Novel type of in situ extraction: Use of solvent containing microcapsules for the bioconversion of 2-phenylethanol from L-phenylalanine by Saccharomyces cerevisiae

Production and purification of 2-phenylethanol by Saccharomyces cerevisiae using tobacco waste extract as a substrate

2-phenylethanol (2-PE), which is extracted naturally from plant or biotechnology processing, is widely used in the food and cosmetics industries. Due to the high cost of 2-PE production, the valorization of waste carbon to produce 2-PE has gained increasing attention. Here, 2-PE was produced by Saccharomyces cerevisiae using tobacco waste extract (TWE) as the substrate. Considering the toxicity of nicotine and its inhibition of 2-PE, the tolerance of S. cerevisiae was first evaluated. The results suggested that the production of 2-PE by S. cerevisiae in TWEs could be carried out at 2·0 mg ml-1 nicotine concentrations and may be inhibited by 1·0 mg ml-1 2-PE. Thus, the compounds in the TWEs prepared at different temperatures were detected, and the results revealed that the TWEs prepared at 140°C contained 2·18 mg ml-1 of nicotine, had total sugar concentrations of 26·8 mg ml-1 and were suitable for 2-PE production. Due to feedback regulation, the 2-PE production was only 1·11 mg ml-1 , and the remaining glucose concentration remained at 13·78 mg ml-1 , which indicated insufficient glucose utilization. Then, in situ product recovery was further implemented to remove this inhibition; the glucose utilization (the remaining concentration decreased to 3·64 mg ml-1 ) increased, and the 2-PE production increased to 1·65 mg ml-1 . The 2-PE produced in the fermentation broth was first isolated by elution from the resin with 75% ethanol and then by removing the impurities with 2·5% activated charcoal, and pure 2-PE was identified by gas chromatography mass spectrometry. The results of this study suggest that TWE could be an alternative carbon source for 2-PE production. This could provide an outlet tobacco waste as well as reducing the price of natural 2-PE, although more strategies need to be explored to improve the production yield of 2-PE by using TWE.

Artificial Cell Encapsulation for Biomaterials and Tissue Bio-Nanoengineering: History, Achievements, Limitations, and Future Work for Potential Clinical Applications and Transplantation

Pancreatic β-cell loss and failure with subsequent deficiency of insulin production is the hallmark of type 1 diabetes (T1D) and late-stage type 2 diabetes (T2D). Despite the availability of parental insulin, serious complications of both types are profound and endemic. One approach to therapy and a potential cure is the immunoisolation of β cells via artificial cell microencapsulation (ACM), with ongoing promising results in human and animal studies that do not depend on immunosuppressive regimens. However, significant challenges remain in the formulation and delivery platforms and potential immunogenicity issues. Additionally, the level of impact on key metabolic and disease biomarkers and long-term benefits from human and animal studies stemming from the encapsulation and delivery of these cells is a subject of continuing debate. The purpose of this review is to summarise key advances in this field of islet transplantation using ACM and to explore future strategies, limitations, and hurdles as well as upcoming developments utilising bioengineering and current clinical trials.

Improvement of 2-phenylethanol production in Saccharomyces cerevisiae by evolutionary and rational metabolic engineering

2-Phenylethanol (2-PE) is a valuable aromatic compound with favorable flavors and good properties, resulting in its widespread application in the cosmetic, food and medical industries. In this study, a mutant strain, AD032, was first obtained by adaptive evolution under 2-PE stress. Then, a fusion protein from the Ehrlich pathway, composed of tyrB from Escherichia coli, kdcA from Lactococcus lactis and ADH2 from Saccharomyces cerevisiae, was constructed and expressed. As a result, 3.14 g/L 2-PE was achieved using L-phenylalanine as a precursor. To further increase 2-PE production, L-glutamate oxidase from Streptomyces overexpression was applied for the first time in our research to improve the supply of α-ketoglutarate in the transamination of 2-PE synthesis. Furthermore, we found that the disruption of the pyruvate decarboxylase encoding gene PDC5 caused an increase in 2-PE production, which has not yet been reported. Finally, assembly of the efficient metabolic modules and process optimization resulted in the strain RM27, which reached 4.02 g/L 2-PE production from 6.7 g/L L-phenylalanine without in situ product recovery. The strain RM27 produced 2-PE (0.8 mol/mol) with L-phenylalanine as a precursor, which was considerably high, and displayed manufacturing potential regarding food safety and process simplification aspects. This study suggests that innovative strategies regarding metabolic modularization provide improved prospects for 2-PE production in food exploitation.

Metabolic Engineering of Escherichia coli for para-Amino-Phenylethanol and para-Amino-Phenylacetic Acid Biosynthesis

Aromatic amines are an important class of chemicals which are used as building blocks for the synthesis of polymers and pharmaceuticals. In this study we establish a de novo pathway for the biosynthesis of the aromatic amines para-amino-phenylethanol (PAPE) and para-amino-phenylacetic acid (4-APA) in Escherichia coli. We combined a synthetic para-amino-l-phenylalanine pathway with the fungal Ehrlich pathway. Therefore, we overexpressed the heterologous genes encoding 4-amino-4-deoxychorismate synthase (pabAB from Corynebacterium glutamicum), 4-amino-4-deoxychorismate mutase and 4-amino-4-deoxyprephenate dehydrogenase (papB and papC from Streptomyces venezuelae) and ThDP-dependent keto-acid decarboxylase (aro10 from Saccharomyces cerevisiae) in E. coli. The resulting para-amino-phenylacetaldehyde either was reduced to PAPE or oxidized to 4-APA. The wild type strain E. coli LJ110 with a plasmid carrying these four genes produced (in shake flask cultures) 11 ± 1.5 mg l⁻¹ of PAPE from glucose (4.5 g l⁻¹). By the additional cloning and expression of feaB (phenylacetaldehyde dehydrogenase from E. coli) 36 ± 5 mg l⁻¹ of 4-APA were obtained from 4.5 g l⁻¹ glucose. Competing reactions, such as the genes for aminotransferases (aspC and tyrB) or for biosynthesis of L-phenylalanine and L-tyrosine (pheA, tyrA) and for the regulator TyrR were removed. Additionally, the E. coli genes aroFBL were cloned and expressed from a second plasmid. The best producer strains of E. coli showed improved formation of PAPE and 4-APA, respectively. Plasmid-borne expression of an aldehyde reductase (yahK from E. coli) gave best values for PAPE production, whereas feaB-overexpression led to best values for 4-APA. In fed-batch cultivation, the best producer strains achieved 2.5 ± 0.15 g l⁻¹ of PAPE from glucose (11% C mol mol-1 glucose) and 3.4 ± 0.3 g l⁻¹ of 4-APA (17% C mol mol⁻¹ glucose), respectively which are the highest values for recombinant strains reported so far.

Phase Equilibrium Investigation on 2-Phenylethanol in Binary and Ternary Systems: Influence of High Pressure on Density and Solid-Liquid Phase Equilibrium

Ionic liquids (ILs) are important new solvents proposed for applications in different separation processes. Herein, an idea of possible use of high pressure in a general strategy of production of 2-phenylethanol (PEA) is discussed. In this work, we present the influence of pressure on the density in binary systems of {1-hexyl-1-methylpyrrolidynium bis{(trifluoromethyl)sulfonyl}imide, [HMPYR][NTf₂], or 1-dodecyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [DoMIM][NTf₂] + PEA} in a wide range of temperatures (298.15-348.15 K) and pressures (0.1-40 MPa). The densities at ambient and high pressures are measured to present the physicochemical properties of the ILs used in the process of separation of PEA from aqueous phase. The Tait equation was used for the correlation of density of one-component and two-component systems as a function of mole fraction, temperature, and pressure. The influence of pressure is not significant. These systems exhibit mainly negative molar excess volumes, V^E. The solid-liquid phase equilibrium (SLE) of [DoMIM][NTf₂] in PEA at atmospheric pressure was measured and compared to the SLE high-pressure results. Additionally, the ternary liquid-liquid phase equilibrium (LLE) at ambient pressure in the {[DoMIM][NTf₂] (1) + PEA (2) + water (3)} at temperature T = 308.15 K was investigated. The solubility of water in the [DoMIM][NTf₂] is quite high in comparison with that measured by us earlier for ILs ( x₃ = 0.403) at T = 308.15 K, which results in not very successful average selectivity of extraction of PEA from the aqueous phase. The [DoMIM][NTf₂] has shown strong interaction with PEA without the immiscibility region. The ternary system revealed Treybal's type phase equilibrium in which two partially miscible binaries ([DoMIM][NTf₂] + water) and (PEA + water) exist. From the results of LLE in the ternary system, the selectivity and the solute distribution ratio of separation of water/PEA were calculated and compared to the results obtained for the ILs measured earlier by us. The popular NRTL model was used to correlate the experimental tie-lines in ternary LLE. These results may help in a new technological project of "in situ" extraction of PEA from aqueous phase during the biosynthesis.

Reconstruction of metabolic module with improved promoter strength increases the productivity of 2-phenylethanol in Saccharomyces cerevisiae

Background

2-phenylethanol (2-PE) is an important aromatic compound with a lovely rose-like scent. Saccharomyces cerevisiae is a desirable microbe for 2-PE production but its natural yield is not high, and one or two crucial genes’ over-expression in S. cerevisiae did not improve 2-PE greatly.

Results

A new metabolic module was established here, in which, permease Gap1p for l-phenylalanine transportation, catalytic enzymes Aro8p, Aro10p and Adh2p in Ehrlich pathway respectively responsible for transamination, decarboxylation and reduction were assembled, besides, glutamate dehydrogenase Gdh2p was harbored for re-supplying another substrate 2-oxoglutarate, relieving product glutamate repression and regenerating cofactor NADH. Due to different promoter strengths, GAP1, ARO8, ARO9, ARO10, ADH2 and GDH2 in the new modularized YS58(G1-A8-A10-A2)-GDH strain enhanced 11.6-, 15.4-, 3.6-, 17.7-, 12.4- and 7.5-folds respectively, and crucial enzyme activities of aromatic aminotransferases and phenylpyruvate decarboxylase were 4.8- and 7-folds respectively higher than that of the control.

Conclusions

Under the optimum medium and cell density, YS58(G1-A8-A10-A2)-GDH presented efficient 2-PE synthesis ability with ~ 6.3 g L⁻¹ of 2-PE titer in 5-L fermenter reaching 95% of conversation ratio. Under fed-batch fermentation, 2-PE productivity at 24 h increased 29% than that of single-batch fermentation. Metabolic modularization with promoter strategy provides a new prospective for efficient 2-PE production.

Regulation of crucial enzymes and transcription factors on 2-phenylethanol biosynthesis via Ehrlich pathway in Saccharomyces cerevisiae

2-Phenylethanol (2-PE) is widely used in food, perfume and pharmaceutical industry, but lower production in microbes and less known regulatory mechanisms of 2-PE make further study necessary. In this study, crucial genes like ARO8 and ARO10 of Ehrlich pathway for 2-PE synthesis and key transcription factor ARO80 in Saccharomyces cerevisiae were re-regulated using constitutive promoter; in the meantime, the effect of nitrogen source in synthetic complete (SC) medium with L-phenylalanine (L-Phe) on Aro8/Aro9 and Aro10 was investigated. The results showed that aromatic aminotransferase activities of ARO8 over-expressing strains were seriously inhibited by ammonia sulfate in SC + Phe medium. Flask fermentation test demonstrated that over-expressing ARO8 or ARO10 led to about 42 % increase in 2-PE production when compared with the control strain. Furthermore, influence of transcription factors Cat8 and Mig1 on 2-PE biosynthesis was explored. CAT8 over-expression or MIG1 deletion increased in the transcription of ARO9 and ARO10. 2-PE production of CAT8 over-expressing strain was 62 % higher than that of control strain. Deletion of MIG1 also led to 2-PE biosynthesis enhancement. The strain of CAT8 over-expression and MIG1 deletion was most effective in regulating expression of ARO9 and ARO10. Analysis of mRNA levels and enzyme activities indicates that transaminase in Ehrlich pathway is the crucial target of Nitrogen Catabolize Repression (NCR). Among the engineering strains, the higher 3.73 g/L 2-PE production in CAT8 over-expressing strain without in situ product recovery suggests that the robust strain has potentiality for commercial exploitation.

Yeast factories for the production of aromatic compounds: from building blocks to plant secondary metabolites

The aromatic amino acid biosynthesis pathway is a source to a plethora of commercially relevant chemicals with very diverse industrial applications. Tremendous efforts in microbial engineering have led to the production of compounds ranging from small aromatic molecular building blocks all the way to intricate plant secondary metabolites. Particularly, the yeast Saccharomyces cerevisiae has been a great model organism given its superior capability to heterologously express long metabolic pathways, especially the ones containing cytochrome P450 enzymes. This review contains a collection of state-of-the-art metabolic engineering work devoted towards unraveling the mechanisms for enhancing the flux of carbon into the aromatic pathway. Some of the molecules discussed include the polymer precursor muconic acid, as well as important nutraceuticals (flavonoids and stilbenoids), and opium-derived drugs (benzylisoquinoline alkaloids).

Microencapsulation using vibrating technology

For over a half a century now, microencapsulation has played a very important role in many industries and in the recent decades, this versatile technology has been applied to numerous biotechnology and medical processes. However, successful application in these areas requires a methodology which has the capability to produce mono-dispersed, homogenous-shaped capsules, with a narrow size distribution, using a short production time. The manufacture of capsules using vibrating technology has gained significant interest mainly due to its simplistic approach to produce homogenous microcapsules with the desired characteristics for biotechnological and medical processes. However, certain limitations still exist for this methodology, which include the inability to manufacture microcapsules at large quantities and/or using highly viscous polymers. In this review, a detailed description of the theoretical and practical aspects behind the production of different types of alginate-based microcapsules, for application in biotechnological and medical processes, using vibrating technology, is given.

Two-phase systems: potential for in situ extraction of microalgal products

Algae are currently used for production of niche products and are becoming increasingly interesting for the production of bulk commodities, such as biodiesel. For the production of these goods to become economically feasible, production costs will have to be lowered by one order of magnitude. The application of two-phase systems could be used to lower production costs. These systems circumvent the costly step of cell harvesting, whilst the product is extracted and prepared for downstream processing. The mechanism of extraction is a fundamental aspect of the practical question whether two-phase systems can be applied for in situ extraction, viz, simultaneous growth, product formation and extraction, or as a separate downstream processing step. Three possible mechanisms are discussed; 1) product excretion 2) cell permeabilization, and 3) cell death. It was shown that in the case of product excretion, the application of two-phase systems for in situ extraction can be very valuable. With permeabilization and cell death, in situ extraction is not ideal, but the application of two-phase systems as downstream extraction steps can be part of a well-designed biorefinery process. In this way, processing costs can be decreased while the product is mildly and selectively extracted. Thus far none of the algal strains used in two-phase systems have been shown to excrete their product; the output has always been the result of cell death. Two-phase systems can be a good approach as a downstream processing step for these species. For future applications of two-phase in situ extraction in algal production processes, either new species that show product excretion should be discovered, or existing species should be modified to induce product excretion.

Synergistic inhibition effect of 2-phenylethanol and ethanol on bioproduction of natural 2-phenylethanol by Saccharomyces cerevisiae and process enhancement

Natural 2-phenylethanol (PEA) could be produced on a large scale by way of bioconversion with yeast from l-phenylalanine. In this work the synergistic inhibition effect of the target product PEA and the byproduct ethanol on the bioconversion rate by Saccharomyces cerevisiae R-UV3 was systematically studied and a new kinetic model with an item representing the synergistic effect was proposed. Optimization strategies to repress the inhibition effect of PEA and ethanol were carried out in the mode of fed-batch culture with ISPR. The glucose concentration was regulated at the level of 0.2±0.1g/L by controlling a suitable respiratory quotient on line, which could limit the accumulation of the ethanol lower than 10g/L. In the presence of resin FD0816 with a weight of 10% of the medium, PEA was removed from the broth and the overall PEA concentration and the space-time yield reached 13.7g/L and 0.39g L(-1) h(-1) respectively. The semi-continuous process with ISPR was performed, in which the replacement of the resin was operated repeatedly when the aqueous PEA was over 2.7g/L and bioconversion continued until the bioactivity of the yeast cells declined, consequently achieving a final overall PEA concentration of 32.5g/L and a space-time yield of 0.45g L(-1) h(-1).

Application of Supercritical Fluid Extraction in Biotechnology

In the present paper recent investigations on the applications of supercritical fluid extraction (SCE) from post fermentation biomass or in situ extraction of inhibitory fermentation products as a promising method for increasing the yield of extraction have been reviewed. Although supercritical CO2 (SC-CO2) is unfriendly, or even toxic, for some living cells and precludes direct fermentation in dense CO2, it does not rule out other useful applications for in situ extraction of inhibitory fermentation products and fractional extraction of biomass constituents. This technique is a highly desirable method for fractional extraction of biomass constituents, and intracellular metabolites due to the potential of system modification by physical parameters and addition of co-solvents to selectively extract compounds of different polarity, volatility and hydrophilicity without any contamination.

Sorption of hydrophobic organic compounds (HOC) in rapeseed oil bodies

Oil-bodies are minute plant organelles (0.5-2.0microm diameter) consisting of an oil core surrounded by a phospholipid monolayer/proteinaceous membrane. Oil-bodies have been isolated from rapeseed seeds and demonstrated to constitute a novel type of micro-capsule suitable for the extraction of hydrophobic organic compounds from aqueous environments. Three hydrophobic pesticides: atrazine (2-chlor-4-ethyl-amino-6-isopropylamino-1,3,5-triazine), carbaryl (1-naphthyl methylcarbamate) and parathion (O,O-diethyl O-(4-nitrophenyl) phosphorothioate), as well as naphthalene and 2-phenylethanol were successfully extracted from aqueous solutions, with absorption in the inner oily core of OB as sorption mechanism. The OB membrane does not represent a barrier for the mass transfer of the compound towards the inner oily core of OB. Moreover, due to very high surface area to volume ratio, oil-bodies exhibit very good mass transfer properties compared with larger synthetic microcapsules or two-phase liquid-liquid extraction (LLE) techniques, which diminishes the need for strong agitation and avoids the formation of difficult to separate stable emulsions.

In situ extraction of intracellular l-asparaginase using thermoseparating aqueous two-phase systems

The feasibility and generic applicability of directly integrating conventional discrete operations of cell disruption by high pressure homogenizer and the product capture by aqueous two-phase extraction (ATPE) system have been demonstrated for the extraction of intracellular L-asparaginase from E. coli. In a side-by-side comparison with the conventional ATPE process, including cell disruption, centrifugal clarification and following ATPE, purification of L-asparaginase via this novel in situ ATPE process yielded a product of L-asparaginase with a higher specific activity of 94.8 U/(mg protein) and a higher yield of 73.3%, both of which in the conventional ATPE process were 78.6 U/(mg protein) and 52.1%, respectively. In the purification of L-asparaginase (pI=4.9), product-debris interactions commonly diminish its recovery. It was demonstrated that immediate extraction of L-asparaginase in ATPE systems when it is released at pH 5.0 during cell disruption effectively increased its recovery in the top phase due to the reduced interaction between L-asparaginase and cell debris and the reduced degradation by contaminated protease. In addition, no clarification step and/or disruptate storage are required in this in situ ATPE, which reduced the number of unit operations and thus shortened the overall process time. This novel process has a good potential for the separation of other intracellular biological products.

Biodegradation of a phenolic mixture in a solid-liquid two-phase partitioning bioreactor

A solid-liquid two-phase partitioning bioreactor (TPPB) in which the non-aqueous phase consisted of polymer (HYTREL) beads was used to degrade a model mixture of phenols [phenol, o-cresol, and 4-chlorophenol (4CP)] by a microbial consortium. In one set of experiments, high concentrations (850 mg l(-1) of each of the three substrates) were reduced to sub-inhibitory levels within 45 min by the addition of the polymer beads, followed by inoculation and rapid (8 h) consumption of the total phenolics loading. In a second set of experiments, the beneficial effect of using polymer beads to launch a fermentation inhibited by high substrate concentrations was demonstrated by adding 1,300 and 2,000 mg l(-1) total substrates (equal concentrations of each phenolic) to a pre-inoculated bioreactor. At these levels, no cell growth and no degradation were observed; however, after adding polymer beads to the systems, the ensuing reduced substrate concentrations permitted complete destruction of the target molecules, demonstrating the essential role played by the polymer sequestering phase when applied to systems facing inhibitory substrate concentrations. In addition to establishing alternative modes of TPPB operation, the present work has demonstrated the differential partitioning of phenols in a mixture between the aqueous and polymeric phases. The polymeric phase was also observed to absorb a degradation intermediate (arising from the incomplete biodegradation of 4CP), which opens the possibility of using solid-liquid TPPBs during biosynthetic transformation to sequester metabolic byproducts.

Production of 2-phenylethanol and 2-phenylethylacetate from L-phenylalanine by coupling whole-cell biocatalysis with organophilic pervaporation

An integrated bioprocess for the production of the natural rose-like aroma compounds, 2-phenylethanol (2-PE) and 2-phenylethylacetate (2-PEAc), from L-phenylalanine (L-phe) with yeasts was investigated. The hydrophobicity of the products leads to product inhibition, which can be compensated by in situ product removal (ISPR). An organophilic pervaporation unit, equipped with a polyoctylmethylsiloxane (POMS) membrane, was coupled via a bypass to a bioreactor and proved to be a suitable technique for the in situ removal of high-boiling products from culture broth. With batch cultures of the thermotolerant yeast Kluyveromyces marxianus CBS 600 in a standard medium at 35 degrees C, the use of pervaporation resulted in a double 2-PE concentration (2.2 g/L) and 1.3 g/L 2-PEAc, which only accumulated transiently in low concentrations during cultivation without ISPR. Using a previously optimized medium, the variation of the temperature from 30 degrees C to 40 degrees C caused an increase in the total conversion yield from 63% to 79%, corresponding to total product concentrations of 5.23 and 5.85 g/L, respectively. In the 40 degrees C batch experiment, the volumetric productivity (2-PE + 2-PEAc) during the exponential phase was 5.2 mmol/L h. While for 2-PE, there is still potential for further optimization, the more hydrophobic 2-PEAc was nearly completely removed from the aqueous culture broth (enrichment factor >400), resulting in highly aroma-enriched permeates. Due to the temperature-correlated performance of the pervaporation, the bioconversion was still efficient even at 45 degrees C (conversion yield: 69%). Surprisingly, at 45 degrees C, the molar ratio of the two products inverted and 2-PEAc turned out to be the main product (4.0 g/L), which opens easy control of the reaction's selectivity by external means. Retrofitting the process with interim heating and cooling equipment to use different temperature levels for cultivation and pervaporation resulted in a decreased yield and product concentration caused by multiple stress factors. The medium composition affected the pervaporation efficiency with molasses acting detrimental.

Integrated bioprocesses

Integrated bioprocesses have been developed to optimise yield and cost-effectiveness of production of low and high molecular weight molecules. Low molecular weight products are removed from the cultivation medium with in situ extraction, in situ adsorption or crystallisation to avoid product inhibition. One-pot processes are being developed to replace two-stage reactions. Recent developments in the integrated purification of high molecular weight products focus mainly on the integration of solid/liquid separation and initial product recovery such as expanded bed adsorption or extraction in aqueous two-phase systems. Additionally, new approaches for a more efficient processing of inclusion bodies have been developed.