Quantification of metabolic limitations during recombinant protein production in Escherichia coli

A recombinant chimeric spider pyriform-aciniform silk with highly tunable mechanical performance

Spider silks are natural protein-based biomaterials which are renowned for their mechanical properties and hold great promise for applications ranging from high-performance textiles to regenerative medicine. While some spiders can produce several different types of silks, most spider silk types - including pyriform and aciniform silks - are relatively unstudied. Pyriform and aciniform silks have distinct mechanical behavior and physicochemical properties, with materials produced using combinations of these silks currently unexplored. Here, we introduce an engineered chimeric fusion protein consisting of two repeat units of pyriform (Py) silk followed by two repeat units of aciniform (W) silk named Py2W2. This recombinant ∼86.5 kDa protein is amenable to expression and purification from Escherichia coli and exhibits high α-helicity in a fluorinated acid- and alcohol-based solution used to form a dope for wet-spinning. Wet-spinning enables continuous fiber production and post-spin stretching of the wet-spun fibers in air or following submersion in water or ethanol leads to increases in optical anisotropy, consistent with increased molecular alignment along the fiber axis. Mechanical properties of the fibers vary as a function of post-spin stretching condition, with the highest extensibility and strength observed in air-stretched and ethanol-treated fibers, respectively, with mechanics being superior to fibers spun from either constituent protein alone. Notably, the maximum extensibility obtained (∼157 ± 38 %) is of the same magnitude reported for natural flagelliform silks, the class of spider silk most associated with being stretchable. Interestingly, Py2W2 is also water-compatible, unlike its constituent Py2. Fiber-state secondary structure correlates well with the observed mechanical properties, with depleted α-helicity and increased β-sheet content in cases of increased strength. Py2W2 fibers thus provide enhanced materials behavior in terms of their mechanics, tunability, and fiber properties, providing new directions for design and development of biomaterials suitable and tunable for disparate applications.

High Cell Density Cultivation Combined with high Specific Enzyme Activity: Cultivation Protocol for the Production of an Amine Transaminase from Bacillus megaterium in E. coli

High cell density cultivation is an established method for the production of various industrially important products such as recombinant proteins. However, these protocols are not always suitable for biocatalytic processes as the focus often lies on biomass production rather than high specific activities of the enzyme inside the cells. In contrast, a range of shake flask protocols are well known with high specific activities but rather low cell densities. To overcome this gap, we established a tailor-made fed-batch protocol combining both aspects: high cell density and high specific activities of heterologously produced enzyme. Using the example of an industrially relevant amine transaminase from Bacillus megaterium, we describe a strategy to optimize the cultivation yield based on the feed rate, IPTG concentration, and post-induction temperature. By adjusting these key parameters, we were able to increase the specific activity by 2.6-fold and the wet cell weight by even 17-fold compared to shake flasks. Finally, we were able to verify our established protocol by transferring it to another experimenter. With that, our optimization strategy can serve as a template for the production of high titers of heterologously produced, active enzymes and might enable the availability of these catalysts for upscaling biocatalytic processes.

Silk fibroin production in Escherichia coli is limited by a positive feedback loop between metabolic burden and toxicity stress

To investigate the metabolic elasticity and production bottlenecks for recombinant silk proteins in Escherichia coli, we performed a comprehensive characterization of one elastin-like peptide strain (ELP) and two silk protein strains (A5 4mer, A5 16mer). Our approach included ¹³C metabolic flux analysis, genome-scale modeling, transcription analysis, and ¹³C-assisted media optimization experiments. Three engineered strains maintained their central flux network during growth, while measurable metabolic flux redistributions (such as the Entner-Doudoroff pathway) were detected. Under metabolic burdens, the reduced TCA fluxes forced the engineered strain to rely more on substrate-level phosphorylation for ATP production, which increased acetate overflow. Acetate (as low as 10 mM) in the media was highly toxic to silk-producing strains, which reduced 4mer production by 43% and 16mer by 84%, respectively. Due to the high toxicity of large-size silk proteins, 16mer's productivity was limited, particularly in the minimal medium. Therefore, metabolic burden, overflow acetate, and toxicity of silk proteins may form a vicious positive feedback loop that fractures the metabolic network. Three solutions could be applied: 1) addition of building block supplements (i.e., eight key amino acids: His, Ile, Phe, Pro, Tyr, Lys, Met, Glu) to reduce metabolic burden; 2) disengagement of growth and production; and 3) use of non-glucose based substrate to reduce acetate overflow. Other reported strategies were also discussed in light of decoupling this positive feedback loop.

Developing a dynamic equilibrium system in Escherichia coli to improve the production of recombinant proteins

The combination of Escherichia coli BL21 (DE3) and the pET expression system is used extensively for the expression of various recombinant proteins (RPs). However, RP overexpression often introduces a growth burden for the host, especially in the case of toxic proteins. The key to solving this problem is to reduce the host burden associated with protein overproduction, which is often achieved by regulating the expression or activity of T7 RNAP or growth-decoupled systems. However, these strategies mainly relieve or interrupt the robbing of host resources, and do not eliminate other types of host burdens in the production process. In this study, we constructed a production system based on a dynamic equilibrium to precisely relieve the host burden and increase the RP production. The system is composed of three modules, including the overexpression of basic growth-related genes (rRNA, RNAP core enzyme, sigma factors), prediction and overexpression of key proteins using the enzyme-constrained model ec_iECBD_1354, and dynamic regulation of growth-related and key protein expression intensity based on a burden-driven promoter. Using this system, the production of many high-burden proteins, including autolysis protein and E. coli membrane proteins, was increased to varying degrees. Among them, the cytosine transporter protein (CodB) was most significantly improved, with a 4.02-fold higher production compared to the wild strain. This system can effectively reduce the optimizing costs, and is suitable for developing various types of RP expression hosts rapidly. KEY POINTS: • The basic growth-related resources can relieve the host burden from recombinant protein. • The enzyme-constrained model can accurately predict key genes to improve yield. • The expression intensity can be dynamically adjusted with changes in burden.

Burden Imposed by Heterologous Protein Production in Two Major Industrial Yeast Cell Factories: Identifying Sources and Mitigation Strategies

Production of heterologous proteins, especially biopharmaceuticals and industrial enzymes, in living cell factories consumes cellular resources. Such resources are reallocated from normal cellular processes toward production of the heterologous protein that is often of no benefit to the host cell. This competition for resources is a burden to host cells, has a negative impact on cell fitness, and may consequently trigger stress responses. Importantly, this often causes a reduction in final protein titers. Engineering strategies to generate more burden resilient production strains offer sustainable opportunities to increase production and profitability for this growing billion-dollar global industry. We review recently reported impacts of burden derived from resource competition in two commonly used protein-producing yeast cell factories: Saccharomyces cerevisiae and Komagataella phaffii (syn. Pichia pastoris). We dissect possible sources of burden in these organisms, from aspects related to genetic engineering to protein translation and export of soluble protein. We also summarize advances as well as challenges for cell factory design to mitigate burden and increase overall heterologous protein production from metabolic engineering, systems biology, and synthetic biology perspectives. Lastly, future profiling and engineering strategies are highlighted that may lead to constructing robust burden-resistant cell factories. This includes incorporation of systems-level data into mathematical models for rational design and engineering dynamical regulation circuits in production strains.

Recombinant expression of insoluble enzymes in Escherichia coli: a systematic review of experimental design and its manufacturing implications

Recombinant enzyme expression in Escherichia coli is one of the most popular methods to produce bulk concentrations of protein product. However, this method is often limited by the inadvertent formation of inclusion bodies. Our analysis systematically reviews literature from 2010 to 2021 and details the methods and strategies researchers have utilized for expression of difficult to express (DtE), industrially relevant recombinant enzymes in E. coli expression strains. Our review identifies an absence of a coherent strategy with disparate practices being used to promote solubility. We discuss the potential to approach recombinant expression systematically, with the aid of modern bioinformatics, modelling, and ‘omics’ based systems-level analysis techniques to provide a structured, holistic approach. Our analysis also identifies potential gaps in the methods used to report metadata in publications and the impact on the reproducibility and growth of the research in this field.

Recent Advances in the Prediction of Pharmacokinetics Properties in Drug Design Studies: A Review

This review presents the main aspects related to pharmacokinetic properties, which are essential for the efficacy and safety of drugs. This topic is very important because the analysis of pharmacokinetic aspects in the initial design stages of drug candidates can increase the chances of success for the entire process. In this scenario, experimental and in silico techniques have been widely used. Due to the difficulties encountered with the use of some experimental tests to determine pharmacokinetic properties, several in silico tools have been developed and have shown promising results. Therefore, in this review, we address the main free tools/servers that have been used in this area, as well as some cases of application. Finally, we present some studies that employ a multidisciplinary approach with synergy between in silico, in vitro, and in vivo techniques to assess ADME properties of bioactive substances, achieving successful results in drug discovery and design.

A new lipase (Alip2) with high potential for enzymatic hydrolysis of the diester diethyladipate to the monoester monoethyladipate

Several putative lipase genes from the genome of the yeast Blastobotrys (Arxula) raffinosifermentans (adeninivorans) LS3 were overexpressed in the yeast itself and screened for the desymmetrization of the dicarboxylic acid diester diethyl adipate (DEA) into the monoester monoethyl adipate (MEA). MEA can serve as a monomeric spacer group for functional polymers used in medical chemistry and dental applications. The selected lipase Alip2-c6hp was intracellularly located. After overexpression of the corresponding gene, it was purified and biochemically characterized using p-nitrophenyl butyrate as the substrate for standard activity tests. In fed-batch cultivation with constructed yeast strain B. raffinosifermentans G1212/YRC102-Alip2-c6h for large scale production of the Alip2-c6hp biocatalyst enzyme activities up to 674 U L^-1 were reached. Several tested diesters were hydrolyzed selectively to monoesters. Under optimized conditions, the purified enzyme Alip2p-c6h converted 96 % of the substrate DEA to MEA within 30 min incubation, whereby only 1.6 % of the unwanted side-product adipic acid (AA) was formed. At room temperature the dicarboxylic acid esters diethyl malonate (DEM), diethyl succinate (DES), dimethyl adipate (DMA) and dimethyl suberate (DMSub) were completely hydrolyzed to their corresponding monoesters. A high yield of 87 % and 25 % could also be achieved with the dioldiesters 1,4-diacetoxybutane (DAB) and diacetoxyhexane (DAH). In conclusion the potential of the lipase Alip2-c6hp expressed in B. raffinosifermentans is very promising for selective hydrolysis of DEA to MEA as well as for the production of other monoesters.

Engineering of a robust Escherichia coli chassis and exploitation for large-scale production processes

In large-scale bioprocesses microbes are exposed to heterogeneous substrate availability reducing the overall process performance. A series of deletion strains was constructed from E. coli MG1655 aiming for a robust phenotype in heterogeneous fermentations with transient starvation. Deletion targets were hand-picked based on a list of genes derived from previous large-scale simulation runs. Each gene deletion was conducted on the premise of strict neutrality towards growth parameters in glucose minimal medium. The final strain of the series, named E. coli RM214, was cultivated continuously in an STR-PFR (stirred tank reactor - plug flow reactor) scale-down reactor. The scale-down reactor system simulated repeated passages through a glucose starvation zone. When exposed to nutrient gradients, E. coli RM214 had a significantly lower maintenance coefficient than E. coli MG1655 (Δm_s = 0.038 g_Glucose/g_CDW/h, p < 0.05). In an exemplary protein production scenario E. coli RM214 remained significantly more productive than E. coli MG1655 reaching 44% higher eGFP yield after 28 h of STR-PFR cultivation. This study developed E. coli RM214 as a robust chassis strain and demonstrated the feasibility of engineering microbial hosts for large-scale applications.

Cascaded processing enables continuous upstream processing with E. coli BL21(DE3)

In many industrial sectors continuous processing is already the golden standard to maximize productivity. However, when working with living cells, subpopulation formation causes instabilities in long-term cultivations. In cascaded continuous cultivation, biomass formation and recombinant protein expression can be spatially separated. This cultivation mode was found to facilitate stable protein expression using microbial hosts, however mechanistic knowledge of this cultivation strategy is scarce. In this contribution we present a method workflow to reduce workload and accelerate the establishment of stable continuous processes with E. coli BL21(DE3) exclusively based on bioengineering methods.

Robotic integration enables autonomous operation of laboratory scale stirred tank bioreactors with model-driven process analysis

Given its geometric similarity to large-scale production plants and the excellent possibilities for precise process control and monitoring, the classic stirred tank bioreactor (STR) still represents the gold standard for bioprocess development at a laboratory scale. However, compared to microbioreactor technologies, bioreactors often suffer from a low degree of process automation and deriving key performance indicators (KPIs) such as specific rates or yields often requires manual sampling and sample processing. A widely used parallelized STR setup was automated by connecting it to a liquid handling system and controlling it with a custom-made process control system. This allowed for the setup of a flexible modular platform enabling autonomous operation of the bioreactors without any operator present. Multiple unit operations like automated inoculation, sampling, sample processing and analysis, and decision making, for example for automated induction of protein production were implemented to achieve such functionality. The data gained during application studies was used for fitting of bioprocess models to derive relevant KPIs being in good agreement with literature. By combining the capabilities of STRs with the flexibility of liquid handling systems, this platform technology can be applied to a multitude of different bioprocess development pipelines at laboratory scale.

Development of a dual specific growth rate-based fed-batch process for production of recombinant human granulocyte colony-stimulating factor in Pichia pastoris

A number of limitations exist for production of human granulocyte colony-stimulating factor (G-CSF) in Pichia pastoris. In this study, two different specific growth rates (0.015 h^-1, 0.01 h^-1) were used sequentially in the mixed substrate feeding period during methanol induction phase to enhance the G-CSF titer in the culture broth. Necessary parameters required for implementing the feeding strategy, such as specific product yield on biomass (Y_P/X) and maintenance coefficient (m) on glycerol, methanol, and sorbitol were estimated using continuous culture technique. Using this strategy, for the same volumetric productivity, about 20% increase in protein titer was achieved over that obtained from the run carried out at a single pre-set value of 0.015 h^-1 alone. Thus, implementation of higher specific growth rate (0.015 h^-1) set during initial stages of the methanol induction phase followed by a lower specific growth rate (0.01 h^-1) helped in achieving increased protein titers.

Metabolic manipulation through CRISPRi and gene deletion to enhance cadaverine production in Escherichia coli

Due to the limiting natural resources, greenhouse effect and global warming crisis, the bio-based chemicals which are environmentally friendly materials have gradually become urgent and important. Cadaverine, a 1,5-diaminopentane (DAP), is widely used as block chemicals for synthesis of biopolymer, which can be produced from lysine by lysine decarboxylase (EC 4.1.1.18) in Escherichia coli. However, the DAP will be further utilized into by-products through downstream genes of speE, puuA, speG and ygjG, which decrease the amount of product. In this study, two approaches including Lambda-Red system for gene knockout, and clustered regularly interspaced short palindromic repeats interference (CRISPRi) for gene knockdown; are explored to manipulate the metabolic flux among 26 genetic E. coli. As a result, CadA driven by inducible T7 promoter accumulated more DAP from CRISPRi targeted on single-gene repressive strains such as BT7AiE, BT7AiP, BT7AiG and BT7AiY. The highest DAP titer and productivity was obtained to 38 g/L and 2.67 g/L/h in BT7AiY (repression of ygjG). We also investigated the co-factor pyridoxal 5'-phosphate (PLP) effect on lysine consumption and DAP production from different E. coli derivatives. In contrast to CRISPRi-mediated strains, 4 genes knockout strain (BT7AdEPGY) deal with 98% lysine consumption and achieved 37.45 g/L DAP and 3.17 g/L/h DAP productivity. The metabolic regulation by CRISPRi is a simple strategy and the results are consistent with gene knockout to manipulate the pathway for DAP production.

Independent component analysis of E. coli's transcriptome reveals the cellular processes that respond to heterologous gene expression

Achieving the predictable expression of heterologous genes in a production host has proven difficult. Each heterologous gene expressed in the same host seems to elicit a different host response governed by unknown mechanisms. Historically, most studies have approached this challenge by manipulating the properties of the heterologous gene through methods like codon optimization. Here we approach this challenge from the host side. We express a set of 45 heterologous genes in the same Escherichia coli strain, using the same expression system and culture conditions. We collect a comprehensive RNAseq set to characterize the host's transcriptional response. Independent Component Analysis of the RNAseq data set reveals independently modulated gene sets (iModulons) that characterize the host response to heterologous gene expression. We relate 55% of variation of the host response to: Fear vs Greed (16.5%), Metal Homeostasis (19.0%), Respiration (6.0%), Protein folding (4.5%), and Amino acid and nucleotide biosynthesis (9.0%). If these responses can be controlled, then the success rate with predicting heterologous gene expression should increase.

Production of Active Recombinant Hyaluronidase Inclusion Bodies from Apis mellifera in E. coli Bl21(DE3) and characterization by FT-IR Spectroscopy

The bacterium E. coli is one of the most important hosts for recombinant protein production. The benefits are high growth rates, inexpensive media, and high protein titers. However, complex proteins with high molecular weight and many disulfide bonds are expressed as inclusion bodies (IBs). In the last decade, the overall perception of these IBs being not functional proteins changed, as enzyme activity was found within IBs. Several applications for direct use of IBs are already reported in literature. While fluorescent proteins or protein tags are used for determination of IB activity to date, direct measurements of IB protein activity are scacre. The expression of recombinant hyaluronidase from Apis mellifera in E. coli BL21(DE3) was analyzed using a face centered design of experiment approach. Hyaluronidase is a hard to express protein and imposes a high metabolic burden to the host. Conditions giving a high specific IB titer were found at 25 °C at low specific substrate uptake rates and induction times of 2 to 4 h. The protein activity of hyaluronidase IBs was verified using (Fourier transform) FT-IR spectroscopy. Degradation of the substrate hyaluronan occurred at increased rates with higher IB concentrations. Active recombinant hyaluronidase IBs can be immediately used for direct degradation of hyaluronan without further down streaming steps. FT-IR spectroscopy was introduced as a method for tracking IB activity and showed differences in degradation behavior of hyaluronan dependent on the applied active IB concentration.

Optimization and scale-up production of Zika virus ΔNS1 in Escherichia coli: application of Response Surface Methodology

Diagnosing Zika virus (ZIKV) infections has been challenging due to the cross-reactivity of induced antibodies with other flavivirus. The concomitant occurrence of ZIKV and Dengue virus (DENV) in endemic regions requires diagnostic tools with the ability to distinguish these two viral infections. Recent studies demonstrated that immunoassays using the C-terminal fragment of ZIKV NS1 antigen (ΔNS1) can be used to discriminate ZIKV from DENV infections. In order to be used in serological tests, the expression/solubility of ΔNS1 and growth of recombinant E. coli strain were optimized by Response Surface Methodology. Temperature, time and IPTG concentration were evaluated. According to the model, the best condition determined in small scale cultures was 21 °C for 20 h with 0.7 mM of IPTG, which predicted 7.5 g/L of biomass and 962 mg/L of ΔNS1. These conditions were validated and used in a 6-L batch in the bioreactor, which produced 6.4 g/L of biomass and 500 mg/L of ΔNS1 in 12 h of induction. The serological ELISA test performed with purified ΔNS1 showed low cross-reactivity with antibodies from DENV-infected human subjects. Denaturation of ΔNS1 decreased the detection of anti-ZIKV antibodies, thus indicating the contribution of conformational epitopes and confirming the importance of properly folded ΔNS1 for the specificity of the serological analyses. Obtaining high yields of soluble ΔNS1 supports the viability of an effective serologic diagnostic test capable of differentiating ZIKV from other flavivirus infections.

The Rocky Road From Fed-Batch to Continuous Processing With E. coli

Escherichia coli still serves as a beloved workhorse for the production of many biopharmaceuticals as it fulfills essential criteria, such as having fast doubling times, exhibiting a low risk of contamination, and being easy to upscale. Most industrial processes in E. coli are carried out in fed-batch mode. However, recent trends show that the biotech industry is moving toward time-independent processing, trying to improve the space-time yield, and especially targeting constant quality attributes. In the 1950s, the term "chemostat" was introduced for the first time by Novick and Szilard, who followed up on the previous work performed by Monod. Chemostat processing resulted in a major hype 10 years after its official introduction. However, enthusiasm decreased as experiments suffered from genetic instabilities and physiology issues. Major improvements in strain engineering and the usage of tunable promotor systems facilitated chemostat processes. In addition, critical process parameters have been identified, and the effects they have on diverse quality attributes are understood in much more depth, thereby easing process control. By pooling the knowledge gained throughout the recent years, new applications, such as parallelization, cascade processing, and population controls, are applied nowadays. However, to control the highly heterogeneous cultivation broth to achieve stable productivity throughout long-term cultivations is still tricky. Within this review, we discuss the current state of E. coli fed-batch process understanding and its tech transfer potential within continuous processing. Furthermore, the achievements in the continuous upstream applications of E. coli and the continuous downstream processing of intracellular proteins will be discussed.

Boosting Recombinant Inclusion Body Production-From Classical Fed-Batch Approach to Continuous Cultivation

State of the art microbial recombinant protein production is regularly performed in fed-batch based cultivations. However, these cultivations suffer from highly time-dependent changes in productivity and product quality, leading to high variations in the downstream process. Continuous biomanufacturing offers the possibility of a time independent process, boosting the time-space-yield of the recombinantly produced protein and further reducing costs for production, also as downstream gets more predictive. In the current work, the continuous production of a pharmaceutically relevant protein in form of an inclusion body in E. coli BL21(DE3) was investigated in single vessel cultivations by varying dilution rates using glycerol as carbon source, inducer (lactose or IPTG) and respective inducer concentrations. Attempts to increase low specific productivities observed in single vessel continuous cultivations, led to the establishment of a continuously operated cascade of two stirred tank reactors to spatially separate biomass formation from recombinant protein production. Process performance was substantially improved compared to a single vessel chemostat culture, as specific productivity and space-time yield were boosted using an optimized cascaded process by about a factor of 100. This study shows the potential of a two-stage continuous process as promising alternative to benchmark fed-batch processes achieving constant inclusion body production at a time-independent level.

Engineered Reversal of Function in Glycolytic Yeast Promoters

Promoters are key components of cell factory design, allowing precise expression of genes in a heterologous pathway. Several commonly used promoters in yeast cell factories belong to glycolytic genes, highly expressed in actively growing yeast when glucose is used as a carbon source. However, their expression can be suboptimal when alternate carbon sources are used, or if there is a need to decouple growth from production. Hence, there is a need for alternate promoters for different carbon sources and production schemes. In this work, we demonstrate a reversal of regulatory function in two glycolytic yeast promoters by replacing glycolytic regulatory elements with ones induced by the diauxic shift. We observe a shift in induction from glucose-rich to glucose-poor medium without loss of regulatory activity, and strong ethanol induction. Applications of these promoters were validated for expression of the vanillin biosynthetic pathway, reaching production of vanillin comparable to pathway designs using strong constitutive promoters.

Quantification of proteomic and metabolic burdens predicts growth retardation and overflow metabolism in recombinant Escherichia coli

Escherichia coli has been the host organism most frequently investigated for efficient recombinant protein production. However, the production of a foreign protein in recombinant E. coli often leads to growth deterioration and elevated secretion of acetic acid. Such observed phenomena have been widely linked with cell stress responses and metabolic burdens originated particularly from the increased energy demand. In this study, flux balance analysis and dynamic flux balance analysis were applied to investigate the observed growth physiology of recombinant E. coli, incorporating the proteome allocation theory and an adjustable maintenance energy level (ATPM) to capture the proteomic and energetic burdens introduced by recombinant protein synthesis. Model predictions of biomass growth, substrate consumption, acetate excretion, and protein production with two different strains were in good agreement with the experimental data, indicating that the constraint on the available proteomic resource and the change in ATPM might be important contributors governing the growth physiology of recombinant strains. The modeling framework developed in this work, currently with several limitations to overcome, offers a starting point for the development of a practical, model-based tool to guide metabolic engineering decisions for boosting recombinant protein production.

Enhancement of NAD(H) pool for formation of oxidized biochemicals in Escherichia coli

The NAD⁺/NADH ratio and the total NAD(H) play important roles for whole-cell biochemical redox transformations. After the carbon source is exhausted, the degradation of NAD(H) could contribute to a decline in the rate of a desired conversion. In this study, methods to slow the native rate of NAD(H) degradation were examined using whole-cell Escherichia coli with two model oxidative NAD⁺-dependent biotransformations. A high phosphate concentration (50 mM) was observed to slow NAD(H) degradation. We also constructed E. coli strains with deletions in genes coding several enzymes involved in NAD⁺ degradation. In shake-flask experiments, the total NAD(H) concentration positively correlated with conversion of xylitol to L-xylulose by xylitol 4-dehydrogenase, and the greatest conversion (80%) was observed using MG1655 nadR nudC mazG/pZE12-xdh/pCS27-nox. Controlled 1-L batch processes comparing E. coli nadR nudC mazG with a wild-type background strain demonstrated a 30% increase in final L-xylulose concentration (5.6 vs. 7.9 g/L) and a 25% increase in conversion (0.53 vs. 0.66 g/g). MG1655 nadR nudC mazG was also examined for the conversion of galactitol to L-tagatose by galactitol 2-dehydrogenase. A batch process using 15 g/L glycerol and 10 g/L galactitol generated over 9.4 g/L L-tagatose, corresponding to 90% conversion and a yield of 0.95 g L-tagatose/g galactitol consumed. The results demonstrate the value of minimizing NAD(H) degradation as a means to improve NAD⁺-dependent biotransformations.

Metabolic division of labor in microbial systems

Metabolic pathways are often engineered in single microbial populations. However, the introduction of heterologous circuits into the host can create a substantial metabolic burden that limits the overall productivity of the system. This limitation could be overcome by metabolic division of labor (DOL), whereby distinct populations perform different steps in a metabolic pathway, reducing the burden each population will experience. While conceptually appealing, the conditions when DOL is advantageous have not been rigorously established. Here, we have analyzed 24 common architectures of metabolic pathways in which DOL can be implemented. Our analysis reveals general criteria defining the conditions that favor DOL, accounting for the burden or benefit of the pathway activity on the host populations as well as the transport and turnover of enzymes and intermediate metabolites. These criteria can help guide engineering of metabolic pathways and have implications for understanding evolution of natural microbial communities.

Impact of Glycerol as Carbon Source onto Specific Sugar and Inducer Uptake Rates and Inclusion Body Productivity in E. coli BL21(DE3)

The Gram-negative bacterium E. coli is the host of choice for a multitude of used recombinant proteins. Generally, cultivation is easy, media are cheap, and a high product titer can be obtained. However, harsh induction procedures using isopropyl β-d-1 thiogalactopyranoside as inducer are often referred to cause stress reactions, leading to a phenomenon known as "metabolic" or "product burden". These high expressions of recombinant proteins mainly result in decreased growth rates and cell lysis at elevated induction times. Therefore, approaches tend to use "soft" or "tunable" induction with lactose and reduce the stress level of the production host. The usage of glucose as energy source in combination with lactose as induction reagent causes catabolite repression effects on lactose uptake kinetics and as a consequence reduced product titer. Glycerol-as an alternative carbon source-is already known to have positive impact on product formation when coupled with glucose and lactose in auto-induction systems, and has been referred to show no signs of repression when cultivated with lactose concomitantly. In recent research activities, the impact of different products on the lactose uptake using glucose as carbon source was highlighted, and a mechanistic model for glucose-lactose induction systems showed correlations between specific substrate uptake rate for glucose or glycerol (qs,C) and the maximum specific lactose uptake rate (qs,lac,max). In this study, we investigated the mechanistic of glycerol uptake when using the inducer lactose. We were able to show that a product-producing strain has significantly higher inducer uptake rates when being compared to a non-producer strain. Additionally, it was shown that glycerol has beneficial effects on viability of cells and on productivity of the recombinant protein compared to glucose.

A novel method to recover inclusion body protein from recombinant E. coli fed-batch processes based on phage ΦX174-derived lysis protein E

Production of recombinant proteins as inclusion bodies is an important strategy in the production of technical enzymes and biopharmaceutical products. So far, protein from inclusion bodies has been recovered from the cell factory through mechanical or chemical disruption methods, requiring additional cost-intensive unit operations. We describe a novel method that is using a bacteriophage-derived lysis protein to directly recover inclusion body protein from Escherichia coli from high cell density fermentation process: The recombinant inclusion body product is expressed by using a mixed feed fed-batch process which allows expression tuning via adjusting the specific uptake rate of the inducing substrate. Then, bacteriophage ΦX174-derived lysis protein E is expressed to induce cell lysis. Inclusion bodies in empty cell envelopes are harvested via centrifugation of the fermentation broth. A subsequent solubilization step reveals the recombinant protein. The process was investigated by analyzing the impact of fermentation conditions on protein E-mediated cell lysis as well as cell lysis kinetics. Optimal cell lysis efficiencies of 99% were obtained with inclusion body titers of >2.0 g/l at specific growth rates higher 0.12 h^-1 and inducer uptake rates below 0.125 g/(g × h). Protein E-mediated cell disruption showed a first-order kinetics with a kinetic constant of -0.8 ± 0.3 h^-1. This alternative inclusion body protein isolation technique was compared to the one via high-pressure homogenization. SDS gel analysis showed 10% less protein impurities when cells had been disrupted via high-pressure homogenization, than when empty cell envelopes including inclusion bodies were investigated. Within this contribution, an innovative technology, tuning recombinant protein production and substituting cost-intensive mechanical cell disruption, is presented. We anticipate that the presented method will simplify and reduce the production costs of inclusion body processes to produce technical enzymes and biopharmaceutical products.

Can microbes compete with cows for sustainable protein production - A feasibility study on high quality protein

An increasing population and their increased demand for high-protein diets will require dramatic changes in the food industry, as limited resources and environmental issues will make animal derived foods and proteins, gradually more unsustainable to produce. To explore alternatives to animal derived proteins, an economic model was built around the genome-scale metabolic network of E. coli to study the feasibility of recombinant protein production as a food source. Using a novel model, we predicted which microbial production strategies are optimal for economic return, by capturing the tradeoff between the market prices of substrates, product output and the efficiency of microbial production. A case study with the food protein, Bovine Alpha Lactalbumin was made to evaluate the upstream economic feasibilities. Simulations with different substrate profiles at maximum productivity were used to explore the feasibility of recombinant Bovine Alpha Lactalbumin production coupled with market prices of utilized materials. We found that recombinant protein production could be a feasible food source and an alternative to traditional sources.

Robust Parameter Identification to Perform the Modeling of pta and poxB Genes Deletion Effect on Escherichia Coli

The aim of this study was to design a robust parameter identification algorithm to characterize the effect of gene deletion on Escherichia coli (E. coli) MG1655. Two genes (pta and poxB) in the competitive pathways were deleted from this microorganism to inhibit pyruvate consumption. This condition deviated the E. coli metabolism toward the Krebs cycle. As a consequence, the biomass, substrate (glucose), lactic, and acetate acids as well as ethanol concentrations were modified. A hybrid model was proposed to consider the effect of gene deletion on the metabolism of E. coli. The model parameters were estimated by the application of a least mean square method based on the instrument variable technique. To evaluate the parametric identifier method, a set of robust exact differentiators, based on the super-twisting algorithm, was implemented. The hybrid model was successfully characterized by the parameters obtained from experimental information of E. coli MG1655. The significant difference between parameters obtained with wild-type strain and the modified (with deleted genes) justifies the application of the parametric identification algorithm. This characterization can be used to optimize the production of different byproducts of commercial interest.

Efficient process development of recombinant human granulocyte colony-stimulating factor (rh-GCSF) production in Escherichia coli

Background:

The protein hormone granulocyte colony-stimulating factor (GCSF) stimulates the production of white blood cells and plays an important role in medical treatment of cancer patients.

Methods:

An efficient process was developed for heterologous expression of the human GCSF in E. coli BL21 (DE3). The feeding rate was adjusted to achieve the maximum attainable specific growth rate under critical value. In this method, specific growth rate was maintained at the maximum value of 0.55 h^-1 at the beginning of feeding to 0.4 h^-1 at the induction time. Recombinant human GCSF (rh-GCSF) was produced as inclusion body. At first, inclusion bodies were released by cell disruption and then washed, solubilized and refolded. Finally, the rh-GCSF was purified by cation exchange chromatography.

Results:

Obviouly, higher specific growth rate decreases process time and consequently increases productivity. The final concentration of biomass and GCSF was achieved 126 g DCW.l^-1 and 32.1 g.l^-1. Also, the final specific yield (Y_P/X) and total productivity of rh-GCSF were obtained 254 mg.g^-1 DCW and 1.83 g.l^-1.h^-1, respectively. According to the available data, this is one of the highest Y_P/X and productivity that has been reported for any human protein which is expressed in E. coli. Recovery yield of purification process was %40 and purity of recombinant protein was over than 99%. The circular dichroism spectra of purified rh-GCSF, Neupogen® and PD-Grastim showed that all proteins have a similar secondary structure.

Conclusion:

Modified exponential feeding strategy for fed-batch cultivation of recombinant E. coli, results in minimum fed-batch duration and maximum productivity.

Metabolic network capacity of Escherichia coli for Krebs cycle-dependent proline hydroxylation

Background

Understanding the metabolism of the microbial host is essential for the development and optimization of whole-cell based biocatalytic processes, as it dictates production efficiency. This is especially true for redox biocatalysis where metabolically active cells are employed because of the cofactor/cosubstrate regenerative capacity endogenous in the host. Recombinant Escherichia coli was used for overproducing proline-4-hydroxylase (P4H), a dioxygenase catalyzing the hydroxylation of free l-proline into trans-4-hydroxy-l-proline with a-ketoglutarate (a-KG) as cosubstrate. In this whole-cell biocatalyst, central carbon metabolism provides the required cosubstrate a-KG, coupling P4H biocatalytic performance directly to carbon metabolism and metabolic activity. By applying both experimental and computational biology tools, such as metabolic engineering and ¹³C-metabolic flux analysis (¹³C-MFA), we investigated and quantitatively described the physiological, metabolic, and bioenergetic response of the whole-cell biocatalyst to the targeted bioconversion and identified possible metabolic bottlenecks for further rational pathway engineering.

Results

A proline degradation-deficient E. coli strain was constructed by deleting the putA gene encoding proline dehydrogenase. Whole-cell biotransformations with this mutant strain led not only to quantitative proline hydroxylation but also to a doubling of the specific trans-4-l-hydroxyproline (hyp) formation rate, compared to the wild type. Analysis of carbon flux through central metabolism of the mutant strain revealed that the increased a-KG demand for P4H activity did not enhance the a-KG generating flux, indicating a tightly regulated TCA cycle operation under the conditions studied. In the wild type strain, P4H synthesis and catalysis caused a reduction in biomass yield. Interestingly, the ΔputA strain additionally compensated the associated ATP and NADH loss by reducing maintenance energy demands at comparably low glucose uptake rates, instead of increasing the TCA activity.

Conclusions

The putA knockout in recombinant E. coli BL21(DE3)(pLysS) was found to be promising for productive P4H catalysis not only in terms of biotransformation yield, but also regarding the rates for biotransformation and proline uptake and the yield of hyp on the energy source. The results indicate that, upon a putA knockout, the coupling of the TCA-cycle to proline hydroxylation via the cosubstrate a-KG becomes a key factor constraining and a target to further improve the efficiency of a-KG-dependent biotransformations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0298-1) contains supplementary material, which is available to authorized users.

Controlling heterologous gene expression in yeast cell factories on different carbon substrates and across the diauxic shift: a comparison of yeast promoter activities

Background

Predictable control of gene expression is necessary for the rational design and optimization of cell factories. In the yeast Saccharomyces cerevisiae, the promoter is one of the most important tools available for controlling gene expression. However, the complex expression patterns of yeast promoters have not been fully characterised and compared on different carbon sources (glucose, sucrose, galactose and ethanol) and across the diauxic shift in glucose batch cultivation. These conditions are of importance to yeast cell factory design because they are commonly used and encountered in industrial processes. Here, the activities of a series of “constitutive” and inducible promoters were characterised in single cells throughout the fermentation using green fluorescent protein (GFP) as a reporter.

Results

The “constitutive” promoters, including glycolytic promoters, transcription elongation factor promoters and ribosomal promoters, differed in their response patterns to different carbon sources; however, in glucose batch cultivation, expression driven by these promoters decreased sharply as glucose was depleted and cells moved towards the diauxic shift. Promoters induced at low-glucose levels (P_HXT7, P_SSA1 and P_ADH2) varied in induction strength on non-glucose carbon sources (sucrose, galactose and ethanol); in contrast to the “constitutive” promoters, GFP expression increased as glucose decreased and cells moved towards the diauxic shift. While lower than several “constitutive” promoters during the exponential phase, expression from the SSA1 promoter was higher in the post-diauxic phase than the commonly-used TEF1 promoter. The galactose-inducible GAL1 promoter provided the highest GFP expression on galactose, and the copper-inducible CUP1 promoter provided the highest induced GFP expression following the diauxic shift.

Conclusions

The data provides a foundation for predictable and optimised control of gene expression levels on different carbon sources and throughout batch fermentation, including during and after the diauxic shift. This information can be applied for designing expression approaches to improve yields, rates and titres in yeast cell factories.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0278-5) contains supplementary material, which is available to authorized users.

An ancient Chinese wisdom for metabolic engineering: Yin-Yang

In ancient Chinese philosophy, Yin-Yang describes two contrary forces that are interconnected and interdependent. This concept also holds true in microbial cell factories, where Yin represents energy metabolism in the form of ATP, and Yang represents carbon metabolism. Current biotechnology can effectively edit the microbial genome or introduce novel enzymes to redirect carbon fluxes. On the other hand, microbial metabolism loses significant free energy as heat when converting sugar into ATP; while maintenance energy expenditures further aggravate ATP shortage. The limitation of cell "powerhouse" prevents hosts from achieving high carbon yields and rates. Via an Escherichia coli flux balance analysis model, we further demonstrate the penalty of ATP cost on biofuel synthesis. To ensure cell powerhouse being sufficient in microbial cell factories, we propose five principles: 1. Take advantage of native pathways for product synthesis. 2. Pursue biosynthesis relying only on pathways or genetic parts without significant ATP burden. 3. Combine microbial production with chemical conversions (semi-biosynthesis) to reduce biosynthesis steps. 4. Create "minimal cells" or use non-model microbial hosts with higher energy fitness. 5. Develop a photosynthesis chassis that can utilize light energy and cheap carbon feedstocks. Meanwhile, metabolic flux analysis can be used to quantify both carbon and energy metabolisms. The fluxomics results are essential to evaluate the industrial potential of laboratory strains, avoiding false starts and dead ends during metabolic engineering.

Exchange of single amino acids at different positions of a recombinant protein affects metabolic burden in Escherichia coli

Background

Escherichia coli is commonly used in academia and industry for expressing recombinant proteins because of its well-characterized molecular genetics and the availability of numerous expression vectors and strains. One important issue during recombinant protein production is the so-called ‘metabolic burden’: the material and energy normally reserved for microbial metabolism which is sapped from the bacterium to produce the recombinant protein. This material and energy drain harms biomass formation and modifies respiration. To the best of our knowledge, no research has investigated so far whether a single amino acid exchange in a recombinant protein affects the metabolic burden phenomenon. Thus, in this study, 15 E. coli BL21(DE3) clones expressing either the fusion tags, a recombinant wild type lipase, or 13 different lipase variants are investigated to quantitatively analyze the respective effects of single amino acid exchanges at different positions on respiration, biomass and protein production of each clone. Therefore, two small-scale online monitoring systems, namely a Respiration Activity MOnitoring System (RAMOS) and a microtiter plate based cultivation system (BioLector) are applied.

Results

Upon expression of all enzyme variants, strong variations were found in the Oxygen Transfer Rate (OTR), biomass and protein (lipase) production of the respective E. coli clones. Two distinct patterns of respiration behavior were observed and, so, the clones could be classified into two groups (Type A and B). Potential factors to explain these patterns were evaluated (e.g. plasmid copy number, inclusion body formation). However, no decisive factor could yet be identified. Five distinct cultivation phases could be determined from OTR curves which give real-time information about carbon source consumption, biomass and protein production. In general, it was found that the quantity of product increased with the duration of active respiration.

Conclusions

This work demonstrates that single amino acid exchanges in a recombinant protein influence the metabolic burden during protein production. The small-scale online monitoring devices RAMOS and BioLector enable the real-time detection of even smallest differences in respiration behavior, biomass and protein production in the E. coli clones investigated. Hence, this study underscores the importance of parallel online monitoring systems to unveil the relevance of single amino acid exchanges for the recombinant protein production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0191-y) contains supplementary material, which is available to authorized users.

Genome engineering for improved recombinant protein expression in Escherichia coli

A metabolic engineering perspective which views recombinant protein expression as a multistep pathway allows us to move beyond vector design and identify the downstream rate limiting steps in expression. In E.coli these are typically at the translational level and the supply of precursors in the form of energy, amino acids and nucleotides. Further recombinant protein production triggers a global cellular stress response which feedback inhibits both growth and product formation. Countering this requires a system level analysis followed by a rational host cell engineering to sustain expression for longer time periods. Another strategy to increase protein yields could be to divert the metabolic flux away from biomass formation and towards recombinant protein production. This would require a growth stoppage mechanism which does not affect the metabolic activity of the cell or the transcriptional or translational efficiencies. Finally cells have to be designed for efficient export to prevent buildup of proteins inside the cytoplasm and also simplify downstream processing. The rational and the high throughput strategies that can be used for the construction of such improved host cell platforms for recombinant protein expression is the focus of this review.

Production of the short peptide surfactant DAMP4 from glucose or sucrose in high cell density cultures of Escherichia coli BL21(DE3)

BACKGROUND

Peptides are increasingly used in industry as highly functional materials. Bacterial production of recombinant peptides has the potential to provide large amounts of renewable and low cost peptides, however, achieving high product titers from Chemically Defined Media (CDM) supplemented with simple sugars remains challenging.

RESULTS

In this work, the short peptide surfactant, DAMP4, was used as a model peptide to investigate production in Escherichia coli BL21(DE3), a classical strain used for protein production. Under the same fermentation conditions, switching production of DAMP4 from rich complex media to CDM resulted in a reduction in yield that could be attributed to the reduction in final cell density more so than a significant reduction in specific productivity. To maximize product titer, cell density at induction was maximized using a fed-batch approach. In fed-batch DAMP4 product titer increased 9-fold compared to batch, while maintaining 60% specific productivity. Under the fed-batch conditions, the final product titer of DAMP4 reached more than 7 g/L which is the highest titer of DAMP4 reported to date. To investigate production from sucrose, sucrose metabolism was engineered into BL21(DE3) using a simple plasmid approach. Using this strain, growth and DAMP4 production characteristics obtained from CDM supplemented with sucrose were similar to those obtained when culturing the parent strain on CDM supplemented with glucose.

CONCLUSIONS

Production of a model peptide was increased to several grams per liter using a CDM medium with either glucose or sucrose feedstock. It is hoped that this work will contribute cost reduction for production of designer peptide surfactants to facilitate their commercial application.

Impacts of high β-galactosidase expression on central metabolism of recombinant Pichia pastoris GS115 using glucose as sole carbon source via (13)C metabolic flux analysis

The yeast Pichia pastoris GS115 is a widely used microbial cell factory for the production of heterologous protein. In order to reveal the impacts of high heterologous protein expression on the central metabolism of Pichia pastoris GS115 using glucose as sole carbon source, we engineered a high β-galactosidase expression strain P. pastoris G1HL and a low expression control strain P. pastoris GHL through controlling the initiation strength of constitutive promoter pGAP. The carbon flux distributions in these two strains were quantified via (13)C metabolic flux analysis. Compared to the control strain, G1HL showed a lower growth rate, a higher flux through glycolysis pathway, a higher flux through pentose phosphate pathway, and a lower flux through by-products secretion pathway. The metabolic flux redistribution in G1HL was thought to compensate the increased redox cofactors and energy demands caused by the high protein expression. Although the fluxes through Krebs cycle in two engineered strains were almost the same, they were significantly lower than those in wild strain. The enhanced expression of β-galactosidase by glutamate supplementation demonstrated the potential of P. pastoris GS115 to catabolize more carbon through the Krebs cycle for even higher protein expression. In conclusion, our work indicates that P. pastoris GS115 can readjusts the central metabolism for higher heterologous protein expression and provides strategies for strain development or process optimization for enhancing production of heterologous protein.

Model based engineering of Pichia pastoris central metabolism enhances recombinant protein production

The production of recombinant proteins is frequently enhanced at the levels of transcription, codon usage, protein folding and secretion. Overproduction of heterologous proteins, however, also directly affects the primary metabolism of the producing cells. By incorporation of the production of a heterologous protein into a genome scale metabolic model of the yeast Pichia pastoris, the effects of overproduction were simulated and gene targets for deletion or overexpression for enhanced productivity were predicted. Overexpression targets were localized in the pentose phosphate pathway and the TCA cycle, while knockout targets were found in several branch points of glycolysis. Five out of 9 tested targets led to an enhanced production of cytosolic human superoxide dismutase (hSOD). Expression of bacterial β-glucuronidase could be enhanced as well by most of the same genetic modifications. Beneficial mutations were mainly related to reduction of the NADP/H pool and the deletion of fermentative pathways. Overexpression of the hSOD gene itself had a strong impact on intracellular fluxes, most of which changed in the same direction as predicted by the model. In vivo fluxes changed in the same direction as predicted to improve hSOD production. Genome scale metabolic modeling is shown to predict overexpression and deletion mutants which enhance recombinant protein production with high accuracy.

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Recombinant protein production in P. pastoris affects the central metabolism.

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A genome scale metabolic model can predict these metabolic flux changes.

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Mutations in central metabolic genes enhanced recombinant protein yield up to 40%.

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These beneficial mutations were predicted by the metabolic model with high accuracy.

Live-cell imaging of Pol II promoter activity to monitor gene expression with RNA IMAGEtag reporters

We describe a ribonucleic acid (RNA) reporter system for live-cell imaging of gene expression to detect changes in polymerase II activity on individual promoters in individual cells. The reporters use strings of RNA aptamers that constitute IMAGEtags (Intracellular MultiAptamer GEnetic tags) that can be expressed from a promoter of choice. For imaging, the cells are incubated with their ligands that are separately conjugated with one of the FRET pair, Cy3 and Cy5. The IMAGEtags were expressed in yeast from the GAL1, ADH1 or ACT1 promoters. Transcription from all three promoters was imaged in live cells and transcriptional increases from the GAL1 promoter were observed with time after adding galactose. Expression of the IMAGEtags did not affect cell proliferation or endogenous gene expression. Advantages of this method are that no foreign proteins are produced in the cells that could be toxic or otherwise influence the cellular response as they accumulate, the IMAGEtags are short lived and oxygen is not required to generate their signals. The IMAGEtag RNA reporter system provides a means of tracking changes in transcriptional activity in live cells and in real time.

Analyzing clonal variation of monoclonal antibody-producing CHO cell lines using an in silico metabolomic platform

Monoclonal antibody producing Chinese hamster ovary (CHO) cells have been shown to undergo metabolic changes when engineered to produce high titers of recombinant proteins. In this work, we have studied the distinct metabolism of CHO cell clones harboring an efficient inducible expression system, based on the cumate gene switch, and displaying different expression levels, high and low productivities, compared to that of the parental cells from which they were derived. A kinetic model for CHO cell metabolism was further developed to include metabolic regulation. Model calibration was performed using intracellular and extracellular metabolite profiles obtained from shake flask batch cultures. Model simulations of intracellular fluxes and ratios known as biomarkers revealed significant changes correlated with clonal variation but not to the recombinant protein expression level. Metabolic flux distribution mostly differs in the reactions involving pyruvate metabolism, with an increased net flux of pyruvate into the tricarboxylic acid (TCA) cycle in the high-producer clone, either being induced or non-induced with cumate. More specifically, CHO cell metabolism in this clone was characterized by an efficient utilization of glucose and a high pyruvate dehydrogenase flux. Moreover, the high-producer clone shows a high rate of anaplerosis from pyruvate to oxaloacetate, through pyruvate carboxylase and from glutamate to α-ketoglutarate, through glutamate dehydrogenase, and a reduced rate of cataplerosis from malate to pyruvate, through malic enzyme. Indeed, the increase of flux through pyruvate carboxylase was not driven by an increased anabolic demand. It is in fact linked to an increase of the TCA cycle global flux, which allows better regulation of higher redox and more efficient metabolic states. To the best of our knowledge, this is the first time a dynamic in silico platform is proposed to analyze and compare the metabolomic behavior of different CHO clones.

Increasing protein production by directed vector backbone evolution

Recombinant protein production in prokaryotic and eukaryotic organisms was a key enabling technology for the rapid development of industrial and molecular biotechnology. However, despite all progress the improvement of protein production is an ongoing challenge and of high importance for cost-effective enzyme production. With the epMEGAWHOP mutagenesis protocol for vector backbone optimization we report a novel directed evolution based approach to increase protein production levels by randomly introducing mutations in the vector backbone. In the current study we validate the epMEGAWHOP mutagenesis protocol for three different expression systems. The latter demonstrated the general applicability of the epMEGAWHOP method. Cellulase and lipase production was doubled in one round of directed evolution by random mutagenesis of pET28a(+) and pET22b(+) vector backbones. Protease production using the vector pHY300PLK was increased ~4-times with an average of ~1.25 mutations per kb vector backbone. The epMEGAWHOP does not require any rational understanding of the expression machinery and can generally be applied to enzymes, expression vectors and related hosts. epMEGAWHOP is therefore from our point of view a robust, rapid and straight forward alternative for increasing protein production in general and for biotechnological applications.

Proline availability regulates proline-4-hydroxylase synthesis and substrate uptake in proline-hydroxylating recombinant Escherichia coli

Microbial physiology plays a crucial role in whole-cell biotransformation, especially for redox reactions that depend on carbon and energy metabolism. In this study, regio- and enantio-selective proline hydroxylation with recombinant Escherichia coli expressing proline-4-hydroxylase (P4H) was investigated with respect to its interconnectivity to microbial physiology and metabolism. P4H production was found to depend on extracellular proline availability and on codon usage. Medium supplementation with proline did not alter p4h mRNA levels, indicating that P4H production depends on the availability of charged prolyl-tRNAs. Increasing the intracellular levels of soluble P4H did not result in an increase in resting cell activities above a certain threshold (depending on growth and assay temperature). Activities up to 5-fold higher were reached with permeabilized cells, confirming that host physiology and not the intracellular level of active P4H determines the achievable whole-cell proline hydroxylation activity. Metabolic flux analysis revealed that tricarboxylic acid cycle fluxes in growing biocatalytically active cells were significantly higher than proline hydroxylation rates. Remarkably, a catalysis-induced reduction of substrate uptake was observed, which correlated with reduced transcription of putA and putP, encoding proline dehydrogenase and the major proline transporter, respectively. These results provide evidence for a strong interference of catalytic activity with the regulation of proline uptake and metabolism. In terms of whole-cell biocatalyst efficiency, proline uptake and competition of P4H with proline catabolism are considered the most critical factors.

Probing the metabolism of an inducible mammalian expression system using extracellular isotopomer analysis

In an effort to quantitatively assess the impact of recombinant protein expression on the primary metabolism of mammalian cells in culture, we have employed an efficient inducible expression system and conducted a comparative study of the intracellular flux map distribution with and without the induction of recombinant protein synthesis. Cells were grown in parallel semi-continuous cultures with various singly and uniformly labeled substrates and the resulting mass isotopomer distributions of lactate and extracellular amino acids were measured by mass spectrometry. These distributions were used to quantify the main intracellular fluxes. The analysis revealed that, under mild hypothermic conditions, the onset of protein expression is correlated with small but significant changes in several key pathways related to ATP and NADPH formation. More specifically, we observed that induced cells exhibit a more efficient utilization of glucose, characterized by an increased flux of pyruvate into the TCA cycle. In contrast, the catabolic rates of most amino acids remained relatively unaffected. Such analysis is instrumental to guide the identification of robust biomarkers of productivity, as well as the development of medium formulations optimized for recombinant protein production.