In heterogeneous catalysis, the phase of a catalytic agent, which is responsible for reducing the activation energy of a reaction, is different from the phase of its reactants or substrates. Often, soluble catalysts are tightly associated with an inert carrier in order to artificially alter their phase. Applying this concept to biocatalysis yields a system in which enzyme molecules are immobilized on a solid support. This often serves to stabilize the enzyme, as well as enhance the recyclability of the enzyme since it is no longer soluble. In this dissertation, two methods of enzyme immobilization are evaluated: adsorption to a solid surface and whole-cell biocatalysis. The latter is then engineered for improved kinetics and functional activity using principles of synthetic biology. Adsorption of a protein to a solid surface is driven by the same thermodynamic factors that are responsible for the folding of a protein. Hydrophobic interactions, ionic interactions, covalent bonding, and weak forces all contribute to minimizing the free energy of a protein, which defines its secondary, tertiary, and quaternary structures. Upon introduction to a surface, these different forces rearrange across the surface of the substrate to minimize the free energy of the system. Many factors influence this behavior, including particle curvature, material properties of the surface, and the stability of the protein. In the preexisting body of work, much of the research performed regarding the effects of thermal stability on adsorption were performed using mutant proteins whose structures were intentionally altered for a range of stabilities. In Chapter 2, we evaluate the effects of thermal stability on adsorption behavior using naturally evolved enzymes from the AKR superfamily, namely AdhD and hAR. These enzymes were selected for their structural homology, but vastly different thermal stabilities. Using these proteins, we demonstrate that the previously held theories of thermostable protein adsorption behavior are not entirely applicable to naturally evolved proteins that are not artificially stabilized. We also propose a modification to the classic 4-state adsorption/desorption model by introducing new pathways and protein states based on our experiments. In addition to physisorption, whole-cell biocatalysis was explored as an enzyme immobilization platform. In general, this can be accomplished by cytosolic expression, periplasmic expression, or surface display. After weighing these options, we chose periplasmic expression in E. coli for our biocatalysts. As for the catalytic component, we selected carbonic anhydrase (CA), a class of Zn+2-binding metalloenzymes that are capable of catalyzing the reversible hydration of CO2. This enzyme was selected for the breadth of applications it can be used for, as well as its ubiquity in nature and extremely fast kinetics. Two isoforms were selected (Cab and Cam) for their respective benefits and were periplasmically expressed using 2 different leader peptides, which we discuss in Chapter 3. The enzyme loading in the periplasm, kinetics, thermal stability, and functional activity are all reported for the resulting whole-cell biocatalysts. We also describe a new method for the measurement of the operational stability of CA-based biocatalysts. Modifications to the whole-cell biocatalysts are described in Chapter 4 and Chapter 5. In Chapter 4, we demonstrate that expression of a viral envelope protein enhances the permeability of the outer membranes of E. coli cells. We characterize this improvement by measuring small-molecule permeance, whole-cell kinetics, and functional activity of the modified biocatalysts. We also quantify this enhancement by applying concepts of porous chemical catalysts to our whole-cells. In doing so, we show improvements in parameters such as the effectiveness factor, Thiele modulus, diffusivity, and permeability. Finally, in Chapter 5 we show enhancement of the functional activity of the whole-cell biocatalysts by displaying small peptides on the outer surfaces of the cells. The modified cells are shown to enhance precipitation of calcium carbonate, a common end product of carbon mineralization. Improved solid formation rates are also reported and possible explanations for these effects are discussed. Overall, this dissertation explores immobilization of enzymes to create heterogeneous biocatalysts. First, the effects of immobilization on enzyme structure, stability, and activity are shown for two different immobilization techniques: adsorption to a solid surface and periplasmic expression in E. coli cells. After characterization, engineering of the whole-cell biocatalysts for improved properties is presented. Finally, future directions for this work are discussed which would advance our understanding of heterogeneous biocatalysts, as well as enhance their utility.

Toxicity of organic solvents to microbial hosts is a major consideration in the economical production of biofuels such as ethanol and especially butanol, with low product concentrations leading to high recovery costs. The key to rational engineering of solvent tolerant microorganisms for such processes lies in obtaining appropriate tolerance genes (modules) suited for different compounds. In this project, a synthetic biology approach was adopted to generate a library of standardised BioBrick parts involved in different stress responses. Using a multiple-assay approach, including a bioluminescence assay, these stress response genes were tested individually and in combination to determine their effects on survival in ethanol, nbutanol, acetone and fermentation inhibitors produced by biomass pre-treatment. A set of tolerance modules was obtained for ethanol and n-butanol. Proof-of-concept tests suggested that ethanol and n-butanol toxicity was mainly due to damage to membrane, cellular proteins and DNA possibly by oxidative stress. No synergistic interactions were observed from a combination of different tolerance genes. Further tests carried out using enzyme and fluorescence-based assays to elucidate the effect of n-butanol on the cell envelope showed that the solvent released lipopolysaccharides from the outer membrane of E. coli and also caused both outer and inner membranes to be leaky. Very high n-butanol concentrations resulted in an altered cell shape and bleb formation suggesting an impairment in cell division and peptidoglycan biosynthesis. The cell membrane was modified by cis-trans isomerisation of unsaturated fatty acids in the phospholipids resulting in a reduction of membrane leakage which in effect, increased n-butanol tolerance in E. coli. In conclusion, results from this research suggest that strategies to protect the membrane and cellular proteins should be included in rational engineering of n-butanol tolerant bacteria.

The emerging field of synthetic biology aims to use artificially designed genetic circuits to program living cells, much as engineers program a computer or control electronic or mechanical systems. This thesis focuses on the design and implementation of synthetic gene circuits in the bacterium Escherichia coli to create new cellular functions, and on the quantitative characterization and modelling of these circuits. Though important in any engineering discipline, quantitative system characterization has been poorly explored in synthetic biology. We have performed a quantitative system characterization by implementing simple gene circuits in Escherichia coli. The work showed that the level and variability of gene expression varied across different cell strains, and we investigated how these effects manifested through the coupled effects of cell division, cellular growth rate, and plasmid copy number regulation. The work suggests that gene circuit modules from a standard library cannot be used universally; the cellular context and the time dependent dynamics must be considered when implementing gene circuits. In order to work precisely as engineering devices, synthetic gene circuits must be appropriately tuned. One standard method of tuning genetic circuits requires altering the DNA sequences by extensive molecular biology work. Part of this thesis focuses on developing easily tunable gene circuits. A set of circuits were developed in E. coli where the shape of the chemically induced signal response curves can be tuned from a band structure to a sigmoidal structure simply by altering the temperature in a single system. Another set of circuits was developed which demonstrate a range of chemically tunable signal response curves along with multiple functions in a single device. One of the ultimate goals of synthetic biology is to program living cell in a human-controlled way. To this end, I developed a set of genetic devices that could work as ‘in cell disease prevention devices,’ preventing an otherwise fatal viral infection in E. coli. The device displays a number of ‘device’ properties: being dormant under normal conditions, detecting the onset of the disease state, turning on automatically to prevent a lethal outcome, and being subject to external deactivation when desired. The combination of design, characterization, and mathematical understanding explored in this work represents a contribution in the direction of developing synthetic biology as a well-founded engineering discipline.

The emerging field of synthetic biology aims to use artificially designed genetic circuits to program living cells, much as engineers program a computer or control electronic or mechanical systems. This thesis focuses on the design and implementation of synthetic gene circuits in the bacterium Escherichia coli to create new cellular functions, and on the quantitative characterization and modelling of these circuits. Though important in any engineering discipline, quantitative system characterization has been poorly explored in synthetic biology. We have performed a quantitative system characterization by implementing simple gene circuits in Escherichia coli. The work showed that the level and variability of gene expression varied across different cell strains, and we investigated how these effects manifested through the coupled effects of cell division, cellular growth rate, and plasmid copy number regulation. The work suggests that gene circuit modules from a standard library cannot be used universally; the cellular context and the time dependent dynamics must be considered when implementing gene circuits. In order to work precisely as engineering devices, synthetic gene circuits must be appropriately tuned. One standard method of tuning genetic circuits requires altering the DNA sequences by extensive molecular biology work. Part of this thesis focuses on developing easily tunable gene circuits. A set of circuits were developed in E. coli where the shape of the chemically induced signal response curves can be tuned from a band structure to a sigmoidal structure simply by altering the temperature in a single system. Another set of circuits was developed which demonstrate a range of chemically tunable signal response curves along with multiple functions in a single device. One of the ultimate goals of synthetic biology is to program living cell in a human-controlled way. To this end, I developed a set of genetic devices that could work as ‘in cell disease prevention devices,’ preventing an otherwise fatal viral infection in E. coli. The device displays a number of ‘device’ properties: being dormant under normal conditions, detecting the onset of the disease state, turning on automatically to prevent a lethal outcome, and being subject to external deactivation when desired. The combination of design, characterization, and mathematical understanding explored in this work represents a contribution in the direction of developing synthetic biology as a well-founded engineering discipline.

The confluence of next generation DNA sequencing and synthesis when combined with the application of concepts such as standardization and modular design has led to the genesis of a new discipline. The nascent field of Synthetic Biology concerns the rational design and construction of genetic circuits, pathways, machines and eventually whole organisms. The immaturity of this field dictates that early research efforts, including this Thesis, describe foundational work towards the creation of tools which make biology more amenable to being engineered. The first part of this Thesis describes an attempt to standardize the measurement of transcriptional promoter activity in E. coli. A method to measure in vivo promoter activity was developed for E. coli and tested in a multi-institution trial. Comparable results were achieved with less than a two-fold range for the measured promoters across eight laboratories. A standardized measurement kit was created and distributed for use by the teams participating in the 2008 international Genetically Engineered Machines competition. Techniques learned measuring the activity of E. coli promoters were applied to a collection of S. cerevisiae strains. Several promoters were measured in synthetic dextrose media and ADH1 was measured in multiple media conditions. The outcome of these experiments is to consider proposing ADH1as the reference promoter in S. cerevisiae. The second aspect of this Thesis describes the construction of artificial organelles in S. cerevisiae. Artificial organelles hold the prospect of being able to insulate synthetic genetic pathways from the cell. Two proteins are essential for the biogenesis of the peroxisome organelle in humans and yeast, Pex3p and Pex19p. Pex3p functions as a peroxisomal membrane receptor for Pex19p, while Pex19p shuttles other peroxisomal proteins to the membrane, including Pex3p, creating a feedback loop. Human Pex19p has previously been shown to dock to yeast Pex3p and a version of yeast Pex19p has been shown to work with human Pex3p as a high degree of evolutionary conservation exists between these proteins. Because of these inter-species protein docking characteristics, there exists the possibility of creating bimodality: the ambition of the work was therefore to create a cell strain which possessed both synthetic “humanized” and natural yeast peroxisomes. An S. cerevisiae BY4741a derivative strain was engineered with fluorophore tagged versions of human (CFP) and yeast (YFP) Pex3p and untagged yeast and human Pex19p proteins. The results indicated the creation of a single population of peroxisomes when a measure of fluorescently imaged CFP and YFP peroxisomes were plotted on a scatter plot. A log of the ratio of CFP to YFP peroxisomes was plotted on a histogram and a normal distribution was found to best fit the curve, indicating a lack of bimodality. Finally, microscopy images of this strain were reviewed and by visual inspection, showed no evidence of distinct human or yeast peroxisomes. This experiment therefore produced no evidence of bimodality when examining the interactions of human and yeast Pex3p and Pex19p proteins. However, the four proteins were shown to interact closely to produce a single population of chimeric human-yeast peroxisomes. The peroxisome-deficient mutant phonotype strain was rescued using human Pex3p and Pex19p.

The enzymatic deconstruction of lignocellulosic plant biomass is performed by specialist microbial species. It is a ubiquitous process within nature and central to the global recycling of carbon and energy. Lignocellulose is a complex heteropolymer, highly recalcitrant and resistant to hydrolysis due to the major polysaccharide cellulose existing as a crystalline lattice, intimately associated with a disordered sheath of hemicellulosic polysaccharides and lignin. In this thesis I aim to transfer the highly efficient cellulolytic mechanism of the bacterium Cellulomonas fimi, to that of a suitably amenable and genetically tractable expression host, in the hopes of better understanding the enzymatic hydrolysis of lignocellulose. Using tools and concepts from molecular biology and synthetic biology, I constructed a library of standardised genetic parts derived from C. fimi, each encoding a known enzymatic activity involved in the hydrolysis of cellulose, mannan or xylan; three of the major polysaccharides present in lignocellulose. Characterization assays were performed on individual parts to confirm enzymatic activity and compare efficiencies against a range of substrates. Results then informed the rational design and construction of parts into modular devices. The resultant genetic devices were introduced into the expression hosts Escherichia coli and Citrobacter freundii, and transformed strains were assayed for the ability to utilize various forms of xylan, mannan and cellulose as a sole carbon source. Results identified devices which when expressed by either host showed growth on the respective carbon sources. Notably, devices with improved activity against amorphous cellulose, crystalline cellulose, mannan and xylan were determined. Recombinant cellulase expressing strains of E. coli and C. freundii were shown capable of both deconstruction and utilization of pure cellulose paper as a sole carbon source. Moreover, this capacity was shown to be entirely unhindered when C. freundii strains were cultured in saline media. These findings show promise in developing C. freundii for bioprocessing of biomass in sea water, so as to reduce the use of fresh water resources and improve sustainability as well as process economics. Work presented in this thesis contributes towards understanding the complementarities and synergies of the enzymes responsible for lignocellulose hydrolysis. Moreover, the research emphasizes the merits of standardizing genetic parts used within metabolic engineering projects and how adopting such design principles can expedite the research process.

We are working to enable the engineering of integrated biological systems. Specifically, we would like to be able to build systems using standard parts that, when combined, have reliable and predictable behavior. Here, we define standard characteristics for describing the absolute physical performance of genetic parts that control gene expression. The first characteristic, PoPS, defines the level of transcription as the number of RNA polymerase molecules that pass a point on DNA each second, on a per DNA copy basis (PoPS = Polymerase Per Second; PoPSdc = PoPS per DNA copy). The second characteristic, RiPS, defines the level of translation as the number of ribosome molecules that pass a point on mRNA each second, on a per mRNA copy basis (RiPS = Ribosomes Per Second; RiPSmc = RiPS per mRNA copy). In theory, it should be possible to routinely combine devices that send and receive PoPS and RiPS signals to produce gene expression-based systems whose quantiative behavior is easy to predict. To begin to evaluate the utility of the PoPS and RIPS framework we are characterizing the performance of a simple gene expression device in E. coli growing at steady state under standard operating conditions; we are using a simple ordinary differential equation model to estimate the steady state PoPS and RiPS levels. / Poster presented at the 2005 ICSB meeting, held at Harvard Medical School in Boston, MA.

Building reliable genetic devices in synthetic biology is still a major challenge despite the various advances that have been made in the field since its inception. In principle, genetic devices with matching input and output expression levels can be assembled from well-characterized genetic parts. In practice, a priori genetic circuit design continues to be difficult in synthetic biology due to the lack of foundational work in this area. Currently, a successful genetic device is typically created by manually building and testing many combinatorial variants of the target device and then picking the best one. While this process is slow and error-prone, as synthetic genetic devices grow in complexity, this approach also becomes unmanageable and impractical. Fluctuations in genetic context have been identified as a major cause of rational genetic circuit design failures. Promoter elements often behave unpredictably as they are moved from the context in which they were originally characterized. Thus, the ordered location of parts in a synthetic device impacts expected performance. Synthetic spacer DNA sequences have been reported to successfully buffer promoters from their neighboring DNA sequence but design rules for these sequences are lacking. I address this problem with a novel method based on a randomized insulator library. I have developed a high-throughput, flow cytometry-based screen that randomly samples from a library of 4^36 potential insulators created in a single cloning step. This method provides precise control over genetic circuit expression. I further show that insulating the promoters in a genetic NOT-gate improves circuit performance and nearly eliminates the effect of the order in which the promoters are organized in the device. This foundational work will help improve the design of reliable genetic devices in E. coli. Finally, automated DNA assembly using liquid-handling robots can help increase the speed at which combinatorial synthetic device variants are assembled. However, these systems require significant investment in optimizing the handling parameters for handling very small volumes of the various liquids in DNA assembly protocols. I have optimized and validated these liquid-handling parameters on the Tecan EVO liquid handling robotic platform. These materials have been made available to the larger community. / 2017-12-03T00:00:00Z

Protein-protein interactions modulate cellular processes in health and disease. However, tracing weak or rare associations or dissociations of proteins is not a trivial task. Kinases are often regulated through interaction partners and, at the same time, themselves regulate cellular interaction networks. The use of kinase domains for creating a synthetic sensor device that reads low concentration protein-protein interactions and amplifies them to a higher concentration interaction which is then translated into a FRET (Fluorescence Resonance Energy Transfer) signal is here proposed. To this end, DNA constructs for interaction amplification (split kinases), positive controls (intact kinase domains), scaffolding proteins and phosphopeptide - SH2-domain modules for the reading of kinase activity were assembled and expression protocols for fusion proteins containing Lyn, Src, and Fak kinase domains in bacterial and in cell-free systems were optimized. Also, two non-overlapping methods for measuring the kinase activity of these proteins were stablished and, finally, a protein-fragment complementation assay with the split-kinase constructs was tested. In conclusion, it has been demonstrated that features such as codon optimization, vector design and expression conditions have an impact on the expression yield and activity of kinase-based proteins. Furthermore, it has been found that the defined PURE cell-free system is insufficient for the active expression of catalytic kinase domains. In contrast, the bacterial co-expression with phosphatases produced active kinase fusion proteins for two out of the three tested Tyrosine kinase domains.

Cellular activities constantly change to precisely respond to their biological needs. In many cases, proteins carry out these activities because they can exhibit graded and dynamic responses to perform an array of cellular functions. To study these biological activities and to repurpose proteins for novel uses such as cell therapies, we must be able to control protein activity with synthetic inducers, such as chemical ligands. Multiple chemical inducers have been employed to achieve protein control, but there remains a need for inducer ligands that minimally interact with endogenous pathways, display high bioavailability, and are absent or minimally present from dietary sources. In this work, we control protein activities with the Hepatitis C virus cis-protease NS3 and its numerous clinically validated, highly specific inhibitors. First, we use NS3 to create a Ligand Inducible Connection (LInC) to chemically control gene expression, protein localization and cell signaling in mammalian cells. We then extend the use of catalytically inactive NS3 as a high affinity binder in conjunction with genetically encoded approaches to inhibit NS3, including peptides and RNA aptamers. Using catalytically inactive NS3, genetically encoded peptides, and small molecule drugs, we conditionally control peptide docking with antiviral drugs. We apply this concept to control mammalian gene expression, cell signaling, enzyme activity, and develop a new mechanism for allosteric regulation of Cre recombinase. Altogether, we have developed a new toolkit for controlling diverse protein activities with highly orthogonal, antiviral drugs. / 2023-05-15T00:00:00Z