Bacterial whole-cell biocatalysts by surface display of enzymes: toward industrial application

Surface displayed MerR increases mercury accumulation by green microalga Chlamydomonas reinhardtii

Mercury is a highly toxic trace metal that can accumulate in aquatic ecosystems and when resent at high concentrations can pose risks to both aquatic life and humans consuming contaminated fish. This research explores the use of the metalloregulatory protein MerR, known for its high affinity and selectivity toward mercury, in a novel application. Through a cell surface engineering approach, MerR was displayed on cells of green alga Chlamydomonas reinhardtii. A hydroxyproline-rich GP1 protein was used as an anchor to construct the engineered strains GP1-MerR that expresses the fluorescent protein mVenus. The surface engineered GP1-MerR strain led up to five folds higher Hg2+ accumulation compared to the WT strain at concentration range from 10-9 to 10-7 M Hg2+. The binding of Hg2+ via MerR was specific and did not get significantly affected by major freshwater water quality variables such as Ca2+ and dissolved organic matter. The presence of other trace metals (Zn2+, Cu2+, Ni2+, Pb2+, Cd2+) in a same concentration range even resulted in 30-40 % increase in the accumulated Hg. Further, the engineered cells also demonstrated the ability to accumulate Hg2+ from the water extracts of the Hg-contaminated sediment samples. These results demonstrate a novel approach utilizing the cell surface display system in C. reinhardtii for its potential application in bioremediation.

Surface expression of carbonic anhydrase on E. coli as a sustainable approach for enzymatic CO2 capture

The utilisation of carbonic anhydrase (CA) in CO2 sequestration is becoming prominent as an efficient, environment friendly and rapid catalyst for capturing CO2 from industrial emissions. However, the application of CA enzyme in soluble form is constrained due to its poor stability in operational conditions of CO2 capture and also production cost of the enzyme. Addressing these limitations, the present study focuses on the surface display of CA from Bacillus halodurans (BhCA) on E coli aiming to contribute to the cost-effectiveness of carbon capture through CA technology. This involved the fusion of the BhCA-encoding gene with the adhesion molecule involved in diffuse adherence (AIDA-I) autotransporter, resulting in the efficient display of BhCA (595 ± 60 U/gram dry cell weight). Verification of the surface display of BhCA was accomplished by conjugating with FITC labelled anti-his antibody followed by fluorescence-activated cell sorting (FACS) and cellular fractionation in conjunction with zymography. Biochemical characterisation of whole-cell biocatalyst revealed a noteworthy enhancement in thermostability, improvement in the thermostability with T1/2 of 90 ± 1.52 minutes at 50 ˚C, 36 ± 2.51 minutes at 60 ˚C and18 ± 1.52 minutes at 80˚C. Surface displayed BhCA displayed remarkable reusability retaining 100% activity even after 15 cycles. Surface displayed BhCA displayed highly alkali stable nature like free counterpart in solution. The alkali stability of the surface-displayed BhCA was comparable to its free counterpart in solution. Furthermore, the study investigated the impact of different metal ions, modulators, and detergents on the whole-cell biocatalysts. The present work represents the first report on surface display of CA utilising the AIDA-1 autotransporter.

Surface Engineering of Escherichia coli to Display Its Phytase (AppA) and Functional Analysis of Enzyme Activities

Escherichia coli phytase (AppA) is widely used as an exogenous enzyme in monogastric animal feed mainly because of its ability to degrade phytic acid or its salt (phytate), a natural source of phosphorus. Currently, successful recombinant production of soluble AppA has been achieved by gene overexpression using both bacterial and yeast systems. However, some methods for the biomembrane immobilization of phytases (including AppA), such as surface display on yeast cells and bacterial spores, have been investigated to avoid expensive enzyme purification processes. This study explored a homologous protein production approach for displaying AppA on the cell surface of E. coli by engineering its outer membrane (OM) for extracellular expression. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of total bacterial lysates and immunofluorescence microscopy of non-permeabilized cells revealed protein expression, whereas activity assays using whole cells or OM fractions indicated functional enzyme display, as evidenced by consistent hydrolytic rates on typical substrates (i.e., p-nitrophenyl phosphate and phytic acid). Furthermore, the in vitro results obtained using a simple method to simulate the gastrointestinal tract of poultry suggest that the whole-cell biocatalyst has potential as a feed additive. Overall, our findings support the notion that biomembrane-immobilized enzymes are reliable for the hydrolysis of poorly digestible substrates relevant to animal nutrition.

A Critical Review on Immobilized Sucrose Isomerase and Cells for Producing Isomaltulose

Isomaltulose is a novel sweetener and is considered healthier than the common sugars, such as sucrose or glucose. It has been internationally recognized as a safe food product and holds vast potential in pharmaceutical and food industries. Sucrose isomerase is commonly used to produce isomaltulose from the substrate sucrose in vitro and in vivo. However, free cells/enzymes were often mixed with the product, making recycling difficult and leading to a significant increase in production costs. Immobilized cells/enzymes have the following advantages including easy separation from products, high stability, and reusability, which can significantly reduce production costs. They are more suitable than free ones for industrial production. Recently, immobilized cells/enzymes have been encapsulated using composite materials to enhance their mechanical strength and reusability and reduce leakage. This review summarizes the advancements made in immobilized cells/enzymes for isomaltulose production in terms of refining traditional approaches and innovating in materials and methods. Moreover, innovations in immobilized enzyme methods include cross-linked enzyme aggregates, nanoflowers, inclusion bodies, and directed affinity immobilization. Material innovations involve nanomaterials, graphene oxide, and so on. These innovations circumvent challenges like the utilization of toxic cross-linking agents and enzyme leakage encountered in traditional methods, thus contributing to enhanced enzyme stability.

Quantitative measurement of cell-surface displayed proteins based on split-GFP assembly

BACKGROUND

Microbial cell surface display technology allows immobilizing proteins on the cell surface by fusing them to anchoring motifs, thereby endowing the cells with diverse functionalities. However, the assessment of successful protein display and the quantification of displayed proteins remain challenging. The green fluorescent protein (GFP) can be split into two non-fluorescent fragments, while they spontaneously assemble and emit fluorescence when brought together through complementation. Based on split-GFP assembly, we aim to: (1) confirm the success display of passenger proteins, (2) quantify the number of passenger proteins displayed on individual cells.

RESULTS

In this study, we propose two innovative methods based on split-green fluorescent protein (split-GFP), named GFP1-10/GFP11 and GFP1-9/GFP10-11 assembly, for the purpose of confirming successful display and quantifying the number of proteins displayed on individual cells. We evaluated the display efficiency of SUMO and ubiquitin using different anchor proteins to demonstrate the feasibility of the two split-GFP assembly systems. To measure the display efficiency of functional proteins, laccase expression was measured using the split-GFP assembly system by co-displaying GFP11 or GFP10-11 tags, respectively.

CONCLUSIONS

Our study provides two split-GFP based methods that enable qualitative and quantitative analyses of individual cell display efficiency with a simple workflow, thus facilitating further comprehensive investigations into microbial cell surface display technology. Both split-GFP assembly systems offer a one-step procedure with minimal cost, simplifying the fluorescence analysis of surface-displaying cells.

Whole-Cell Display of Phospholipase D in Escherichia coli for High-Efficiency Extracellular Phosphatidylserine Production

Phospholipids are widely utilized in various industries, including food, medicine, and cosmetics, due to their unique chemical properties and healthcare benefits. Phospholipase D (PLD) plays a crucial role in the biotransformation of phospholipids. Here, we have constructed a super-folder green fluorescent protein (sfGFP)-based phospholipase D (PLD) expression and surface-display system in Escherichia coli, enabling the surface display of sfGFP-PLDr34 on the bacteria. The displayed sfGFP-PLDr34 showed maximum enzymatic activity at pH 5.0 and 45 °C. The optimum Ca2+ concentrations for the transphosphatidylation activity and hydrolysis activity are 100 mM and 10 mM, respectively. The use of displayed sfGFP-PLDr34 for the conversion of phosphatidylcholine (PC) and L-serine to phosphatidylserine (PS) showed that nearly all the PC was converted into PS at the optimum conditions. The displayed enzyme can be reused for up to three rounds while still producing detectable levels of PS. Thus, Escherichia coli/sfGFP-PLD shows potential for the feasible industrial-scale production of PS. Moreover, this system is particularly valuable for quickly screening higher-activity PLDs. The fluorescence of sfGFP can indicate the expression level of the fused PLD and changes that occur during reuse.

Split-GFP complementation at the bacterial cell surface for antibody-free labeling and quantification of heterologous protein display

The split-GFP system is a versatile tool with numerous applications, but it has been underutilized for the labeling of heterologous surface-displayed proteins. By inserting the 16 amino acid sequence of the GFP11-tag between a protein of interest and an autotransporter protein, it is possible to present a protein at the outer membrane of gram-negative bacteria and to fluorescently label it by complementation with externally added GFP1-10. The labeled cells could be clearly discerned from cells without the protein of interest using flow cytometry and the insertion of the GFP11-tag caused no significant alteration of the catalytic activity for the tested model enzyme CsBglA. Furthermore, the amount of the protein of interest on the cells could be quantified by comparing the green fluorescence resulting from the complementation to that of standards with known concentrations. This allows a precise characterization of whole-cell biocatalysts, which is difficult with existing methods. The split-GFP complementation approach was shown to be specific, in a similar manner as commercial antibodies. It is cost-efficient, minimizes the possibility of adverse effects on protein expression or solubility, and can be performed at high throughput.

Combining lipid-mimicking-enabled transition metal and enzyme-mediated catalysis at the cell surface of E. coli

Being an essential multifunctional platform and interface to the extracellular environment, the cell membrane constitutes a valuable target for the modification and manipulation of cells and cellular behavior, as well as for the implementation of artificial, new-to-nature functionality. While bacterial cell surface functionalization via expression and presentation of recombinant proteins has extensively been applied, the corresponding application of functionalizable lipid mimetics has only rarely been reported. Herein, we describe an approach to equip E. coli cells with a lipid-mimicking, readily membrane-integrating imidazolium salt and a corresponding NHC-palladium complex that allows for flexible bacterial membrane surface functionalization and enables E. coli cells to perform cleavage of propargyl ethers present in the surrounding cell medium. We show that this approach can be combined with already established on-surface functionalization, such as bacterial surface display of enzymes, i.e. laccases, leading to a new type of cascade reaction. Overall, we envision the herein presented proof-of-concept studies to lay the foundation for a multifunctional toolbox that allows flexible and broadly applicable functionalization of bacterial membranes.

Enhancing the organic solvent resistance of ω-amine transaminase for enantioselective synthesis of (R)-(+)-1(1-naphthyl)-ethylamine

BACKGROUND

Biocatalysis in high-concentration organic solvents has been applied to produce various industrial products with many advantages. However, using enzymes in organic solvents often suffers from inactivation or decreased catalytic activity and stability. An R-selective ω-amine transaminase from Aspergillus terreus (AtATA) exhibited activity toward 1-acetylnaphthalene. However, AtATA displayed unsatisfactory organic solvent resistance, which is required to enhance the solubility of the hydrophobic substrate 1-acetylnaphthalene. So, improving the tolerance of enzymes in organic solvents is essential.

MAIN METHODS AND RESULTS

The method of regional random mutation combined with combinatorial mutation was used to improve the resistance of AtATA in organic solvents. Enzyme surface areas are structural elements that undergo reversible conformational transitions, thus affecting the stability of the enzyme in organic solvents. Herein, three surface areas containing three loops were selected as potential mutation regions. And the "best" mutant T23I/T200K/P260S (M3) was acquired. In different concentrations of dimethyl sulfoxide (DMSO), the catalytic efficiency (kcat /Km ) toward 1-acetylnaphthalene and the stability (half-life t1/2 ) were higher than the wild-type (WT) of AtATA. The results of decreased Root Mean Square Fluctuation (RMSF) values via 20-ns molecular dynamics (MD) simulations under 15%, 25%, 35%, and 45% DMSO revealed that mutant M3 had lower flexibility, acquiring a more stable protein structure and contributing to its organic solvents stability than WT. Furthermore, M3 was applied to convert 1-acetylnaphthalene for synthesizing (R)-(+)-1(1-naphthyl)-ethylamine ((R)-NEA), which was an intermediate of Cinacalcet Hydrochloride for the treatment of secondary hyperthyroidism and hypercalcemia. Moreover, in a 20-mL scale-up experiment, 10 mM 1-acetylnaphthalene can be converted to (R)-NEA with 85.2% yield and a strict R-stereoselectivity (enantiomeric excess (e.e.) value >99.5%) within 10 h under 25% DMSO.

CONCLUSION

The beneficial mutation sites were identified to tailor AtATA's organic solvents stability via regional random mutation. The "best" mutant T23I/T200K/P260S (M3) holds great potential application for the synthesis of (R)-NEA.

Display of Oligo-α-1,6-Glycosidase from Exiguobacterium sibiricum on the Surface of Escherichia coli Cells

Cell-surface display using anchor motifs of outer membrane proteins allows exposure of target peptides and proteins on the surface of microbial cells. Previously, we obtained and characterized highly catalytically active recombinant oligo-α-1,6-glycosidase from the psychrotrophic bacterium Exiguobacterium sibiricum (EsOgl). It was also shown that the autotransporter AT877 from Psychrobacter cryohalolentis and its deletion variants efficiently displayed type III fibronectin (¹⁰Fn3) domain 10 on the surface of Escherichia coli cells. The aim of the work was to obtain an AT877-based system for displaying EsOgl on the surface of bacterial cells. The genes for the hybrid autotransporter EsOgl877 and its deletion mutants EsOgl877Δ239 and EsOgl877Δ310 were constructed, and the enzymatic activity of EsOgl877 was investigated. Cells expressing this protein retained ~90% of the enzyme maximum activity within a temperature range of 15-35°C. The activity of cells expressing EsOgl877Δ239 and EsOgl877Δ310 was 2.7 and 2.4 times higher, respectively, than of the cells expressing the full-size AT. Treatment of cells expressing EsOgl877 deletion variants with proteinase K showed that the passenger domain localized to the cell surface. These results can be used for further optimization of display systems expressing oligo-α-1,6-glycosidase and other heterologous proteins on the surface of E. coli cells.

Whole-cell bioconversion using non-Leloir transglycosylation reactions: a review

Microbial biocatalysts are evolving technological tools for glycosylation research in food, feed and pharmaceuticals. Advances in bioengineered Leloir and non-Leloir carbohydrate-active enzymes allow for whole-cell biocatalysts to curtail production costs of purified enzymes while enhancing glucan synthesis through continued enzyme expression. Unlike sugar nucleotide-dependent Leloir glycosyltransferases, non-Leloir enzymes require inexpensive sugar donors and can be designed to match the high value, yield and selectivity of the former. This review addresses the current state of bacterial cell-based production of glucans and glycoconjugates via transglycosylation, and describes how alterations made to microbial hosts to surpass purified enzymes as the preferred mode of catalysis are steadily being acquired through genetic engineering, rational design and process optimization. A comprehensive exploration of relevant literature has been summarized to describe whole-cell biocatalysis in non-Leloir glycosylation reactions with various donors and acceptors, and the characterization, application and latest developments in the optimization of their use.

30th Annual GP2A Medicinal Chemistry Conference

The Group for the Promotion of Pharmaceutical Chemistry in Academia (GP2A) held their 30th annual conference in August 2022 in Trinity College Dublin, Ireland. There were 9 keynote presentations, 10 early career researcher presentations and 41 poster presentations.

Yeast Cell Surface Engineering of a Nicotinamide Riboside Kinase for the Production of β-Nicotinamide Mononucleotide via Whole-Cell Catalysis

β-Nicotinamide mononucleotide (NMN) has been widely used as a nutraceutical for self-medication. The one-step conversion of nicotinamide riboside (NR) to β-NMN has been considered to be the most promising synthetic route for β-NMN. Here, human nicotinamide riboside kinase 2 (NRK-2) was functionally displayed on the cell surface of Saccharomyces cerevisiae EBY100, forming a whole-cell biocatalyst (Whole-cell NRK-2). Whole-cell NRK-2 could convert nicotinamide riboside (NR) to β-NMN efficiently in the presence of ATP and Mg²⁺, with a maximal activity of 64 IU/g (dry weight) and a K_m of 3.5 μM, similar to that of free NRK-2 reported previously. To get the best reaction conditions for β-NMN synthesis, the amounts of NR, ATP, and Mg²⁺ used, pH, and temperature for the synthetic reaction were optimized. Using Whole-cell NRK-2 as the catalyst under the optimized conditions, β-NMN synthesized from NR reached a maximal conversion rate of 98.2%, corresponding to 12.6 g/L of β-NMN in the reaction mixture, which was much higher than those of synthetic processes reported. Additionally, Whole-cell NRK-2 had good pH stability and thermostability, required no complicated treatments before or after use, and could be reused in sequential production. Therefore, this study provided a safe, stable, highly effective, and low-cost biocatalyst for the preparation of β-NMN, which has great potential in industrial production.

Development of highly efficient whole-cell catalysts of cis-epoxysuccinic acid hydrolase by surface display

Bacterial cis-epoxysuccinic acid hydrolases (CESHs) are intracellular enzymes used in the industrial production of enantiomeric tartaric acids. The enzymes are mainly used as whole-cell catalysts because of the low stability of purified CESHs. However, the low cell permeability is the major drawback of the whole-cell catalyst. To overcome this problem, we developed whole-cell catalysts using various surface display systems for CESH[L] which produces L(+)-tartaric acid. Considering that the display efficiency depends on both the carrier and the passenger, we screened five different anchoring motifs in Escherichia coli. Display efficiencies are significantly different among these five systems and the InaPbN-CESH[L] system has the highest whole-cell enzymatic activity. Conditions for InaPbN-CESH[L] production were optimized and a maturation step was discovered which can increase the whole-cell activity several times. After optimization, the total activity of the InaPbN-CESH[L] surface display system is higher than the total lysate activity of an intracellular CESH[L] overexpression system, indicating a very high CESH[L] display level. Furthermore, the whole-cell InaPbN-CESH[L] biocatalyst exhibited good storage stability at 4 °C and considerable reusability. Thereby, an efficient whole-cell CESH[L] biocatalyst was developed in this study, which solves the cell permeability problem and provides a valuable system for industrial L(+)-tartaric acid production.

Display of a novel carboxylesterase CarCby on Escherichia coli cell surface for carbaryl pesticide bioremediation

Background

Carbamate pesticides have been widely used in agricultural and forestry pest control. The large-scale use of carbamates has caused severe toxicity in various systems because of their toxic environmental residues. Carbaryl is a representative carbamate pesticide and hydrolase/carboxylesterase is the initial and critical enzyme for its degradation. Whole-cell biocatalysts have become a powerful tool for environmental bioremediation. Here, a whole cell biocatalyst was constructed by displaying a novel carboxylesterase/hydrolase on the surface of Escherichia coli cells for carbaryl bioremediation.

Results

The carCby gene, encoding a protein with carbaryl hydrolysis activity was cloned and characterized. Subsequently, CarCby was displayed on the outer membrane of E. coli BL21(DE3) cells using the N-terminus of ice nucleation protein as an anchor. The surface localization of CarCby was confirmed by SDS–PAGE and fluorescence microscopy. The optimal temperature and pH of the engineered E. coli cells were 30 °C and 7.5, respectively, using pNPC4 as a substrate. The whole cell biocatalyst exhibited better stability and maintained approximately 8-fold higher specific enzymatic activity than purified CarCby when incubated at 30 °C for 120 h. In addition, ~ 100% and 50% of the original activity was retained when incubated with the whole cell biocatalyst at 4 ℃ and 30 °C for 35 days, respectively. However, the purified CarCby lost almost 100% of its activity when incubated at 30 °C for 134 h or 37 °C for 96 h, respectively. Finally, approximately 30 mg/L of carbaryl was hydrolyzed by 200 U of the engineered E. coli cells in 12 h.

Conclusions

Here, a carbaryl hydrolase-containing surface-displayed system was first constructed, and the whole cell biocatalyst displayed better stability and maintained its catalytic activity. This surface-displayed strategy provides a new solution for the cost-efficient bioremediation of carbaryl and could also have the potential to be used to treat other carbamates in environmental bioremediation.

Supplementary information

The online version contains supplementary material available at 10.1186/s12934-022-01821-5.

Enzyme cascade converting cyclohexanol into ε-caprolactone coupled with NADPH recycling using surface displayed alcohol dehydrogenase and cyclohexanone monooxygenase on E. coli

The application of enzymes as biocatalysts in industrial processes has great potential due to their outstanding stereo‐, regio‐ and chemoselectivity. Using autodisplay, enzymes can be immobilized on the cell surface of Gram‐negative bacteria such as Escherichia coli. In the present study, the surface display of an alcohol dehydrogenase (ADH) and a cyclohexanone monooxygenase (CHMO) on E. coli was investigated. Displaying these enzymes on the surface of E. coli resulted in whole‐cell biocatalysts accessible for substrates without further purification. An apparent maximal reaction velocity V_MAX(app) for the oxidation of cyclohexanol with the ADH whole‐cell biocatalysts was determined as 59.9 mU ml⁻¹. For the oxidation of cyclohexanone with the CHMO whole‐cell biocatalysts a V_MAX(app) of 491 mU ml⁻¹ was obtained. A direct conversion of cyclohexanol to ε‐caprolactone, which is a known building block for the valuable biodegradable polymer polycaprolactone, was possible by combining the two whole‐cell biocatalysts. Gas chromatography was applied to quantify the yield of ε‐caprolactone. 1.12 mM ε‐caprolactone was produced using ADH and CHMO displaying whole‐cell biocatalysts in a ratio of 1:5 after 4 h in a cell suspension of OD_578nm 10. Furthermore, the reaction cascade as applied provided a self‐sufficient regeneration of NADPH for CHMO by the ADH whole‐cell biocatalyst.

Biodegradation of λ-cyhalothrin through cell surface display of bacterial carboxylesterase

Pyrethroids are the third widespread used insecticides globally which have been extensively applied in agricultural or household environments. Due to continuous applications, pyrethroids have been detected both in living cells and environments. The permanent exposure to pyrethroids have caused substantial health risks and ecosystem concerns. In this work, a λ-cyhalothrin (one kind of pyrethroid insecticides) degrading bacterium Bacillus velezensis sd was isolated and a carboxylesterase gene, CarCB2 was characterized. A whole cell biocatalyst was developed for λ-cyhalothrin biodegradation by displaying CarCB2 on the surface of Escherichia coli cells. CarCB2 was successfully displayed and functionally expressed on E. coli cells with optimal pH and temperature of 7.5 and 30 °C, using p-NPC₄ as substrate, respectively. The whole cell biocatalyst exhibited better stability than the purified CarCB2, and approximately 120%, 60% or 50% of its original activity at 4 °C, 30 °C or 37 °C over a period of 35 d was retained, respectively. No enzymatic activity was detected when incubated the purified CarCB2 at 30 °C for 120 h, or 37 °C for 72 h, respectively. Additionally, 30 mg/L of λ-cyhalothrin was degraded in citrate-phosphate buffer by 10 U of the whole cell biocatalyst in 150 min. This work reveals that the whole cell biocatalyst affords a promising approach for efficient biodegradation of λ-cyhalothrin, and might have the potential to be applied in further environmental bioremediation of other different kinds of pyrethroid insecticides.

Application of Corynebacterium glutamicum engineering display system in three generations of biorefinery

The fermentation production of platform chemicals in biorefineries is a sustainable alternative to the current petroleum refining process. The natural advantages of Corynebacterium glutamicum in carbon metabolism have led to C. glutamicum being used as a microbial cell factory that can use various biomass to produce value-added platform chemicals and polymers. In this review, we discussed the use of C. glutamicum surface display engineering bacteria in the three generations of biorefinery resources, and analyzed the C. glutamicum engineering display system in degradation, transport, and metabolic network reconstruction models. These engineering modifications show that the C. glutamicum engineering display system has great potential to become a cell refining factory based on sustainable biomass, and further optimizes the inherent properties of C. glutamicum as a whole-cell biocatalyst. This review will also provide a reference for the direction of future engineering transformation.

Whole-Cell Display of Phosphotransferase in Escherichia coli for High-Efficiency Extracellular ATP Production

Adenosine triphosphate (ATP), as a universal energy currency, takes a central role in many biochemical reactions with potential for the synthesis of numerous high-value products. However, the high cost of ATP limits industrial ATP-dependent enzyme-catalyzed reactions. Here, we investigated the effect of cell-surface display of phosphotransferase on ATP regeneration in recombinant Escherichia coli. By N-terminal fusion of the super-folder green fluorescent protein (sfGFP), we successfully displayed the phosphotransferase of Pseudomonas brassicacearum (PAP-Pb) on the surface of E. coli cells. The catalytic activity of sfGFP-PAP-Pb intact cells was 2.12 and 1.47 times higher than that of PAP-Pb intact cells, when the substrate was AMP and ADP, respectively. The conversion of ATP from AMP or ADP were up to 97.5% and 80.1% respectively when catalyzed by the surface-displayed enzyme at 37 °C for only 20 min. The whole-cell catalyst was very stable, and the enzyme activity of the whole cell was maintained above 40% after 40 rounds of recovery. Under this condition, 49.01 mg/mL (96.66 mM) ATP was accumulated for multi-rounds reaction. This ATP regeneration system has the characteristics of low cost, long lifetime, flexible compatibility, and great robustness.

A Modular System for the Rapid Comparison of Different Membrane Anchors for Surface Display on Escherichia coli

Comparison of different membrane anchor motifs for the surface display of a protein of interest (passenger) is crucial for achieving the best possible performance. However, generating genetic fusions of the passenger to various membrane anchors is time-consuming. We herein employ a recently developed modular display system, in which the membrane anchor and the passenger are expressed separately and assembled in situ via SpyCatcher and SpyTag interaction, to readily combine a model passenger cytochrome P450 BM3 (BM3) with four different membrane anchors (Lpp-OmpA, PgsA, INP and AIDA-I). This approach has the significant advantage that passengers and membrane anchors can be freely combined in a modular fashion without the need to generate direct genetic fusion constructs in each case. We demonstrate that the membrane anchors impact not only cell growth and membrane integrity, but also the BM3 surface display capacity and whole-cell biocatalytic activity. The previously used Lpp-OmpA as well as PgsA were found to be efficient for the display of BM3 via SpyCatcher/SpyTag interaction. Our strategy can be transferred to other user-defined anchor and passenger combinations and could thus be used for acceleration and improvement of various applications involving cell surface display.

Whole-cell catalysis by surface display of fluorinase on Escherichia coli using N-terminal domain of ice nucleation protein

Background

Fluorinases play a unique role in the production of fluorine-containing organic molecules by biological methods. Whole-cell catalysis is a better choice in the large-scale fermentation processes, and over 60% of industrial biocatalysis uses this method. However, the in vivo catalytic efficiency of fluorinases is stuck with the mass transfer of the substrates.

Results

A gene sequence encoding a protein with fluorinase function was fused to the N-terminal of ice nucleation protein, and the fused fluorinase was expressed in Escherichia coli BL21(DE3) cells. SDS-PAGE and immunofluorescence microscopy were used to demonstrate the surface localization of the fusion protein. The fluorinase displayed on the surface showed good stability while retaining the catalytic activity. The engineered E.coli with surface-displayed fluorinase could be cultured to obtain a larger cell density, which was beneficial for industrial application. And 55% yield of 5′-fluorodeoxyadenosine (5′-FDA) from S-adenosyl-L-methionine (SAM) was achieved by using the whole-cell catalyst.

Conclusions

Here, we created the fluorinase-containing surface display system on E.coli cells for the first time. The fluorinase was successfully displayed on the surface of E.coli and maintained its catalytic activity. The surface display provides a new solution for the industrial application of biological fluorination.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01697-x.

Characterization of three glutamate decarboxylases from Bacillus spp. for efficient γ-aminobutyric acid production

BACKGROUND

Gamma-aminobutyric acid (GABA) is an important bio-product used in pharmaceuticals and functional foods and as a precursor of the biodegradable plastic polyamide 4. Glutamate decarboxylase (GAD) converts L-glutamate (L-Glu) into GABA via decarboxylation. Compared with other methods, develop a bioconversion platform to produce GABA is of considerable interest for industrial use.

RESULTS

Three GAD genes were identified from three Bacillus strains and heterologously expressed in Escherichia coli BL21 (DE3). The optimal reaction temperature and pH values for three enzymes were 40 °C and 5.0, respectively. Of the GADs, GADZ11 had the highest catalytic efficiency towards L-Glu (2.19 mM^- 1 s^- 1). The engineered E. coli strain that expressed GADZ11 was used as a whole-cell biocatalyst for the production of GABA. After repeated use 14 times, the cells produced GABA with an average molar conversion rate of 98.6% within 14 h.

CONCLUSIONS

Three recombinant GADs from Bacillus strains have been conducted functional identification. The engineered E. coli strain heterologous expressing GADZ1, GADZ11, and GADZ20 could accomplish the biosynthesis of L-Glu to GABA in a buffer-free reaction at a high L-Glu concentration. The novel engineered E. coli strain has the potential to be a cost-effective biotransformation platform for the industrial production of GABA.

Fe(III)-complex mediated bacterial cell surface immobilization of eGFP and enzymes

We report a facile and reversible method to immobilize a broad range of His6-tagged proteins on the E. coli cell surface through Fe(iii)-metal complexes. A His6-tagged eGFP and four His6-tagged enzymes were successfully immobilized on the cell surface. Additionally, a hydrogel sheath around E. coli cells was generated by immobilized His6-tagged HRP.

Genetically Programmable Microbial Assembly

Engineered microbial communities show promise in a wide range of applications, including environmental remediation, microbiome engineering, and synthesis of fine chemicals. Here we present methods by which bacterial aggregates can be directed into several distinct architectures by inducible surface expression of heteroassociative protein domains (SpyTag/SpyCatcher and SynZip17/18). Programmed aggregation can be used to activate a quorum-sensing circuit, and aggregate size can be tuned via control of the amount of the associative protein displayed on the cell surface. We further demonstrate reversibility of SynZip-mediated assembly by addition of soluble competitor peptide. Genetically programmable bacterial assembly provides a starting point for the development of new applications of engineered microbial communities in environmental technology, agriculture, human health, and bioreactor design.

Biological approaches practised using genetically engineered microbes for a sustainable environment: A review

Conventional methods used to remediate toxic substances from the environment have failed drastically, and thereby, advancement in newer remediation techniques can be one of the ways to improve the quality of bioremediation. The increased environmental pollution led to the exploration of microorganisms and construction of genetically engineered microbes (GEMs) for pollution abatement through bioremediation. The present review deals with the successful bioremediation techniques and approaches practised using genetically modified or engineered microbes. In the present scenario, physical and chemical strategies have been practised for the remediation of domestic and industrial wastes but these techniques are expensive and toxic to the environment. Involving engineered microbes can provide a much safer and cost effective strategy in comparison with the other techniques. With the aid of biotechnology and genetic engineering, GEMs are designed by transforming microbes with a more potent protein to overexpress the desired character. GEMs such as bacteria, fungi and algae have been used to degrade oil spills, camphor, hexane, naphthalene, toluene, octane, xylene, halobenzoates, trichloroethylene etc. These engineered microbes are more potent than the natural strains and have higher degradative capacities with quick adaptation for various pollutants as substrates or cometabolize. The road ahead for the implementation of genetic engineering to produce such organisms for the welfare of the environment and finally, public health is indeed long and worthwhile.

Cell-surface engineering of yeasts for whole-cell biocatalysts

Due to the unique advantages comparing with traditional free enzymes and chemical catalysis, whole-cell biocatalysts have been widely used to catalyze reactions effectively, simply and environment friendly. Cell-surface display technology provides a novel and effective approach for improved whole-cell biocatalysts expressing heterologous enzymes on the cell surface. They can overcome the substrate transport limitation of the intracellular expression and provide the enzymes with enhanced properties. Among all the host surface-displaying microorganisms, yeast is ideally suitable for constructing whole cell-surface-displaying biocatalyst, because of the large cell size, the generally regarded as safe (GRAS) status, and the perfect post-translational processing of secreted proteins. Yeast cell-surface display system has been a promising and powerful method for development of novel and improved engineered biocatalysts. In this review, the characterization and principles of yeast cell-surface display and the applications of yeast cell-surface display in engineered whole-cell biocatalysts as well as the improvement of the enzyme efficiency are summarized and discussed.

Protective Immunity Against Enterotoxigenic Escherichia coli by Oral Vaccination of Engineered Lactococcus lactis

Enterotoxigenic Escherichia coli (ETEC) is one of the leading causes of diarrhea in children globally, and thus suitable vaccines are desired. Antigen display on lactic acid bacteria is a reliable approach for efficient oral vaccination and preventing bowel diseases. To develop an oral vaccine against ETEC, the gene of the binding domain from heat-labile toxin (LTB), a key ETEC virulence factor, was codon-optimized and cloned into a construct containing a signal peptide and an anchor for display on L. lactis. Bioinformatics analysis showed a codon adaptation index of 0.95 for the codon-optimized gene. Cell surface expression of LTB was confirmed by transmission electron microscopy and blotting. White New Zealand rabbits were immunized per os (PO) with the recombinant L. lactis, and the antibody titers were assayed with ELISA. In vitro neutralization assay was performed using mouse adrenal tumor cells and rabbit ileal loop test was performed as the in vivo assay. ELISA results indicated that oral administration of the engineered L. lactis elicited a significant production of IgA in the intestine. In vitro neutralization assay showed that the effect of the toxin could be neutralized with 500 µg/ml of IgG isolated from the oral vaccine group. Furthermore, the dose of ETEC causing fluid accumulation in the ileal loop test showed a tenfold increase in rabbits immunized with either recombinant L. lactis or LTB protein compared to other groups. Our results imply that recombinant L. lactis could potentially be an effective live oral vaccine against ETEC toxicity.

Process intensification using immobilized enzymes for the development of white biotechnology

Process intensification of biocatalysed reactions using different techniques such as microwaves, ultrasound, hydrodynamic cavitation, ionic liquids, microreactors and flow chemistry in various industries is critically analysed and future directions provided.

Autodisplay of human PIP5K1α lipid kinase on Escherichia coli and inhibitor testing

The human phosphatidylinositol 4-phosphate 5-kinase type I α (hPIP5K1α) plays a major role in the PI3K/AKT/mTOR signaling pathway. As it has been shown before that hPIP5K1α is involved in the development of different types of cancer in particular prostate cancer, inhibitors of the enzyme might be a new option for the treatment of this disease. Here we report on the expression of hPIP5K1α on the surface of E. coli using Autodisplay. Autodisplay is defined as the surface display of a recombinant protein on a gramnegative bacterium by the autotransporter secretion pathway. After verification of surface expression, enzyme activity of whole cells displaying hPIP5K1α was determined by a capillary electrophoresis based assay. When using cells at an OD₅₇₈ of 2.5, the artificial substrate phosphatidylinositol4-phosphate (PI(4)P) fluorescein was converted by a rate of 10.7 ± 0.2 fmol/min. Using this substrate inhibition of three pyranobenzoquinone type compounds was tested. The most active compound was 4-(2-amino-3-cyano-6-hydroxy-5,8-dioxo-7-undecyl-5,8-dihydro-4H-chromen-4-yl) benzoic acid with an IC₅₀ value of 8.6 μM. Because until now, all attempts to purify hPIP5K1α failed, we suggest the use of whole cells of E. coli displaying the enzyme as a convenient tool for inhibitor identification.

Heterologous expression of EUGT11 from Oryza sativa in Pichia pastoris for highly efficient one-pot production of rebaudioside D from rebaudioside A

Rebaudioside D is a promising sweetener due to its zero calorie and high sweetness. Here, a transglucosylase gene eugt11 from Oryza sativa was for the first time expressed in Pichia pastoris, and transformant XE-3 showed the highest expression levels in pH 5.5 BMMY media containing 0.75% methanol. The affinity-purified EUGT11 from XE-3 displayed the highest activity at pH 6.0-6.5 and 45 °C, compared to pH 8.5 and 35 °C for EUGT11 from Escherichia coli. One-pot synthesis with orthogonal design was employed to optimize the rebaudioside D production using XE-3, and the initial pH 7.0 of the medium appears to be a significant factor and delivers the highest conversion efficiency. A two-step temperature-control strategy was developed, and a conversion rate of 95.31% was achieved at 28/35 °C vs. 62.41% in a one-step process at 28 °C. This study provides a high-efficient whole-cell biocatalysts technology for the sweetener production.

Naked Bacterium: Emerging Properties of a Surfome-Streamlined Pseudomonas putida Strain

Environmental bacteria are most often endowed with native surface-attachment programs that frequently conflict with efforts to engineer biofilms and synthetic communities with given tridimensional architectures. In this work, we report the editing of the genome of Pseudomonas putida KT2440 for stripping the cells of most outer-facing structures of the bacterial envelope that mediate motion, binding to surfaces, and biofilm formation. To this end, 23 segments of the P. putida chromosome encoding a suite of such functions were deleted, resulting in the surface-naked strain EM371, the physical properties of which changed dramatically in respect to the wild type counterpart. As a consequence, surface-edited P. putida cells were unable to form biofilms on solid supports and, because of the swimming deficiency and other alterations, showed a much faster sedimentation in liquid media. Surface-naked bacteria were then used as carriers of interacting partners (e.g., Jun-Fos domains) ectopically expressed by means of an autotransporter display system on the now easily accessible cell envelope. Abstraction of individual bacteria as adhesin-coated spherocylinders enabled rigorous quantitative description of the multicell interplay brought about by thereby engineered physical interactions. The model was then applied to parametrize the data extracted from automated analysis of confocal microscopy images of the experimentally assembled bacterial flocks for analyzing their structure and distribution. The resulting data not only corroborated the value of P. putida EM371 over the parental strain as a platform for display artificial adhesins but also provided a strategy for rational engineering of catalytic communities.

Applications of Bacillus subtilis Spores in Biotechnology and Advanced Materials

The bacterium Bacillus subtilis has long been an important subject for basic studies. However, this organism has also had industrial applications due to its easy genetic manipulation, favorable culturing characteristics for large-scale fermentation, superior capacity for protein secretion, and generally recognized as safe (GRAS) status. In addition, as the metabolically dormant form of B. subtilis, its spores have attracted great interest due to their extreme resistance to many environmental stresses, which makes spores a novel platform for a variety of applications. In this review, we summarize both conventional and emerging applications of B. subtilis spores, with a focus on how their unique characteristics have led to innovative applications in many areas of technology, including generation of stable and recyclable enzymes, synthetic biology, drug delivery, and material sciences. Ultimately, this review hopes to inspire the scientific community to leverage interdisciplinary approaches using spores to address global concerns about food shortages, environmental protection, and health care.

Potential use of two aryl sulfotransferase cell-surface display systems to detoxify the endocrine disruptor bisphenol A

Bisphenol A (BPA) is one of the most common toxic endocrine disruptors in the environment. A fast, efficient and environmental-friendly method for BPA detoxification is urgently needed. In this study, we show that the enzymatic transformation of BPA into a non-estrogenic BPA sulfate can be performed by the aryl sulfotransferase (ASTB) from Desulfitobacterium hafniense. We developed and compared two Escherichia coli ASTB cell-surface displaying systems using the outer membrane porin F (OprF) and the lipoprotein outer membrane A (Lpp-OmpA) as carriers. The surface localization of both fusion proteins was confirmed by Western blot and flow cytometry analysis as well as the enzymatic activity assay of the outer membrane fractions. Unfortunately, Lpp-OmpA-ASTB cells had an adverse effect on cell growth. In contrast, the OprF-ASTB cell biocatalyst was stable, expressing 70% of enzyme activity for 7 days. It also efficiently sulfated 90% of 5 mM BPA (1 mg/mL) in wastewater within 6 h.

Surface Display of Complex Enzymes by in Situ SpyCatcher-SpyTag Interaction

The display of complex proteins on the surface of cells is of great importance for protein engineering and other fields of biotechnology. Herein, we describe a modular approach, in which the membrane anchor protein Lpp-OmpA and a protein of interest (passenger) are expressed independently as genetically fused SpyCatcher and SpyTag units and assembled in situ by post-translational coupling. Using fluorescent proteins, we first demonstrate that this strategy allows the construct to be installed on the surface of E. coli cells. The scope of our approach was then demonstrated by using three different functional enzymes, the stereoselective ketoreductase Gre2p, the homotetrameric glucose 1-dehydrogenase GDH, and the bulky heme- and diflavin-containing cytochrome P450 BM3 (BM3). In all cases, the SpyCatcher-SpyTag method enabled the generation of functional whole-cell biocatalysts, even for the bulky BM3, which could not be displayed by conventional fusion with Lpp-OmpA. Furthermore, by using a GDH variant carrying an internal SpyTag, the system could be used to display an enzyme with unmodified N- and C-termini.

Functional Display of an Amoebic Chitinase in Escherichia coli Expressing the Catalytic Domain of EhCHT1 on the Bacterial Cell Surface

Poor solubility is the main drawback of the direct industrial exploitation of chitin, the second most abundant biopolymer after cellulose. Chemical methods are conventional to solubilize chitin from natural sources. Enzymatic hydrolysis of soluble chitinous substrates is a promising approach to obtain value-added by-products, such as N-acetylglucosamine units or low molecular weight chito-oligomers. Protein display on the bacterial membrane remains attractive to produce active enzymes anchored to a biological surface. The Lpp-OmpA system, a gene fusion of the Lpp signal sequence with the OmpA transmembrane region, represents the traditional system for targeting enzymes to the E. coli surface. EhCHT1, the amoebic chitinase, exhibits an efficient endochitinolytic activity and significant biochemical features, such as stability over a wide range of pH values. Using an extended Lpp-OmpA system as a protein carrier, we engineered E. coli to express the catalytic domain of EhCHT1 on the surface and assess the endochitinase activity as a trait. Engineered bacteria showed a consistent hydrolytic rate over a typical substrate, suggesting that the displayed enzyme has operational stability. This study supports the potential of biomembrane-associated biocatalysts as a reliable technology for the hydrolysis of soluble chitinous substrates.

Recombinant expression of nanobodies and nanobody-derived immunoreagents

Antibody fragments for which the sequence is available are suitable for straightforward engineering and expression in both eukaryotic and prokaryotic systems. When produced as fusions with convenient tags, they become reagents which pair their selective binding capacity to an orthogonal function. Several kinds of immunoreagents composed by nanobodies and either large proteins or short sequences have been designed for providing inexpensive ready-to-use biological tools. The possibility to choose among alternative expression strategies is critical because the fusion moieties might require specific conditions for correct folding or post-translational modifications. In the case of nanobody production, the trend is towards simpler but reliable (bacterial) methods that can substitute for more cumbersome processes requiring the use of eukaryotic systems. The use of these will not disappear, but will be restricted to those cases in which the final immunoconstructs must have features that cannot be obtained in prokaryotic cells. At the same time, bacterial expression has evolved from the conventional procedure which considered exclusively the nanobody and nanobody-fusion accumulation in the periplasm. Several reports show the advantage of cytoplasmic expression, surface-display and secretion for at least some applications. Finally, there is an increasing interest to use as a model the short nanobody sequence for the development of in silico methodologies aimed at optimizing the yields, stability and affinity of recombinant antibodies.

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There is an increasing request for immunoreagents based on nanobodies.

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The multiplicity of their applications requires constructs with different structural complexity.

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Alternative expression methods are necessary to achieve such structural requirements.

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In silico optimization of nanobody biophysical characteristics becomes more and more reliable.

A simple and rapid protein purification method based on cell-surface display of SUMO-fused recombinant protein and Ulp1 protease

The development of novel methods for highly efficient protein purification remains a research focus in the biotechnology field because conventional purification approaches, including affinity purification, gel filtration, and ion-exchange chromatography, require complex manipulation steps and are costly. Here, we describe a simple and rapid protein purification strategy in which the SUMO tag and Ulp1 protease are surface-displayed separately on Escherichia coli cells. After protein induction, the cells are harvested, resuspended in cleavage buffer, and incubated together for cleavage. In this approach, the surface-displayed Ulp1 cleaves the membrane-anchored SUMO fusion protein, resulting in the release of the target protein from the C-terminal of SUMO into the solution. The bacterial cells harboring SUMO and Ulp1 on their surfaces can be easily removed by centrifugation. To evaluate the purification method, we used red fluorescent protein (mCherry). Purified mCherry protein (7.72 ± 1.05 mg from 1 L of bacterial culture) was obtained after only 30 min of incubation. The protein purity was higher than 80%, and could be further improved (> 90%) by simple ultrafiltration. This study offers a promising and simple strategy for the purification of recombinant protein in its native form that requires only cleavage and centrifugation steps.

High-Throughput, Lysis-Free Screening for Sulfatase Activity Using Escherichia coli Autodisplay in Microdroplets

Directed evolution of enzymes toward improved catalytic performance has become a powerful tool in protein engineering. To be effective, a directed evolution campaign requires the use of high-throughput screening. In this study we describe the development of an ultra high-throughput lysis-free procedure to screen for improved sulfatase activity by combining microdroplet-based single-variant activity sorting with E. coli autodisplay. For the first step in a 4-step screening procedure, we quantitatively screened >10⁵ variants of the homodimeric arylsulfatase from Silicibacter pomeroyi (SpAS1), displayed on the E. coli cell surface, for improved sulfatase activity using fluorescence activated droplet sorting. Compartmentalization of the fluorescent reaction product with living E. coli cells autodisplaying the sulfatase variants ensured the continuous linkage of genotype and phenotype during droplet sorting and allowed for direct recovery by simple regrowth of the sorted cells. The use of autodisplay on living cells simplified and reduced the degree of liquid handling during all steps in the screening procedure to the single event of simply mixing substrate and cells. The percentage of apparent improved variants was enriched >10-fold as a result of droplet sorting. We ultimately identified 25 SpAS1 variants with improved performance toward 4-nitrophenyl sulfate (up to 6.2-fold) and/or fluorescein disulfate (up to 30-fold). In SpAS1 variants with improved performance toward the bulky fluorescein disulfate, many of the beneficial mutations occur in residues that form hydrogen bonds between α-helices in the C-terminal oligomerization region, suggesting a previously unknown role for the dimer interface in shaping the substrate binding site of SpAS1.

Autodisplay of alpha amylase from Bacillus megaterium in E. coli for the bioconversion of starch into hydrogen, ethanol and succinic acid

In this work, the expression of an α-amylase from Bacillus megaterium on the cell surface of Escherichia coli strains WDHA (Δ hycA and Δ ldhA) and WDHFP (Δ hycA, Δ frdD and Δ pta) by the autodisplay adhesin involved in diffuse adherence (AIDA) system was carried out with the purpose to confer the ability to E. coli strains to degrade starch and thus produce hydrogen, ethanol and succinic acid. For the characterization of the biocatalyst, the effect of temperature (30-70 °C), pH (3-6) and CaCl₂ concentration (0-25 mM), as well as the thermostability of the biocatalyst (55-80 °C) at several time intervals (15-60 min) were evaluated. The results showed that the biocatalyst had a maximum activity at 55 °C and pH 4.5. Calcium was required for the activity as well for the thermal stability of the biocatalyst. The calculated Vmax and Km values were 0.24 U/cm³ and 5.8 mg/cm³, respectively. Furthermore, a set of anaerobic batch fermentations was carried out using 10 g/dm³ of starch and 1 g/dm³ of glucose as carbon sources in 120 cm³ serological bottles, using WDHA and WDHFP strains harboring the pAIDA-amyA plasmid. The hydrogen production for WDHA was 1056.06 cm³/dm³ and the succinic acid yield was 0.68 g/g_starch, whereas WDHFP strain produced 1689.68 cm³/dm³ of hydrogen and an ethanol yield of 0.28 g/g_starch. This work represents a promising strategy to improve the exploitation of starchy biomass for the production of biofuels (hydrogen and ethanol) or succinate without the need of a pre-saccharification process.

Functional Autodisplay of Phenolic Acid Decarboxylase using a GDSL Autotransporter on Escherichia coli for Efficient Catalysis of 4-Hydroxycinnamic Acids to Vinylphenol Derivatives

Bioproduction of vinylphenol derivatives, such as 4-vinylguaiacol (4-VG) and 4-vinylphenol (4-VP), from 4-hydroxycinnamic acids, such as ferulic acid (FA) and p-coumaric acid (pCA), employing whole cells expressing phenolic acid decarboxylases (PAD) as a biocatalyst has attracted much attention in recent years. However, the accumulation of 4-VG or 4-VP in the cell may cause high cytotoxicity to Escherichia coli (E. coli) and consequently cell death during the process. In this study, we firstly report the functional display of a phenolic acid decarboxylase (BLPAD) from Bacillus licheniformis using a GDSL autotransporter from Pseudomonas putida on the cell surface of E. coli. Expression and localization of BLPAD on E. coli were verified by SDS-PAGE and protease accessibility. The PelB signal peptide is more effective in guiding the translocation of BLPAD on the cell surface than the native signal peptide of GDSL, and the cell surface displaying BLPAD activity reached 19.72 U/OD600. The cell surface displaying BLPAD showed good reusability and retained 63% of residual activity after 7 cycles of repeated use. In contrast, the residual activity of the intracellular expressing cells was approximately 11% after 3 cycles of reuse. The molar bioconversion yields of 72.6% and 80.4% were achieved at the concentration of 300 mM of FA and pCA in a biphasic toluene/Na2HPO4–citric acid buffer system, respectively. Its good reusability and efficient catalysis suggested that the cell surface displaying BLPAD can be used as a whole-cell biocatalyst for efficient production of 4-VG and 4-VP.

Improving the autotransporter-based surface display of enzymes in Pseudomonas putida KT2440

Pseudomonas putida can be used as a host for the autotransporter‐mediated surface display of enzymes (autodisplay), resulting in whole‐cell biocatalysts with recombinant functionalities on their cell envelope. The efficiency of autotransporter‐mediated secretion depends on the N‐terminal signal peptide as well as on the C‐terminal translocator domain of autotransporter fusion proteins. We set out to optimize autodisplay for P. putida as the host bacterium by comparing different signal peptides and translocator domains for the surface display of an esterase. The translocator domain did not have a considerable effect on the activity of the whole‐cell catalysts. In contrast, by using the signal peptide of the P. putida outer membrane protein OprF, the activity was more than 12‐fold enhanced to 638 mU ml⁻¹ OD⁻¹ compared with the signal peptide of V. cholerae CtxB (52 mU ml⁻¹ OD⁻¹). This positive effect was confirmed with a β‐glucosidase as a second example enzyme. Here, cells expressing the protein with N‐terminal OprF signal peptide showed more than fourfold higher β‐glucosidase activity (181 mU ml⁻¹ OD⁻¹) than with the CtxB signal peptide (42 mU ml⁻¹ OD⁻¹). SDS‐PAGE and flow cytometry analyses indicated that the increased activities correlated with an increased amount of recombinant protein in the outer membrane and a higher number of enzymes detectable on the cell surface.