A novel strategy for enhancing extracellular secretion of recombinant proteins in Escherichia coli

Metabolic engineering of Escherichia coli for upcycling of polyethylene terephthalate waste to vanillin

Polyethylene terephthalate (PET) waste presents a significant environmental challenge due to its durability and resistance to degradation. Innovative approaches for upcycling PET waste into high-value chemicals can mitigate these issues while contributing to a circular economy. In this study, we developed a multi-enzyme cascade system in E. coli to convert PET-derived monomer terephthalic acid (TPA) into vanillin (VAN). The metabolic engineering approach was then employed to increase VAN production, including 1) inhibition of VAN degradation by knocking out endogenous aldehyde reductases and alcohol dehydrogenases and 2) enhancement of TPA uptake by modifying membrane proteins to increase cell permeability. The engineered E. coli demonstrated a VAN production of 658.55 mg/L from 1992 mg/L of TPA with a molar conversion rate of 71.1 %, representing the highest production of VAN using TPA as the substrate. Additionally, the engineered E. coli effectively converted post-consumer PET waste into VAN under mild conditions, with the highest production of 259.2 mg/L in 20× diluted PET hydrolysates, highlighting its potential for application in PET waste upcycling. This approach not only provides an environmentally friendly alternative to traditional chemical synthesis but also offers substantial economic benefits by transforming low-value waste into high-value chemicals.

Manufacturing of the highly active thermophile PETases PHL7 and PHL7mut3 using Escherichia coli

BACKGROUND

The global plastic waste crisis requires combined recycling strategies. One approach, enzymatic degradation of PET waste into monomers, followed by re-polymerization, offers a circular economy solution. However, challenges remain in producing sufficient amounts of highly active PET-degrading enzymes without costly downstream processes.

RESULTS

Using the growth-decoupled enGenes eX-press V2 E. coli strain, pH, induction strength and feed rate were varied in a factorial-based optimization approach, to find the best-suited production conditions for the PHL7 enzyme. This led to a 40% increase in activity of the fermentation supernatant. Optimization of the expression construct resulted in a further 4-fold activity gain. Finally, the identified improvements were applied to the production of the more active and temperature stable enzyme variant, PHL7mut3. The unpurified fermentation supernatant of the PHL7mut3 fermentation was able to completely degrade our PET film sample after 16 h of incubation at 70 °C at an enzyme loading of only 0.32 mg enzyme per g of PET.

CONCLUSIONS

In this research, we present an optimized process for the extracellular production of thermophile and highly active PETases PHL7 and PHL7mut3, eliminating the need for costly purification steps. These advancements support large-scale enzymatic recycling, contributing to solving the global plastic waste crisis.

Extracellular Secretion and Simple Purification of Bacterial Collagen from Escherichia coli

Because of structural similarities with type-I animal collagen, recombinant bacterial collagen-like proteins have been progressively used as a source of collagen for biomaterial applications. However, the intracellular expression combined with current costly and time-consuming chromatography methods for purification makes the large-scale production of recombinant bacterial collagen challenging. Here, we report the use of an adapted secretion pathway, used natively byEscherichia colito secrete curli fibers, for extracellular secretion of the bacterial collagen. We confirmed that a considerable fraction of expressed collagen (∼70%) is being secreted freely into the extracellular medium, with an initial purity of ∼50% in the crude culture supernatant. To simplify the purification of extracellular collagen, we avoided cell lysis and used cross-flow filtration or acid precipitation to concentrate the voluminous supernatant and separate the collagen from impurities. We confirmed that the secreted collagen forms triple helical structures, using Sirius Red staining and circular dichroism. We also detected collagen biomarkers via Raman spectroscopy, further supporting that the recombinant collagen forms a stable triple helical conformation. We further studied the effect of the isolation methods on the morphology and secondary structure, concluding that the final collagen structure is process-dependent. Overall, we show that the curli secretion system can be adapted for extracellular secretion of the bacterial collagen, eliminating the need for cell lysis, which simplifies the collagen isolation process and enables a simple cost-effective method with potential for scale-up.

Extracellular secretion of a cutinase with polyester-degrading potential by E. coli using a novel signal peptide from Amycolatopsis mediterranei

Recent studies in this laboratory showed that an extracellular cutinase from A. mediterranei (AmCut) was able to degrade the plastics polycaprolactone and polybutylene succinate. Such plastics can be slow to degrade in soils due to a lack of efficient polyester degrading organisms. AmCut also showed potential for the biocatalytic synthesis of esters by reverse hydrolysis. The gene for AmCut has an upstream leader sequence whose transcript is not present in the purified enzyme. In this study, we show using predictive modelling, that this sequence codes for an N-terminal signal peptide that directs transmembrane expression via the Sec secretion pathway. E. coli is a useful host for recombinant enzymes used in biocatalysis due to the ease of genetic manipulation in this organism, which allows tuning of enzymes for specific applications, by mutagenesis. When a truncated GST-tagged AmCut gene (lacking its signal peptide) was expressed in E. coli, all cutinase activity was observed in the cytosolic fraction. However, when GST-tagged AmCut was expressed in E. coli along with its native signal peptide, cutinase activity was observed in both the periplasmic space and the culture medium. This finding revealed that the native signal peptide of a Gram-positive organism (AmCut) was being recognised by the Gram-negative (E. coli) Sec transmembrane transport system. AmCut was transported into E. coli's periplasmic space from where it was released into the culture medium. Surprisingly, the presence of a bulky GST tag at the N-terminus of the signal peptide did not hinder transmembrane targeting. Although the periplasmic targeting was unexpected, it is not unprecedented due to the conservation of the Sec pathway across species. It was more surprising that AmCut was secreted from the periplasmic space into the culture medium. This suggests that extracellular AmCut translocation across the E. coli outer membrane may involve non-classical secretion pathways. This tuneable recombinant E. coli expressing extracellular AmCut may be useful for degradation of polyester substrates in the environment; this and other applications are discussed.

Extracellular production of azurin from Pseudomonas aeruginosa in the presence of Triton X-100 or Tween 80

Azurin which is a bacterial secondary metabolite has attracted much attention as potential anticancer agent in recent years. This copper-containing periplasmic redox protein supresses the tumor growth selectively. High-level secretion of proteins into the culture medium offers a significant advantage over periplasmic or cytoplasmic expression. The aim of this study was to investigate the effect of nonionic surfactants on the expression of the Pseudomonas aeruginosa azurin. Different concentrations of Triton X-100 and Tween 80 were used as supplements in growth media and extracellular azurin production was stimulated by both surfactants. According to western blot analysis results, in the presence of Triton X-100, maximum azurin expression level was achieved with 96 h of incubation at 1% concentration, and 48 h at 2% concentration. On the other hand, maximum azurin expression level was achieved in the presence of 1% Tween 80 at 72 h incubation. This study suggested for the first time a high level of azurin secretion from P. aeruginosa in the presence of Triton X-100 or Tween 80, which would be advantageous for the purification procedure.

Combining fermentation to produce O-succinyl-l-homoserine and enzyme catalysis for the synthesis of l-methionine in one pot

The biosynthetic pathway of l-methionine in microorganisms was complex and regulated at multiple levels. In this study, a two-step method for l-methionine production combined fermentation and biocatalysis was realized in one pot. The O-succinyl-l-homoserine (OSH) producing strain Escherichia coli W3110(DE3) ΔIJB∗TrcmetL/pTrc-metA^fbr-Trc-thrA^fbr-yjeH (ΔIJB) was constructed initially. OSH in the fermentation supernatant was then converted to l-methionine in the presence of O-succinyl-l-homoserine sulfhydrylase (OSHS) and sodium methanethiol. The titer of l-methionine could reach 21.1 g/L after 88 h (84 h fermentation and 4 h catalysis) in a two-step method (process 1). In a one-pot two-strain system (process 2), two strains ΔIJB and E. coli BL21(DE3)/pET28b-OSHS-cutinase were co-cultured, and 8.24 g/L l-methionine was obtained. In another one-pot one-strain system (process 3), strain E. coli ΔIJB/pET28b-OSHS-cutinase could co-express OSHS and cutinase during ΔIJB fermentation at the same time, obtaining 13.6 g/L l-methionine in a 5 L fermentor after 84 h. By comparing the three processes for l-methionine production based on the process 1, the simplified process in process 3 provided in this study showed potent in the large-scale production of l-methionine with convenient handling and production efficiency, but further works still need to be carried out to improve the l-methionine production.

Enhancing extracellular production of recombinant proteins in Escherichia coli by co-expressing with a haloarchaeal protein containing a putative LolA-like domain

Escherichia coli represents one of the most widely used hosts for recombinant protein production, but its limited capacity for producing extracellular proteins is often cited as a drawback. NJ7G_0991 is an extracellular protein of the haloarchaeon Natrinema sp. J7-2 and comprises a signal peptide, a putative LolA-like domain, and a C-terminal domain of unknown function. Here, we found that the full-length (0991) and the C-terminal domain-deletion variant (0991ΔC) of NJ7G_0991, but not its signal peptide-deletion variant (0991ΔS), were efficiently released into the culture supernatant of E. coli without extensive cell lysis as determined by β-galactosidase activity assay. After lysozyme treatment, E. coli cells producing 0991 or 0991ΔC, but not 0991ΔS, were converted from rod-shaped forms to spheres, suggesting that the secretion of 0991 or 0991ΔC into the periplasm leads to an increase of outer membrane permeability of E. coli. A pelB signal peptide was fused to the N-terminus of the LolA-like domain, and the resulting variant PelB-0991ΔC could be released into the culture supernatant of E. coli more efficiently than 0991ΔC. By using PelB-0991ΔC as a co-expression partner, the extracellular production level of a recombinant thermostable subtilase WF146 could be enhanced by up to 14-fold, and the extracellular concentration of an active site variant of WF146 (WF146-SA) reached up to 129 mg/l. To the best of our knowledge, this is the first report on archaeal protein-based co-expression system for extracellular production of recombinant proteins in E. coli. KEY POINTS: • The haloarchaeal protein NJ7G_0991 can be efficiently released into the culture supernatant of E. coli. • The recombinant NJ7G_0991 increases the outer membrane permeability of E. coli. • The LolA-like domain of NJ7G_0991 can be used as a co-expression partner to improve extracellular production of recombinant proteins in E. coli.

Monitoring and control of E. coli cell integrity

Soluble expression of recombinant proteins in E. coli is often done by translocation of the product across the inner membrane (IM) into the periplasm, where it is retained by the outer membrane (OM). While the integrity of the IM is strongly coupled to viability and impurity release, a decrease in OM integrity (corresponding to increased "leakiness") leads to accumulation of product in the extracellular space, strongly impacting the downstream process. Whether leakiness is desired or not, differential monitoring and control of IM and OM integrity are necessary for an efficient E. coli bioprocess in compliance with the guidelines of Quality by Design and Process Analytical Technology. In this review, we give an overview of relevant monitoring tools, summarize the research on factors affecting E. coli membrane integrity and provide a brief discussion on how the available monitoring technology can be implemented in real-time control of E. coli cultivations.

A Fructan Sucrase Secreted Extracellular and Purified in One-Step by Gram-Positive Enhancer Matrix Particles

Fructan sucrase is a kind of biological enzyme that catalyzes the synthesis of fructan, and fructan is a polysaccharide product with important industrial application value. In this study, the Fructan sucrase gene of Bacillus subtilis was cloned to plasmid PET-28A-ACMA-Z, and three clones were obtained after the transformation of Escherichia coli BL21, namely BS-FF, BSO, and BS. The clones BS-FF and BSO secreted the recombinant enzymes outside the cells, while the clone BS expressed them inside the cells. The induction experiment results showed that the optimum IPTG concentration in the medium was 0.5 mM and 1.0 mM for clones BS-FF and BSO, respectively, while the incubation conditions were at 28 °C for 8 h. The recombinant fructan sucrase was purified one step using a material called GEM particles. The results indicated that 95.25% of fructan sucrase expressed by the clone BS-FF could be secreted into the extracellular area, and even 98.78% by the clone BSO. With the above purification system, the receiving rate of the recombinant enzyme for clones BS-FF and BSO was 97.70% and 84.99%, respectively. As for the bioactivity of recombinant fructan sucrase, the optimum temperature and pH were 50 °C and 5.6, respectively. The Km and Vmax of it were 33.96 g/L and 0.63 g/(L·min), respectively. The engineered strains with the high extracellular secretion of fructan sucrase were constructed, and a one-step method for the purification of the recombinant enzyme was established. The results might provide a novel selection for the enzymatic production of fructan on a large scale.

Downregulation of T7 RNA polymerase transcription enhances pET-based recombinant protein production in Escherichia coli BL21 (DE3) by suppressing autolysis

Escherichia coli BL21 (DE3) is an excellent and widely used host for recombinant protein production. Many variant hosts were developed from BL21 (DE3), but improving the expression of specific proteins remains a major challenge in biotechnology. In this study, we found that when BL21 (DE3) overexpressed glucose dehydrogenase (GDH), a significant industrial enzyme, severe cell autolysis was induced. Subsequently, we observed this phenomenon in the expression of 10 other recombinant proteins. This precludes a further increase of the produced enzyme activity by extending the fermentation time, which is not conducive to the reduction of industrial enzyme production costs. Analysis of membrane structure and messenger RNA expression analysis showed that cells could underwent a form of programmed cell death (PCD) during the autolysis period. However, blocking three known PCD pathways in BL21 (DE3) did not completely alleviate autolysis completely. Consequently, we attempted to develop a strong expression host resistant to autolysis by controlling the speed of recombinant protein expression. To find a more suitable protein expression rate, the high- and low-strength promoter lacUV5 and lac were shuffled and recombined to yield the promoter variants lacUV5-1A and lac-1G. The results showed that only one base in lac promoter needs to be changed, and the A at the +1 position was changed to a G, resulting in the improved host BL21 (DE3-lac1G), which resistant to autolysis. As a consequence, the GDH activity at 43 h was greatly increased from 37.5 to 452.0 U/ml. In scale-up fermentation, the new host was able to produce the model enzyme with a high rate of 89.55 U/ml/h at 43 h, compared to only 3 U/ml/h achieved using BL21 (DE3). Importantly, BL21 (DE3-lac1G) also successfully improved the production of 10 other enzymes. The engineered E. coli strain constructed in this study conveniently optimizes recombinant protein overexpression by suppressing cell autolysis, and shows great potential for industrial applications.

Evolution of Escherichia coli Expression System in Producing Antibody Recombinant Fragments

Antibodies and antibody-derived molecules are continuously developed as both therapeutic agents and key reagents for advanced diagnostic investigations. Their application in these fields has indeed greatly expanded the demand of these molecules and the need for their production in high yield and purity. While full-length antibodies require mammalian expression systems due to the occurrence of functionally and structurally important glycosylations, most antibody fragments and antibody-like molecules are non-glycosylated and can be more conveniently prepared in E. coli-based expression platforms. We propose here an updated survey of the most effective and appropriate methods of preparation of antibody fragments that exploit E. coli as an expression background and review the pros and cons of the different platforms available today. Around 250 references accompany and complete the review together with some lists of the most important new antibody-like molecules that are on the market or are being developed as new biotherapeutics or diagnostic agents.

A novel knock out strategy to enhance recombinant protein expression in Escherichia coli

Background

The expression of recombinant proteins triggers a stress response which downregulates key metabolic pathway genes leading to a decline in cellular health and feedback inhibition of both growth and protein expression. Instead of individually upregulating these downregulated genes or improving transcription rates by better vector design, an innovative strategy would be to block this stress response thereby ensuring a sustained level of protein expression.

Results

We postulated that the genes which are commonly up-regulated post induction may play the role of signalling messengers in mounting the cellular stress response. We identified those genes which have no known downstream regulatees and created knock outs which were then tested for GFP expression. Many of these knock outs showed significantly higher expression levels which was also sustained for longer periods. The highest product yield (Y_p/x) was observed in a BW25113ΔcysJ knock out (Y_p/x 0.57) and BW25113ΔelaA (Y_p/x 0.49), whereas the Y_p/x of the control W3110 strain was 0.08 and BW25113 was 0.16. Double knock out combinations were then created from the ten best performing single knock outs leading to a further enhancement in expression levels. Out of 45 double knock outs created, BW25113ΔelaAΔyhbC (Y_p/x 0.7) and BW25113ΔcysJΔyhbC (Y_p/x 0.64) showed the highest increase in product yield compared to the single gene mutant strains. We confirmed the improved performance of these knock outs by testing and obtaining higher levels of recombinant asparaginase expression, a system better suited for analysing sustained expression since it gets exported to the extracellular medium.

Conclusion

Creating key knock outs to block the CSR and enhance expression is a radically different strategy that can be synergistically combined with traditional methods of improving protein yields thus helping in the design of superior host platforms for protein expression.

Principle and potential applications of the non-classical protein secretory pathway in bacteria

In addition to the extracellular proteins secreted by known secretory pathways, a number of cytoplasmic proteins without predicable or known signal sequences or secretory motifs have been found in the extracellular milieu, and were consequently classified as non-classically secreted proteins. Non-classical protein secretion is considered to be a general, conserved cellular phenomenon in both eukaryotes and prokaryotes. There are several research hotspots on the non-classical protein secretory pathway, and the most important two of them are the recognition principle of substrate proteins and possible secretory mechanisms. To date, researchers have made some progress in understanding the characteristics of these proteins. For example, it was discovered that many non-classically secreted proteins exist and are secreted in multimeric form. Some of these proteins prefer to be clustered and exported at the poles and the septum of the cell. The majority of these proteins play different functions when they are in the intra- and extracellular environments, and several of their functions are related to survival and pathogenicity. Furthermore, non-classically secreted proteins can be used as leading proteins to guide a POI (protein of interest) out of the cells, which provides a novel strategy for protein secretion with potential applications in the industry. Summarizing these findings, this review emphasizes the hot spots related to non-classically secreted proteins in bacteria, lists the most important hypotheses on the selection and secretion mechanisms of non-classically secreted proteins, and put forward their potential applications.

Extracellular production of recombinant N-glycosylated anti-VEGFR2 monobody in leaky Escherichia coli strain

OBJECTIVE

To improve the production yield of N-glycosylated anti-VEGFR2 (vascular endothelial growth factor receptor 2) monobody (FN3_VEGFR2-Gly) in lpp knockout Escherichia coli cells harboring Campylobacter jejuni N-glycosylation pathway.

RESULTS

The leaky CLM37-Δlpp strain efficiently secreted FN3_VEGFR2-Gly into culture medium. The extracellular levels of glycosylated FN3_VEGFR2-Gly in CLM37-Δlpp culture medium were approximately 11 and 15 times higher compared to those in CLM37 cells via IPTG and auto-induction, respectively. In addition, the highest level of total glycosylated FN3_VEGFR2-Gly (70 ± 3.4 mg/L) was found in culture medium via auto-induction. Furthermore, glycosylated FN3_VEGFR2-Gly was more stable than unglycosylated FN3_VEGFR2-Gly in this expression system, but their bioactivities were relatively similar.

CONCLUSIONS

Lpp knockout leaky E. coli strain combined with auto-induction method can enhance the extracellular production of homogenous N-glycosylated FN3_VEGFR2-Gly, and facilitate the downstream protein purification. The findings of this study may provide practical implications for the large-scale production and cost-effective harvesting of N-glycosylation proteins.

A novel autolysis system for extracellular production and direct immobilization of a phospholipase D fused with cellulose binding domain

Background

Several types of phospholipases have been described in phospholipids modification. The majority of phospholipase D (PLD) superfamily members can catalyze two separate reactions: the hydrolysis of phospholipids to produce phosphatidic acid (PA) and the transphosphatidylation of phosphatidyl groups into various phosphatidyl alcohols to produce modified phospholipids. Transphosphatidylation is a useful biocatalytic method for the synthesis of functional phospholipids from lecithin or phosphatidylcholine (PC), which are both easily accessible. Different PLD coding genes have been cloned from various sources from viral, prokaryotic, and eukaryotic organisms. Despite the catalytic potential of PLD, their low productivity has hampered their practical applications, probably because PLD, which is highly toxic to the host cells, when transformation of the PLD genes into the host cells, degrade PLs in the cell membrane. In this study, we designed a novel two-step expression system to produce and secrete recombinant PLD in extracellular medium, cellulose-binding domains as an affinity fused with PLD for immobilization and purification proteins.

Results

The engineered BL21 (DE3) host strain, which harbored the final expression vector pET28a-PLD-CBD-araC-ESN, was induced by IPTG and L-arabinose, the cell density decreased rapidly over a 2 h period and the enzymes released into the extracellular medium accounts owned 81.75% hydrolytic activity. Scanning electron microscopy results showed that there were obvious structural changes on the cell surface. The extracellularly secreted PLD-CBD powder was used to catalyze the transphosphatidylation reaction synthesis of phosphatidylserine, 2.3 U enzymes reacted for 12 h, during which the conversion rate reached 99% with very few by-products being produced. When the fused protein PLD-CBD immobilized on microcrystalline cellulose, the enzymes can be cycle used five times with 26% conversion rate was preserved.

Conclusions

This study introduced an effective method for use in the expression of recombinant proteins and their extracellular secretion that simplifies the steps of sonication and purification and demonstrates great potential in the industrial application of enzymes. Cellulose as the most abundant renewable biomass resources in nature, and the cost is low, used for PLD immobilization make it more simple, effective and sustainable.

Secretion of recombinant proteins from E. coli

The microorganism Escherichia coli is commonly used for recombinant protein production. Despite several advantageous characteristics like fast growth and high protein yields, its inability to easily secrete recombinant proteins into the extracellular medium remains a drawback for industrial production processes. To overcome this limitation, a multitude of approaches to enhance the extracellular yield and the secretion efficiency of recombinant proteins have been developed in recent years. Here, a comprehensive overview of secretion mechanisms for recombinant proteins from E. coli is given and divided into three main sections. First, the structure of the E. coli cell envelope and the known natural secretion systems are described. Second, the use and optimization of different one- or two-step secretion systems for recombinant protein production, as well as further permeabilization methods are discussed. Finally, the often-overlooked role of cell lysis in secretion studies and its analysis are addressed. So far, effective approaches for increasing the extracellular protein concentration to more than 10 g/L and almost 100% secretion efficiency exist, however, the large range of optimization methods and their combinations suggests that the potential for secretory protein production from E. coli has not yet been fully realized.

Enhanced extracellular production of recombinant proteins in Escherichia coli by co-expression with Bacillus cereus phospholipase C

BACKGROUND

Our laboratory has reported a strategy for improving the extracellular production of recombinant proteins through co-expression with Thermobifida fusca cutinase, which increases membrane permeability via its phospholipid hydrolysis activity. However, the foam generated by the lysophospholipid product makes the fermentation process difficult to control in a fermentor. Phospholipase C (PLC) catalyzes the hydrolysis of phospholipids to produce sn1,2-diacylglycerides and organic phosphate, which do not induce foam formation. Therefore, co-expression with Bacillus cereus PLC was investigated as a method to improve the extracellular production of recombinant proteins.

RESULTS

When B. cereus PLC was expressed in Escherichia coli without its signal peptide, 95.3% of the total PLC activity was detected in the culture supernatant. PLC expression enhanced membrane permeability without obvious cell lysis. Then, six test enzymes, three secretory and three cytosolic, were co-expressed with B. cereus PLC. The enhancement of extracellular production correlated strongly with the molecular mass of the test enzyme. Extracellular production of Streptomyces sp. FA1 xylanase (43 kDa), which had the lowest molecular mass among the secretory enzymes, was 4.0-fold that of its individual expression control. Extracellular production of glutamate decarboxylase (51 kDa), which had the lowest molecular mass among the cytosolic enzymes, reached 26.7 U/mL; 88.3% of the total activity produced. This strategy was effectively scaled up using a 3-L fermentor. No obvious foam was generated during this fermentation process.

CONCLUSIONS

This is the first study to detail the enhanced extracellular production of recombinant proteins through co-expression with PLC. This new strategy, which is especially appropriate for lower molecular mass proteins, allows large-scale protein production in an easily controlled fermentation process.

Extracellular expression of glutamate decarboxylase B in Escherichia coli to improve gamma-aminobutyric acid production

Escherichia coli overexpressing glutamate decarboxylase GadB can produce gamma-aminobutyric acid with addition of monosodium glutamate. The yield and productivity of gamma-aminobutyric acid might be significantly improved if the overexpressed GadB in E. coli cells can be excreted outside, where it can directly transforms monosodium glutamate to gamma-aminobutyric acid. In this study, GadB was fused to signal peptides TorA or PelB, respectively, and overexpressed in E. coli BL21(DE3). It was found that TorA could facilitate GadB secretion much better than PelB. Conditions for GadB secretion and gamma-aminobutyric acid production were optimized in E. coli BL21(DE3)/pET20b-torA-gadB, leading the secretion of more than half of the overexpressed GadB. Fed-batch fermentation for GadB expression and gamma-aminobutyric acid production of BL21(DE3)/pET20b-torA-gadB was sequentially performed in one fermenter; 264.4 and 313.1 g/L gamma-aminobutyric acid were obtained with addition of monosodium glutamate after 36 and 72 h, respectively.

Improving the thermostability and enhancing the Ca(2+) binding of the maltohexaose-forming α-amylase from Bacillus stearothermophilus

The thermostability of the maltohexaose-forming α-amylase from Bacillus stearothermophilus (AmyMH) without added Ca(2+) was improved through structure-based rational design in this study. Through comparison of a homologous model structure of AmyMH with the crystal structure of the thermostable α-amylase from Bacillus licheniformis, Ser242, which located at the beginning of fourth α-helix of the central (β/α)8 barrel was selected for mutation to improve thermostability. In addition, an amide-containing side chain (Asn193) and a loop in domain B (ΔIG mutation), which have been proven to be important for thermostability in corresponding position of other α-amylases, were also investigated. Five mutants carrying the mutations ΔIG, N193F, S242A, ΔIG/N193F, and ΔIG/N193F/S242A were generated and their proteins characterized. The most thermostable mutant protein, ΔIG/N193F/S242A, exhibited a 26-fold improvement in half-life at 95°C compared to the wild-type enzyme without added Ca(2+). Mutant ΔIG/N193F/S242A also exhibited substantially better activity and stability in the presence of the chelator EDTA, demonstrating enhanced Ca(2+) binding. These results suggest that mutant ΔIG/N193F/S242A has potential for use in the industrial liquefaction of starch.

Extracellular expression of natural cytosolic arginine deiminase from Pseudomonas putida and its application in the production of L-citrulline

The Pseudomonas putida arginine deiminase (ADI), a natural cytosolic enzyme, and Thermobifida fusca cutinase were co-expressed in Escherichia coli, and the optimized cutinase gene was used for increasing its expression level. 90.9% of the total ADI protein was released into culture medium probably through a nonspecific leaking mechanism caused by the co-expressed cutinase. The enzymatic properties of the extracellular ADI were found to be similar to those of ADI prepared by conventional cytosolic expression. Extracellular production of ADI was further scaled up in a 3-L fermentor. When the protein expression was induced by IPTG (25.0μM) and lactose (0.1gL(-1)h(-1)) at 30°C, the extracellular ADI activity reached 101.2UmL(-1), which represented the highest ADI production ever reported. In addition, the enzymatic synthesis of l-citrulline was performed using the extracellularly expressed ADI, and the conversion rate reached 100% with high substrate concentration at 650gL(-1).

Extracellular expression of Thermobifida fusca cutinase with pelB signal peptide depends on more than type II secretion pathway in Escherichia coli

Our previous studies demonstrated that Thermobifida fusca cutinase is released into culture medium when expressed without a signal peptide in Escherichia coli, and this extracellular expression results from an enhanced membrane permeability caused by cutinase's phospholipid hydrolase activity. The present study investigated whether this phenomenon would also occur during the expression of cutinase fused to pelB signal peptide (pelB-cutinase). Secretion of fusion proteins of this type is generally believed to occur via type II secretion pathway. The results showed that when pelB-cutinase was expressed in a secB knockout strain, which has a defective type II secretion pathway, there was still a large amount of cutinase in the culture medium. Additional experiments confirmed that the periplasmic and cytoplasmic fractions of the expressing cells had hydrolytic activity toward phosphatidyl ethanolamine, and the recombinant cells showed correspondingly improved membrane permeability. All these phenomena were also observed in the parent E. coli strain. Moreover, the secretion efficiency of the inactive cutinase mutant was found to be significantly lower than that of pelB-cutinase in the parent E. coli. Based on these results, the phospholipid hydrolase activity of pelB-cutinase must play a larger role in its extracellular production than does type II secretion pathway.

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.

Biosorption and biodegradation of triphenyltin by Stenotrophomonas maltophilia and their influence on cellular metabolism

Triphenyltin (TPT), an endocrine disruptor, is polluting the global environment through its worldwide use. However, information concerning the mechanisms of TPT biodegradation and cellular metabolism is severely limited. Therefore, these processes were elucidated through experiments involving TPT biosorption and degradation, intracellular metabolite analysis, nutrient use, ion and monosaccharide release, cellular membrane permeability and protein concentration quantification. The results verified that TPT was initially adsorbed by the cell surface of Stenotrophomonas maltophilia and was subsequently transported and degraded intracellularly with diphenyltin and monophenyltin production. Cl(-), Na(+), arabinose and glucose release, membrane permeability and the extracellular protein concentration increased during TPT treatment, whereas K(+) and PO4(3-) utilization and intracellular protein concentration declined. The biosorption, degradation and removal efficiencies of TPT at 0.5mgL(-1) by 0.3gL(-1) viable cells at 10 d were 3.8, 77.8 and 86.2%, respectively, and the adsorption efficiency by inactivated cells was 72.6%.