrial F0F1 was reconstituted in proteoliposomes by a new procedure. MF0F1 was inserted in preformed reverse phase evaporation vesicles of large diameters prepared from asolectin (MF0F1-REV). Reconstitution was mediated by Triton X-100, which was subsequently removed by treatment with Bio-Beads. Parameters which resulted in optimal reconstitution were described. The MF0F1-REV proteoliposomes catalyzed an exchange between Pi and ATP and were capable of proton pumping. Both reactions were inhibited by oligomycin and uncoupler of oxidative phosphorylation. The range of Pi-ATP exchange activity of the proteoliposomes (70-110 nmol min[-1] mg[-1]) compared favorably with activities obtained in vesicles reconstituted by cholate dialysis or cholate dilution. The most important aspect of this method is that, unlike other reconstitution methods, exogenous F1 and other coupling factors are not required to obtain high Pi-ATP exchange activity by MF0F1-REV. This simple and rapid reconstitution procedure should be useful for future studies dealing with functional analysis of MF0F1.