Enzymatic and whole cell catalysis: finding new strategies for old processes

Whole cell biocatalysts: essential workers from Nature to the industry

Microorganisms have been exposed to a myriad of substrates and environmental conditions throughout evolution resulting in countless metabolites and enzymatic activities. Although mankind have been using these properties for centuries, we have only recently learned to control their production, to develop new biocatalysts with high stability and productivity and to improve their yields under new operational conditions. However, microbial cells still provide the best known environment for enzymes, preventing conformational changes in the protein structure in non-conventional medium and under harsh reaction conditions, while being able to efficiently regenerate necessary cofactors and to carry out cascades of reactions. Besides, a still unknown microbe is probably already producing a compound that will cure cancer, Alzeihmer's disease or kill the most resistant pathogen. In this review, the latest developments in screening desirable activities and improving production yields are discussed.

Establishment of Bioprocess for Synthesis of Nicotinamide by Recombinant Escherichia coli Expressing High-Molecular-Mass Nitrile Hydratase

Application of engineered bacteria expressing nitrile hydratase for the production of amide is getting tremendous attention due to the rapid development of recombinant DNA technique. This study evaluated the effect of 3-cyanopyridine concentrations on nicotinamide production using recombinant Escherichia coli strain (BAG) expressing high-molecular-mass nitrile hydratase from Rhodococcus rhodochrous J1, and established proper process of whole-cell catalysis of 3-cyanopyridine and high cell-density cultivation. The process of substrate fed-batch was applied in the production of nicotinamide, and the concentration of product reached 390 g/L under the condition of low cell-density. After the high cell-density cultivation of BAG in 5 L bioreactor, the OD₆₀₀ of cell attained 200 and the total activity reached 2813 U/mL. Different high density of BAG after fermentation in the tank was used to catalyze 3-cyanopyridine, and the concentration of nicotinamide reached to 508 g/L in just 60 min. The productivity of BAG was 212% higher than that of R. rhodochrous J1, and it is possible that BAG is able to achieve industrial production of nicotinamide.

Construction of engineered Saccharomyces cerevisiae strain to improve that whole-cell biocatalytic production of melibiose from raffinose

There are excessive by-products in the biocatalysis process of this whole-cell biocatalytic production of melibiose from raffinose with current Saccharomyces cerevisiae strains. To solve this problem, we constructed engineered strains based on a liquor yeast (S. cerevisiae) via gene deletion (mel1 gene), heterologous integration (fsy1 or/and ffzi1 gene from Candida magnoliae), and gene overexpression (gcr1 gene). Functional verification showed that deletion of the mel1 gene led to elimination of the reactions catalyzed by α-galactosidase, as well as elimination of the degradation of melibiose and the formation of galactose by-product. Insertion of the fsy1 or/and ffzi1 gene and overexpression of the gcr1 gene could contribute to fructose transport for enhancing the biopurification rate of the fructose by-product. Compared with the wild-type strain, the optimal engineered strain of MP8 (Δmel1::fsy1 _cm ::ffzi1 _cm ::gcr1 _sc ) had improved about 30% on yield, 31% on productivity, and 36% on purity of the melibiose product.

Production of itaconate by whole-cell bioconversion of citrate mediated by expression of multiple cis-aconitate decarboxylase (cadA) genes in Escherichia coli

Itaconate, a C₅ unsaturated dicarboxylic acid, is an important chemical building block that is used in manufacturing high-value products, such as latex and superabsorbent polymers. Itaconate is produced by fermentation of sugars by the filamentous fungus Aspergillus terreus. However, fermentation by A. terreus involves a long fermentation period and the formation of various byproducts, resulting in high production costs. E. coli has been developed as an alternative for producing itaconate. However, fermentation of glucose gives low conversion yields and low productivity. Here, we report the whole-cell bioconversion of citrate to itaconate with enhanced aconitase and cis-aconitate decarboxylase activities by controlling the expression of multiple cadA genes. In addition, this bioconversion system does not require the use of buffers, which reduces the production cost and the byproducts released during purification. Using this whole-cell bioconversion system, we were able to catalyze the conversion of 319.8 mM of itaconate (41.6 g/L) from 500 mM citrate without any buffer system or additional cofactors, with 64.0% conversion in 19 h and a productivity of 2.19 g/L/h. Our bioconversion system suggests very high productivity for itaconate production.

Two-Step Production of Phenylpyruvic Acid from L-Phenylalanine by Growing and Resting Cells of Engineered Escherichia coli: Process Optimization and Kinetics Modeling

Phenylpyruvic acid (PPA) is widely used in the pharmaceutical, food, and chemical industries. Here, a two-step bioconversion process, involving growing and resting cells, was established to produce PPA from l-phenylalanine using the engineered Escherichia coli constructed previously. First, the biotransformation conditions for growing cells were optimized (l-phenylalanine concentration 20.0 g·L-1, temperature 35°C) and a two-stage temperature control strategy (keep 20°C for 12 h and increase the temperature to 35°C until the end of biotransformation) was performed. The biotransformation conditions for resting cells were then optimized in 3-L bioreactor and the optimized conditions were as follows: agitation speed 500 rpm, aeration rate 1.5 vvm, and l-phenylalanine concentration 30 g·L-1. The total maximal production (mass conversion rate) reached 29.8 ± 2.1 g·L-1 (99.3%) and 75.1 ± 2.5 g·L-1 (93.9%) in the flask and 3-L bioreactor, respectively. Finally, a kinetic model was established, and it was revealed that the substrate and product inhibition were the main limiting factors for resting cell biotransformation.

Molecular Mechanisms Contributing to the Growth and Physiology of an Extremophile Cultured with Dielectric Heating

The effect of microwave frequency electromagnetic fields on living microorganisms is an active and highly contested area of research. One of the major drawbacks to using mesophilic organisms to study microwave radiation effects is the unavoidable heating of the organism, which has limited the scale (<5 ml) and duration (<1 h) of experiments. However, the negative effects of heating a mesophile can be mitigated by employing thermophiles (organisms able to grow at temperatures of >60°C). This study identified changes in global gene expression profiles during the growth of Thermus scotoductus SA-01 at 65°C using dielectric (2.45 GHz, i.e., microwave) heating. RNA sequencing was performed on cultures at 8, 14, and 24 h after inoculation to determine the molecular mechanisms contributing to long-term cellular growth and survival under microwave heating conditions. Over the course of growth, genes associated with amino acid metabolism, carbohydrate metabolism, and defense mechanisms were upregulated; the number of repressed genes with unknown function increased; and at all time points, transposases were upregulated. Genes involved in cell wall biogenesis and elongation were also upregulated, consistent with the distinct elongated cell morphology observed after 24 h using microwave heating. Analysis of the global differential gene expression data enabled the identification of molecular processes specific to the response of T. scotoductus SA-01 to dielectric heating during growth.

IMPORTANCE

The residual heating of living organisms in the microwave region of the electromagnetic spectrum has complicated the identification of radiation-only effects using microorganisms for 50 years. A majority of the previous experiments used either mature cells or short exposure times with low-energy high-frequency radiation. Using global differential gene expression data, we identified molecular processes unique to dielectric heating using Thermus scotoductus SA-01 cultured over 30 h in a commercial microwave digestor. Genes associated with amino acid metabolism, carbohydrate metabolism, and defense mechanisms were upregulated; the number of repressed genes with unknown function increased; and at all time points, transposases were upregulated. These findings serve as a platform for future studies with mesophiles in order to better understand the response of microorganisms to microwave radiation.

Overexpression of transport proteins improves the production of 5-aminovalerate from l-lysine in Escherichia coli

Bacterial transporters mediate the exchanges between intracellular and extracellular environments. Modification of transport route could be applied to speed up the metabolic reactions and promote the production of aimed compounds. Herein, lysine 2-monooxygenase (DavB) and δ-aminovaleramidase (DavA) were co-expressed in Escherichia coli BL21(DE3) to produce nylon-5 monomer 5-aminovalerate from l-lysine. Then, PP2911 (4-aminobutyrate transporter in Pseudomonas putida) and LysP (the lysine specific permease in E. coli) were overexpressed to promote 5-aminovalerate production using whole cells of recombinant E. coli. The constructed E. coli strain overexpressing transport proteins exhibited good 5-aminovalerate production performance and might serve as a promising biocatalyst for 5-aminovalerate production from l-lysine. This strategy not only shows an efficient process for the production of nylon monomers but also might be used in production of other chemicals.

Stimulation of acetoin production in metabolically engineered Lactococcus lactis by increasing ATP demand

Having a sufficient supply of energy, usually in the form of ATP, is essential for all living organisms. In this study, however, we demonstrate that it can be beneficial to reduce ATP availability when the objective is microbial production. By introducing the ATP hydrolyzing F₁-ATPase into a Lactococcus lactis strain engineered into producing acetoin, we show that production titer and yield both can be increased. At high F₁-ATPase expression level, the acetoin production yield could be increased by 10 %; however, because of the negative effect that the F₁-ATPase had on biomass yield and growth, this increase was at the cost of volumetric productivity. By lowering the expression level of the F₁-ATPase, both the volumetric productivity and the final yield could be increased by 5 % compared to the reference strain not overexpressing the F₁-ATPase, and in batch fermentation, it was possible to convert 176 mM (32 g/L) of glucose into 146.5 mM (12.9 g/L) acetoin with a yield of 83 % of the theoretical maximum. To further demonstrate the potential of the cell factory developed, we complemented it with the lactose plasmid pLP712, which allowed for growth and acetoin production from a dairy waste stream, deproteinized whey. Using this cheap and renewable feedstock, efficient acetoin production with a titer of 157 mM (14 g/L) acetoin was accomplished.

Recent advances in whole cell biocatalysis techniques bridging from investigative to industrial scale

Recent advances in biocatalysis have strongly boosted its recognition as a valuable addition to traditional chemical synthesis routes. As for any catalytic process, catalyst's costs and stabilities are of highest relevance for the economic application in chemical manufacturing. Employing biocatalysts as whole cells circumvents the need of cell lysis and enzyme purification and hence strongly cuts on cost. At the same time, residual cell wall components can shield the entrapped enzyme from potentially harmful surroundings and aid to enable applications far from natural enzymatic environments. Further advantages are the close proximity of reactants and catalysts as well as the inherent presence of expensive cofactors. Here, we review and comment on benefits and recent advances in whole cell biocatalysis.

Finding and Producing Probiotic Glycosylases for the Biocatalysis of Ginsenosides: A Mini Review

Various microorganisms have been widely applied in nutraceutical industries for the processing of phytochemical conversion. Specifically, in the Asian food industry and academia, notable attention is paid to the biocatalytic process of ginsenosides (ginseng saponins) using probiotic bacteria that produce high levels of glycosyl-hydrolases. Multiple groups have conducted experiments in order to determine the best conditions to produce more active and stable enzymes, which can be applicable to produce diverse types of ginsenosides for commercial applications. In this sense, there are various reviews that cover the biofunctional effects of multiple types of ginsenosides and the pathways of ginsenoside deglycosylation. However, little work has been published on the production methods of probiotic enzymes, which is a critical component of ginsenoside processing. This review aims to investigate current preparation methods, results on the discovery of new glycosylases, the application potential of probiotic enzymes and their use for biocatalysis of ginsenosides in the nutraceutical industry.

Production of Rebaudioside A from Stevioside Catalyzed by the Engineered Saccharomyces cerevisiae

Rebaudioside A has superior taste quality among the steviol glycosides extracted from Stevia rebaudiana leaves. Given its high purity as a general-purpose sweetener, rebaudioside A has received significant attention and has been widely applied in food and beverages in recent decades. Stevioside is one of the major steviol glycosides and can be converted to rebaudioside A by the uridine-diphosphate dependent glucosyltransferase UGT76G1 in S. rebaudiana. To explore the applicability of and limits in producing rebaudioside A from stevioside through whole-cell biocatalysis, the engineered Saccharomyces cerevisiae expressing UGT76G1, using a newly constructed constitutive expression vector, was used as the whole-cell biocatalyst. Citrate was added to the reaction mixture to allow metabolic regulation when glucose was fed to provide the activated sugar donor UDP-glucose for glycosylation of stevioside in vivo. In an evaluation of the whole-cell reaction parameters involving cell permeability, temperature, pH, citrate and Mg(2+) concentrations, and glucose feeding, production of 1160.5 mg/L rebaudioside A from 2 g/L stevioside was achieved after 48 h without supplementation of extracellular UDP-glucose.

Enhanced bioconversion rate and released substrate inhibition in (R)-phenylephrine whole-cell bioconversion via partial acetone treatment

An approach was developed to enhance the efficiency for the bioconversion of 1-(3-hydroxyphenyl)-2-(methyamino)-ethanone to (R)-phenylephrine. The strain Serratia marcescens N10612, giving the benefit of 99% enantiomeric excess in (R)-PE conversion, was used. The fermentation was devised to harvest cells with high hydrophobic prodigiosin content inside the cells. Then, the partial acetone extraction was applied to remove prodigiosin from the cells. The treatment was found to increase the cells conversion rate without loss of the cells NADPH redox system. When using 50% (v/v) acetone for 5min, the processed cells can give a specific conversion rate of 16.03μmol/h/g-cells. As compared the treated cells with cells under the basal medium, the maximum reaction rate (Vmax) increased from 6.69 to 10.27 (μmol/h/g-cells), the dissociation constant (Km) decreased from 0.236 to 0.167mM and the substrate inhibition constant (KSi) increased from 0.073 to 1.521mM. The 20-fold increase in substrate inhibition constant referred to a great release from the substrate inhibition for the use of S. marcescens N10612 in the bioconversion, which would greatly benefit the bioconversion to be industrialized.

Significantly improved solvent tolerance of Escherichia coli by global transcription machinery engineering

BACKGROUND

Escherichia coli has emerged as a promising platform microorganism to produce biofuels and fine chemicals of industrial interests. Certain obstacles however remain to be overcome, among which organic-solvent tolerance is a crucial one.

RESULTS

We used global transcription machinery engineering (gTME) to improve the organic-solvent tolerance (OST) of E. coli JM109. A mutant library of σ(70) encoded by rpoD was screened under cyclohexane pressure. E. coli JM109 strain harboring σ(70) mutant C9 was identified with capability of tolerating 69 % cyclohexane. The rpoD mutant contains three amino-acid substitutes and a stop-codon mutation, resulting a truncated sequence containing regions σ(1.1) and σ(1.2). Total protein difference produced by E. coli JM109 strain harboring C9 was examined with 2D-PAGE, and 204 high-abundant proteins showed over twofold variation under different solvent stress.

CONCLUSIONS

Our results show that several genes (gapA, sdhB, pepB and dppA) play critical roles in enhanced solvent tolerance of E. coli, mainly involving in maintaining higher intracellular energy level under solvent stress. Global transcription machinery engineering is therefore a feasible and efficient approach for engineering strain with enhanced OST-phenotype.

Glycosylation and sulfation of emodin by Gliocladium deliquescens NRRL 1086

The present study was designed to explore the substrate scope and biocatalytic capability of Gliocladium deliquescens NRRL 1086 on phenolic natural products. Emodin was subjected to the fermentation culture of Gliocladium deliquescens NRRL 1086 according to the standard two-stage protocol. The biotransformation process was monitored by HPLC-DAD-MS, the main product was isolated by column chromatography, and the structure was elucidated on the basis of NMR spectroscopy. Emodin could be fully metabolized by Gliocladium deliquescens NRRL 1086, resulting in high yield of emodin 6-O-β-D-glucopyranoside and small amount of sulfated product. In conclusion, our results may provide a convenient method to prepare emodin 6-O-β-D-glucopyranoside and the microbe catalyzed glucosylation/sulfation will give an inspiration to pharmacokinetic model studies in vitro.

Biocatalytic Production of Trehalose from Maltose by Using Whole Cells of Permeabilized Recombinant Escherichia coli

Trehalose is a non-reducing disaccharide, which can protect proteins, lipid membranes, and cells from desiccation, refrigeration, dehydration, and other harsh environments. Trehalose can be produced by different pathways and trehalose synthase pathway is a convenient, practical, and low-cost pathway for the industrial production of trehalose. In this study, 3 candidate treS genes were screened from genomic databases of Pseudomonas and expressed in Escherichia coli. One of them from P. stutzeri A1501 exhibited the best transformation ability from maltose into trehalose and the least byproduct. Thus, whole cells of this recombinant E. coli were used as biocatalyst for trehalose production. In order to improve the conversion rate of maltose to trehalose, optimization of the permeabilization and biotransformation were carried out. Under optimal conditions, 92.2 g/l trehalose was produced with a high productivity of 23.1 g/(l h). No increase of glucose was detected during the whole course. The biocatalytic process developed in this study might serve as a candidate for the large scale production of trehalose.

Biotechnological applications of microbial bioconversions

Modern research has focused on the microbial transformation of a huge variety of organic compounds to obtain compounds of therapeutic and/or industrial interest. Microbial transformation is a useful tool for producing new compounds, as a consequence of the variety of reactions for natural products. This article describes the production of many important compounds by biotransformation. Emphasis is placed on reporting the metabolites that may be of special interest to the pharmaceutical and biotechnological industries, as well as the practical aspects of this work in the field of microbial transformations.

Biosynthesis of D-xylulose 5-phosphate from D-xylose and polyphosphate through a minimized two-enzyme cascade

Sugar phosphates cannot be produced easily by microbial fermentation because negatively-charged compounds cannot be secreted across intact cell membrane. D-xylulose 5-phosphate (Xu5P), a very expensive sugar phosphate, was synthesized from D-xylose and polyphosphate catalyzed by enzyme cascades in one pot. The synthetic enzymatic pathway comprised of xylose isomerase and xylulokinase was designed to produce Xu5P, along with a third enzyme, polyphosphate kinase, responsible for in site ATP regeneration. Due to the promiscuous activity of the ATP-based xylulokinase from a hyperthermophilic bacterium Thermotoga maritima on polyphosphate, the number of enzymes in the pathway was minimized to two without polyphosphate kinase. The reactions catalyzed by the two-enzyme and three-enzyme pathways were compared for Xu5P production, and the reaction conditions were optimized by examining effects of reaction temperature, enzyme ratio and substrate concentration. The optimized two-enzyme system produced 32 mM Xu5P from 50 mM xylose and polyphosphate after 36 h at 45°C. Biosynthesis of less costly Xu5P from D-xylose and polyphosphate could be highly feasible via this minimized two-enzyme pathway.

Ultrasound-assisted (R)-phenylephrine whole-cell bioconversion by S. marcescens N10612

The strain Serratia marcescens N10612 is used to perform the bioconversion of 1-(3-hydroxyphenyl)-2-(methyamino)-ethanone (HPMAE) to (R)-phenylephrine ((R)-PE), which is an ephedrine drug substitute. The use of an ultrasound approach is found to improve the efficiency of the (R)-PE bioconversion. The optimization of the (R)-PE bioconversion is carried out by means of statistical experiment design. The optimal conditions obtained are 1.0mM HPMAE, 18.68 g/L glucose and ultrasound power of 120 W, where the predicted specific rate of the (R)-PE bioconversion is 31.46 ± 2.22 (ìmol/h/g-cells) and the experimental specific rate is 33.27 ± 1.46 (ìmol/h/g-cells), which is 3-fold higher than for the operation under ultrasound power of 200 W (11.11 ìmol/h/g-cells) and 4.3-fold higher than for the shaking operation (7.69 ìmol/h/g-cells). The kinetics study of the bioconversion also shows that under the ultrasound operation, the optimal rate (Vmax) of the (R)-PE bioconversion increases from 7.69 to 11.11 (μmol/h/g-cells) and the substrate inhibition constant (KSi) increases from 1.063 mM for the shaking operation to 1.490 mM for ultrasound operation.

Overexpression of artificially fused bifunctional enzyme 4CL1-CCR: a method for production of secreted 4-hydroxycinnamaldehydes in Escherichia coli

BACKGROUND

4-Hydroxycinnamaldehydes are important intermediates in several secondary metabolism pathways, including those involved in the biosynthesis of phenolic acids, flavonoids, terpenoids and monolignols. They are also involved in the biosynthesis and degradation of lignins, which are important limiting factors during the processes of papermaking and biofuel production. Access to these aromatic polymers is necessary to explore the secondary biometabolic pathways they are involved in. Coniferaldehyde, sinapaldehyde, p-coumaraldehyde and caffealdehyde are members of the 4-hydroxycinnamaldehyde family. Although coniferaldehyde and sinapaldehyde can be purchased from commercial sources, p-coumaraldehyde and caffealdehyde are not commercially available. Therefore, there is increasing interest in producing 4-hydroxycinnamaldehydes. Here, we attempted to produce 4-hydroxycinnamaldehydes using engineered Escherichia coli.

RESULTS

4-Coumaric acid: coenzyme A ligase (4CL1) and cinnamoyl coenzyme A reductase (CCR) were fused by means of genetic engineering to generate an artificial bifunctional enzyme, 4CL1-CCR, which was overexpressed in cultured E. coli supplemented with phenylpropanoic acids. Three 4-hydroxycinnamaldehydes, p-coumaraldehyde, caffealdehyde and coniferaldehyde, were thereby biosynthesized and secreted into the culture medium. The products were extracted and purified from the culture medium, and identically characterized by the HPLC-PDA-ESI-MSn. The productivity of this new metabolic system were 49 mg/L for p-coumaraldehyde, 19 mg/L for caffealdehyde and 35 mg/L for coniferaldehyde. Extracellular hydroxycinnamoyl-coenzyme A thioesters were not detected, indicating that these thioesters could not pass freely through the cellular membrane. The fusion enzyme 4CL1-CCR can catalyze sequential multistep reactions, thereby avoiding the permeability problem of intermediates, which reveals its superiority over a mixture of individual native enzymes. Moreover, we have described a highly sensitive and selective method for separation and identification of phenylpropanoic acids and their corresponding cinnamaldehydes in the present paper. The feasibility of this method has been proven in the application of the method to the analysis of the metabolites of whole-cell catalysts.

CONCLUSIONS

We have established a bioconversion pathway for the microbial production of valuable 4-hydroxycinnamaldehydes from phenylpropanoic acids. This biotransformation method is both convenient and environmentally friendly, and provides new insights into the biosynthesis of natural plant secondary products.

Efficient enzymatic production of rebaudioside A from stevioside

Stevioside and rebaudioside A are the chief diterpene glycosides present in the leaves of Stevia rebaudiana. Rebaudioside A imparts a desirable sweet taste, while stevioside produces a residual bitter aftertaste. Enzymatic synthesis of rebaudioside A from stevioside can increase the ratio of rebaudioside A to stevioside in steviol glycoside products, providing a conceivable strategy to improve the organoleptic properties of steviol glycoside products. Here, we demonstrate the efficient conversion of stevioside to rebaudioside A by coupling the activities of recombinant UDP-glucosyltransferase UGT76G1 from S. rebaudiana and sucrose synthase AtSUS1 from Arabidopsis thaliana. The conversion occurred via regeneration of UDP-glucose by AtSUS1. UDP was applicable as the initial material instead of UDP-glucose for UDP-glucose recycling. The amount of UDP could be greatly reduced in the reaction mixture. Rebaudioside A yield in 30 h with 2.4 mM stevioside, 7.2 mM sucrose, and 0.006 mM UDP was 78%.

Non-Conventional Yeasts Whole Cells as Efficient Biocatalysts for the Production of Flavors and Fragrances

The rising consumer requests for natural flavors and fragrances have generated great interest in the aroma industry to seek new methods to obtain fragrance and flavor compounds naturally. An alternative and attractive route for these compounds is based on bio-transformations. In this review, the application of biocatalysis by Non Conventional Yeasts (NCYs) whole cells for the production of flavor and fragrances is illustrated by a discussion of the production of different class of compounds, namely Aldehydes, Ketones and related compounds, Alcohols, Lactones, Terpenes and Terpenoids, Alkenes, and Phenols.

Bioconversion of vitamin D3 to calcifediol by using resting cells of Pseudonocardia sp

OBJECTIVES

Resting cells of Pseudonocardia sp. KCTC 1029BP were used for the bioconversion of vitamin D3 to calcifediol which is widely used to treat osteomalacia and is industrially produced by chemical synthesis.

RESULTS

To obtain the maximum bioconversion yield of calcifediol by the microbial conversion of vitamin D3, a two-step optimization process was used, including the Plackett-Burman and the central composite designs. Six variables, namely agitation speed, aeration rate, resting cell concentration, vitamin D3 concentration, temperature, and pH, were monitored. Of these, aeration rate, resting cell concentration, and temperature were selected as key variables for calcifediol production and were optimized using the central composite design. Optimal bioconversion conditions obtained were as follows: aeration rate of 0.2 vvm, resting cell concentration of 4.7% w/v, and temperature of 33 °C.

CONCLUSION

Using the optimal conditions, 356 mg calcifediol l(-1) was obtained with a bioconversion yield of 59.4% in a 75 l fermentor. These are the highest values reported to date.

Effectiveness of direct immobilization of bacterial cells onto material surfaces using the bacterionanofiber protein AtaA

The bacterionanofiber protein AtaA, a member of the trimeric autotransporter adhesin family found in Acinetobacter sp. Tol 5, is responsible for the nonspecific, high adhesiveness and autoagglutination of this strain. Previously, we introduced the ataA gene into the nonadhesive Acinetobacter strain ST-550, which conferred high adhesiveness to this strain, immobilized its cells, and improved indigo productivity due to enhanced tolerance to the toxic substrate. In this study, we again demonstrated the effectiveness of this new microbial immobilization method using AtaA in a number of conditions. AtaA enabled the effective immobilization of growing, resting, and lyophilized cells of a type strain of Acinetobacter, ADP1, which is also intrinsically nonadhesive, onto the surface of several kinds of support ranging from artificial to natural materials and from hydrophobic polyurethane to hydrophilic glass. Immobilization with AtaA enabled exclusive cell growth in the support space and only a few cells existed in the bulk medium. Immobilization of resting cells drastically increased cell concentration, depending on the support material; dry cells of approximately 110 g/L could be immobilized onto glass wool. Finally, we demonstrated that ADP1 cells immobilized on polyurethane foam can undergo at least 10 repetitive reactions without inactivation during a 5-h period. Even after drying and storing for 3 days, the immobilized cells showed enzymatic activity and an ester hydrolysis reaction was repeated by simply transferring the support with the cells into a fresh reaction buffer.

Enantioselective bioreductive preparation of chiral halohydrins employing two newly identified stereocomplementary reductases

Two robust stereocomplementary carbonyl reductases (DhCR andCgCR) for preparation of hylohydrins were identified through rescreening the carbonyl reductase toolbox.

Sporopollenin as an efficient green support for covalent immobilization of a lipase

Aminoalkyl functionalised sporopollenin exine capsules have been used to immobilizeCandida antarcticalipase B using a covalent diimine-based linker.

Citrobacter amalonaticus phytase on the cell surface of Pichia pastoris exhibits high pH stability as a promising potential feed supplement

Phytase expressed and anchored on the cell surface of Pichia pastoris avoids the expensive and time-consuming steps of protein purification and separation. Furthermore, yeast cells with anchored phytase can be used as a whole-cell biocatalyst. In this study, the phytase gene of Citrobacter amalonaticus was fused with the Pichia pastoris glycosylphosphatidylinositol (GPI)-anchored glycoprotein homologue GCW61. Phytase exposed on the cell surface exhibits a high activity of 6413.5 U/g, with an optimal temperature of 60°C. In contrast to secreted phytase, which has an optimal pH of 5.0, phytase presented on the cell surface is characterized by an optimal pH of 3.0. Moreover, our data demonstrate that phytase anchored on the cell surface exhibits higher pH stability than its secreted counterpart. Interestingly, our in vitro digestion experiments demonstrate that phytase attached to the cell surface is a more efficient enzyme than secreted phytase.

Using ionic liquids in whole-cell biocatalysis for the nucleoside acylation

Background

The use of biocatalysts has become an increasingly attractive alternative to traditional chemical methods, due to the high selectivity, mild reaction conditions and environmentally-friendly processes in nonaqueous catalysis of nucleosids. However, the extensive use of organic solvents may generally suffer from sever drawbacks such as volatileness and toxicity to the environment and lower activity of the biocatalyst. Recently, ionic liquids are considered promising solvents for nonaqueous biocatalysis of polyhydroxyl compounds as ILs are environmental-friendly.

Results

In this research, we developed new IL-containing reaction systems for synthesis of long chain nucleoside ester catalyzed by Pseudomonas fluorescens whole-cells. Various ILs exerted significant but different effects on the bio-reaction. And their effects were closely related with both the anions and cations of the ILs. Use of 10% [BMI][PF₆]/THF gave high reaction efficiency of arabinocytosine laurate synthesis, in which the initial rate, product yield and 5′-regioselectivity reached 2.34 mmol/L·h, 81.1% and >99%, respectively. Furthermore, SEM analysis revealed that ILs can alter the cell surface morphology, improve the permeability of cell envelopes and thus facilitate the mass transfer of substrates to the active sites of cell-bound enzymes.

Conclusion

Our research demonstrated the potential of ILs as promising reaction medium for achieving highly efficient and regioselective whole-cell catalysis.

Influence of organic solvents on catalytic behaviors and cell morphology of whole-cell biocatalysts for synthesis of 5'-arabinocytosine laurate

A whole-cell based method was developed for the regioselective synthesis of arabinocytosine laurate. Among the seven kinds of bacteria strains tested in the acylation reaction, Pseudomonas fluorescens gave the highest productivity and a higher 5′-regioselectivity than 99%. Compared with pure organic solvents, the use of organic solvent mixtures greatly promoted the yield of the whole-cell catalyzed reaction, but showed little influence on the 5′-regioselectivity. Of all the tested solvent mixtures, the best reaction result was found in isopropyl ether/pyridine followed by isopentanol/pyridine. However, the whole-cells showed much lower thermostability in isopropyl ether/pyridine than in THF-pyridine. To better understand the toxic effects of the organic solvents on P. fluorescens whole-cells and growing cells were further examined. Significant influences of organic solvents on the biomass of the cells were found, which differed depending on the type of solvents used. SEM analysis visually revealed the changes in the surface morphology of whole-cells and growing cells cultured in media containing various organic solvents, in terms of surface smoothness, bulges and changed cell sizes. Results demonstrated that organic toxicity to cell structure played an important role in whole-cell mediated catalysis.

Enantioselective, continuous (R)- and (S)-2-butanol synthesis: achieving high space-time yields with recombinant E. coli cells in a micro-aqueous, solvent-free reaction system

The stereoselective production of (R)- or (S)-2-butanol is highly challenging. A potent synthesis strategy is the biocatalytic asymmetric reduction of 2-butanone applying alcohol dehydrogenases. However, due to a time-dependent racemisation process, high stereoselectivity is only obtained at incomplete conversion after short reaction times. Here, we present a solution to this problem: by using a continuous process, high biocatalytic selectivity can be achieved while racemisation is suppressed successfully. Furthermore, high conversion was achieved by applying recombinant, lyophilised E. coli cells hosting Lactobacillus brevis alcohol dehydrogenase in a micro-aqueous solvent-free continuous reaction system. The optimisation of residence time (τ) and 2-butanone concentration boosted both conversion (>99%) and enantiomeric excess (ee) of (R)-2-butanol (>96%). When a residence time of only τ=3.1 min was applied, productivity was extraordinary with a space-time yield of 2278±29g/(L×d), thus exceeding the highest values reported to date by a factor of more than eight. The use of E. coli cells overexpressing an ADH of complementary stereoselectivity yielded a synthesis strategy for (S)-2-butanol with an excellent ee (>98%). Although conversion was only moderate (up to 46%), excellent space-time yields of up to 461g/(L×d) were achieved. The investigated concept represents a synthesis strategy that can also be applied to other biocatalytic processes where racemisation poses a challenge.

Formate production through carbon dioxide hydrogenation with recombinant whole cell biocatalysts

The biological conversion of CO2 and H2 into formate offers a sustainable route to a valuable commodity chemical through CO2 fixation, and a chemical form of hydrogen fuel storage. Here we report the first example of CO2 hydrogenation utilising engineered whole-cell biocatalysts. Escherichia coli JM109(DE3) cells transformed for overexpression of either native formate dehydrogenase (FDH), the FDH from Clostridium carboxidivorans, or genes from Pyrococcus furiosus and Methanobacterium thermoformicicum predicted to express FDH based on their similarity to known FDH genes were all able to produce levels of formate well above the background, when presented with H2 and CO2, the latter in the form of bicarbonate. In the case of the FDH from P. furiosus the yield was highest, reaching more than 1 g L(-1)h(-1) when a hydrogen-sparging reactor design was used.