Kinetics of ATP synthesis catalyzed by the H(+)-ATPase from chloroplasts (CF0F1) reconstituted into liposomes and coreconstituted with bacteriorhodopsin

Towards Synthetic Cells with Self-Producing Energy

Autonomous generation of energy, specifically adenosine triphosphate (ATP), is critical for sustaining the engineered functionalities of synthetic cells constructed from the bottom-up. In this mini-review, we categorize studies on ATP-producing synthetic cells into three different approaches: photosynthetic mechanisms, mitochondrial respiration mimicry, and utilization of non-conventional approaches such as exploiting synthetic metabolic pathways. Within this framework, we evaluate the strengths and limitations of each approach and provide directions for future research endeavors. We also introduce a concept of building ATP-generating synthetic organelle that will enable us to mimic cellular respiration in a simpler way than current strategies. This review aims to highlight the importance of energy self-production in synthetic cells, providing suggestions and ideas that may help overcome some longstanding challenges in this field.

Light energy transduction in liposome-based artificial cells

In this work we review the latest strategies for the bottom-up assembly of energetically autonomous artificial cells capable of transducing light energy into chemical energy and support internalized metabolic pathways. Such entities are built by taking inspiration from the photosynthetic machineries found in nature which are purified and reconstituted directly in the membrane of artificial compartments or encapsulated in form of organelle-like structures. Specifically, we report and discuss recent examples based on liposome-technology and multi-compartment (nested) architectures pointing out the importance of this matter for the artificial cell synthesis research field and some limitations and perspectives of the bottom-up approach.

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.

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.

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.

Catalytic site occupancy during ATP synthase catalysis

An early proposal was that for rapid ATP synthesis by the rotational ATP synthase, a specific second site must bind ADP and P(i), and for rapid ATP hydrolysis a different second site must bind ATP. Such bi-site activation was considered to occur whether or not an ADP or ATP was at a third site. In contrast, a more recent proposal is that rapid ATP hydrolysis requires that all three sites have bound ADP or ATP present. However, discovery that one second site binds ADP better than ATP, together with other data and considerations support the earlier proposal. The retention or rebinding of ADP can explain why three sites fill during hydrolysis as ATP concentration is increased although bi-site activation still prevails.

Light-driven production of ATP catalysed by F0F1-ATP synthase in an artificial photosynthetic membrane

Energy-transducing membranes of living organisms couple spontaneous to non-spontaneous processes through the intermediacy of protonmotive force (p.m.f.)--an imbalance in electrochemical potential of protons across the membrane. In most organisms, p.m.f. is generated by redox reactions that are either photochemically driven, such as those in photosynthetic reaction centres, or intrinsically spontaneous, such as those of oxidative phosphorylation in mitochondria. Transmembrane proteins (such as the cytochromes and complexes I, III and IV in the electron-transport chain in the inner mitochondrial membrane) couple the redox reactions to proton translocation, thereby conserving a fraction of the redox chemical potential as p.m.f. Many transducer proteins couple p.m.f. to the performance of biochemical work, such as biochemical synthesis and mechanical and transport processes. Recently, an artificial photosynthetic membrane was reported in which a photocyclic process was used to transport protons across a liposomal membrane, resulting in acidification of the liposome's internal volume. If significant p.m.f. is generated in this system, then incorporating an appropriate transducer into the liposomal bilayer should make it possible to drive a non-spontaneous chemical process. Here we report the incorporation of F0F1-ATP synthase into liposomes containing the components of the proton-pumping photocycle. Irradiation of this artificial membrane with visible light results in the uncoupler- and inhibitor-sensitive synthesis of adenosine triphosphate (ATP) against an ATP chemical potential of approximately 12 kcal mol(-1), with a quantum yield of more than 7%. This system mimics the process by which photosynthetic bacteria convert light energy into ATP chemical potential.

Turnover number of Escherichia coli F0F1 ATP synthase for ATP synthesis in membrane vesicles

The rate of ATP synthesized by the ATP synthase (F0F1-ATPase) is limited by the rate of energy production via the respiratory chain, when measured in everted membrane vesicles of an Escherichia coli atp wild-type strain. After energization of the membranes with NADH, fractional inactivation of F0F1 by the covalent inhibitor N,N'-dicyclohexylcarbodiimide allowed the rate of ATP synthesis/mol remaining active ATP synthase complexes to increase; the active ATP synthase complexes were calculated using ATP hydrolysis rates as the defining parameter. In addition, variation of the assay temperature revealed an increase of the ATP synthesis rate up to a temperature of 37 degrees C, the optimal growth temperature of E. coli. In parallel, the amount of F0F1 complexes present in membrane vesicles was determined by immunoquantitation to be 3.3 +/- 0.3% of the membrane protein for cells grown in rich medium and 6.6 +/- 0.3% for cells grown in minimal medium with glycerol as sole carbon and energy source. Based on these data, a turnover number for ATP synthesis of 270 +/- 40 s(-1) could be determined in the presence of 5% active F0F1 complexes. Therefore, these studies demonstrate that the ATP synthase complex of E. coli has, with respect to maximum rates, the same capacity as the corresponding enzymes of eukaryotic organells.

A monitoring system for energy transduction by bacteriorhodopsin liposomes

A computer-controlled system and custom software are described that collect information and perform computations to quantify important parameters of energy transduction during the conversion of photons into a proton electrochemical gradient (delta mu H+) by bacteriorhodopsin (BR)-liposomes. The strong actinic light used to energize the BR-liposomes causes several serious problems for the approaches commonly used to measure these parameters. This paper identifies these problems and presents solutions that permit the acquisition of the desired information, namely, the initial (1st sec) rate and total extent of H+ translocation, rate of H+ leakage (driven by an existing delta mu H+), external, internal and delta pH values, and delta psi values. The system is presented with representative experimental data.

Blocking one non-catalytic ADP binding site results in complete inhibition of the F-type ATPase from the thermophilic Bacillus PS3

The F-type ATPase, TF0F1, from the thermophilic Bacillus PS3, which is free of nucleotides after isolation, was specifically loaded with one 2-azido ADP on a non-catalytic site. The enzyme was covalently modified to various extents and the rate of ATP synthesis and ATP hydrolysis was measured. Both ATP synthesis and ATP hydrolysis extrapolated to zero for covalently binding one nucleotide per enzyme. This was interpreted such that the non-catalytic sites are involved in the coupled catalytic process.

ATP synthesis by the F0F1-ATPase from the thermophilic Bacillus PS3 co-reconstituted with bacteriorhodopsin into liposomes. Evidence for stimulation of ATP synthesis by ATP bound to a noncatalytic binding site

F-type ATPase from the thermophilic Bacillus PS3, TF0F1, which was essentially free of bound nucleotides after isolation and purification, was co-reconstituted into liposomes with the light-driven proton pump bacteriorhodopsin. The time course of the light-induced ATP synthesis was biphasic; an initial slow phase accelerated to a final steady-state rate two to three times faster. Adding ATP before initiating the reaction suppressed the slow phase, suggesting that the state of occupancy of specific sites by ATP regulated the synthetic activity of TF0F1. Incubating the purified TF0F1 with ADP and ATP revealed one ADP and two ATP binding sites that were stable to gel filtration. We analyzed the time courses of light-induced ATP synthesis for the enzyme with different nucleotide content, after co-reconstitution into liposomes with bacteriorhodopsin. The two ATP sites were identified to have regulatory function. A complex containing TF0F1.ADP, 1:1, was co-reconstituted with various quantities of ATP to obtain a range of molar ratios of TF0F1.ADP:ATP of between 1:0 and 1:1.7. It was found that the initial rate of ATP synthesis increased with the level of ATP bound to the enzyme. After binding one ATP, a stimulation of ATP synthesis by a factor of 2 was observed. The second ATP site also exhibited regulatory properties. It stimulated ATP synthesis but to a much smaller extent; the stimulation did not exceed 20%. Binding of the photoreactive analogues 2-azido-[alpha-32P]ADP and 2-azido-[alpha-32P]ATP to the TF0F1 and their effects on the rate of ATP synthesis are described further.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of phospholipids with proteins and peptides. New advances IV

1. The review deals with the newest achievements in the field of the various interactions between phospholipids and proteins and peptides. 2. Interactions are classified according to the hydrophobic, hydrophilic or mixed character of the interactive forces. 3. The effect of the interaction on the structure and biological activity of the interacting molecular assemblies is also discussed.

Co-reconstitution of plasma membrane H(+)-ATPase from yeast and bacteriorhodopsin into liposomes. ATP hydrolysis as a function of external and internal pH

The H(+)-ATPase from the plasma membrane of Saccharomyces cerevisiae was isolated and purified. The enzyme was reconstituted with bacteriorhodopsin into asolectin liposomes by detergent dialysis at a molar ratio of 1 H(+)-ATPase to 50 bacteriorhodopsins. The overall orientation of the proteins is such that proton pumping to the vesicle interior occurs upon illumination and after addition of ATP. All liposomes which contain H(+)-ATPase also contain bacteriorhodopsin. The rate of ATP hydrolysis was measured as function of pH in the dark and during illumination of the proteoliposomes. The pH dependency can be described by the protonation of a monovalent group from the outside with an apparent pK of 7.3 and the deprotonation of a monovalent group at the inside with an apparent pK of 3.7. Inside and outside refer to the orientation of the H(+)-ATPase in the liposomes which is opposite to that occurring in vivo. It is concluded that the first step in the reaction cycle is the binding of a proton from the cytosol which is followed by ATP binding, ATP hydrolysis on the enzyme and the release of ADP and phosphate, and finally the proton is released from the enzyme into the external medium.