One-Pot Production of Natural 2-Phenylethanol fromL-Phenylalanine via Cascade Biotransformations

Biotechnological 2-Phenylethanol Production: Recent Developments

2-Phenylethanol (2-PE) is a key flavor compound with a rose-like scent, used in the cosmetics, perfume, home care and food industries. This aroma compound can be obtained naturally from various flowers, however chemical synthesis is the most used route to meet market demand. The increasing interest in natural products has led to the development of more environmentally friendly alternatives for 2-PE production through biotechnological approaches. The most efficient approach involves the biotransformation of L-phenylalanine into 2-PE via the Ehrlich pathway, a process observed in different microorganisms such as yeasts and bacteria. 2-PE produced by this way can be considered as natural. However, due to the toxicity of the aroma to the producing microorganism, low production yields are typically obtained, motivating efforts to develop production processes that can overcome this bottleneck, enhance 2-PE yields and reduce the production costs. This review presents and discusses the latest advances in the bioproduction of 2-PE through microbial fermentation, in terms of producing strains, the optimization of cultivation processes, strategies to mitigate product toxicity, and the use of low value feedstocks. Novel applications for 2-PE are also highlighted.

Structural basis of the Meinwald rearrangement catalysed by styrene oxide isomerase

Membrane-bound styrene oxide isomerase (SOI) catalyses the Meinwald rearrangement-a Lewis-acid-catalysed isomerization of an epoxide to a carbonyl compound-and has been used in single and cascade reactions. However, the structural information that explains its reaction mechanism has remained elusive. Here we determine cryo-electron microscopy (cryo-EM) structures of SOI bound to a single-domain antibody with and without the competitive inhibitor benzylamine, and elucidate the catalytic mechanism using electron paramagnetic resonance spectroscopy, functional assays, biophysical methods and docking experiments. We find ferric haem b bound at the subunit interface of the trimeric enzyme through H58, where Fe(III) acts as the Lewis acid by binding to the epoxide oxygen. Y103 and N64 and a hydrophobic pocket binding the oxygen of the epoxide and the aryl group, respectively, position substrates in a manner that explains the high regio-selectivity and stereo-specificity of SOI. Our findings can support extending the range of epoxide substrates and be used to potentially repurpose SOI for the catalysis of new-to-nature Fe-based chemical reactions.

Whole-Cell Bioconversion Systems for Efficient Synthesis of Monolignols from L-Tyrosine in Escherichia coli

Monolignols and their derivatives exhibit various pharmaceutical and physiological characteristics, such as antioxidant and anti-inflammatory properties. However, they remain difficult to synthesize. In this study, we engineered several whole-cell bioconversion systems with carboxylate reductase (CAR)-mediated pathways for efficient synthesis of p-coumaryl, caffeyl, and coniferyl alcohols from l-tyrosine in Escherichia coli BL21 (DE3). By overexpressing the l-tyrosine ammonia lyase from Flavobacterium johnsoniae (FjTAL), carboxylate reductase from Segniliparus rugosus (SruCAR), alcohol dehydrogenase YqhD and hydroxylase HpaBC from E. coli, and caffeate 3-O-methyltransferase (COMT) from Arabidopsis thaliana, three enzyme cascades FjTAL-SruCAR-YqhD, FjTAL-SruCAR-YqhD-HpaBC, and FjTAL-SruCAR-YqhD-HpaBC-COMT were constructed to produce 1028.5 mg/L p-coumaryl alcohol, 1015.3 mg/L caffeyl alcohol, and 411.4 mg/L coniferyl alcohol from 1500, 1500, and 1000 mg/L l-tyrosine, with productivities of 257.1, 203.1, and 82.3 mg/L/h, respectively. This work provides an efficient strategy for the biosynthesis of p-coumaryl, caffeyl, and coniferyl alcohols from l-tyrosine.

Recent developments in enzymatic and microbial biosynthesis of flavor and fragrance molecules

Flavors and fragrances are an important class of specialty chemicals for which interest in biomanufacturing has risen during recent years. These naturally occurring compounds are often amenable to biosynthesis using purified enzyme catalysts or metabolically engineered microbial cells in fermentation processes. In this review, we provide a brief overview of the categories of molecules that have received the greatest interest, both academically and industrially, by examining scholarly publications as well as patent literature. Overall, we seek to highlight innovations in the key reaction steps and microbial hosts used in flavor and fragrance manufacturing.

Co-Immobilization of ADH and GDH on Metal-Organic-Framework: An Effective Biocatalyst for Asymmetric Reduction of Ketones

Chiral alcohols are not only important building blocks of various bioactive natural compounds and pharmaceuticals, but can serve as synthetic precursors for other valuable organic chemicals, thus the synthesis of these products is of great importance. Bio-catalysis represents one effective way to obtain these molecules, however, the weak stability and high cost of enzymes often hinder its broad application. In this work, we designed a biological nanoreactor by embedding alcohol dehydrogenase (ADH) and glucose dehydrogenase (GDH) in metal-organic-framework ZIF-8. The biocatalyst ADH&GDH@ZIF-8 could be applied to the asymmetric reduction of a series of ketones to give chiral alcohols in high yields (up to 99 %) and with excellent enantioselectivities (>99 %). In addition, the heterogeneous biocatalyst could be recycled and reused at least four times with slight activity decline. Moreover, E. coli containing ADH and GDH was immobilized by ZIF-8 to form biocatalyst E. coli@ZIF-8, which also exhibits good catalytic behaviours. Finally, the chiral alcohols are further converted to marketed drugs (R)-Fendiline, (S)-Rivastigmine and NPS R-568 respectively.

Styrene Oxide Isomerase-Catalyzed Meinwald Rearrangement in Cascade Biotransformations: Synthesis of Chiral and/or Natural Chemicals

Styrene oxide isomerase (SOI) catalyzes the Meinwald rearrangement of aryl epoxides to carbonyl compounds via a 1,2-trans-shift in a stereospecific manner. A number of cascade biotransformations with SOI-catalyzed epoxide isomerization as a key step have been developed to convert readily available substrates into valuable chiral chemicals. Cascade conversion of terminal or internal alkenes into chiral acids, alcohols or amines was achieved, which involved SOI-catalyzed enantio-retentive isomerization of terminal epoxides via 1,2-H shift, or internal epoxides via 1,2-methyl shift. SOI-involved cascades were also developed to convert racemic epoxides into chiral acids or amines via dynamic kinetic resolution. Additionally, combining SOI-catalyzed isomerization with enantioselective C-C bond forming enzymes enabled the synthesis of chiral amino acids or amino alcohols from racemic epoxides. Finally, integration of SOI-involved cascades with biosynthesis pathways allowed for the direct utilization of renewable substrates for the sustainable synthesis of high-value natural chemicals such as alcohols, acids, and esters.

Enzyme-Catalyzed Meinwald Rearrangement with an Unusual Regioselective and Stereospecific 1,2-Methyl Shift

The Meinwald rearrangement is a synthetically useful reaction but often lacks regioselectivity and stereocontrol. A significant challenge in the Meinwald rearrangement of internal epoxides is the non-regioselective migration of different substituents to give a mixture of products. Herein, an enzyme-catalyzed regioselective and stereospecific 1,2-methyl shift in the Meinwald rearrangement of internal epoxides is reported. Styrene oxide isomerase (SOI) catalyzed the unique isomerization of internal epoxides through 1,2-methyl shift without 1,2-hydride shift to give the corresponding aldehydes and a cyclic ketone as the sole product. SOI-catalyzed isomerization showed high stereospecificity, fully retaining the stereoconfiguration. The synthetic utility of this enzymatic Meinwald rearrangement was demonstrated by its incorporation into three new types of enantioselective cascades, to convert trans-β-methyl styrenes into the corresponding R-configured alcohols, acids, or amines in high ee and yield.

Bioproduction of Natural Phenethyl Acetate, Phenylacetic Acid, Ethyl Phenylacetate, and Phenethyl Phenylacetate from Renewable Feedstock

Natural phenethyl acetate (PEA), phenylacetic acid (PAA), ethyl phenylacetate (Et-PA), and phenethyl phenylacetate (PE-PA) are highly desirable aroma chemicals, but with limited availability and high price. Here, green, sustainable, and efficient bioproduction of these chemicals as natural products from renewable feedstocks was developed. PEA and PAA were synthesized from l-phenylalanine (l-Phe) via novel six- and five-enzyme cascades, respectively. Whole-cell-based cascade biotransformation of 100 mm l-Phe in a two-phase system (aqueous/organic: 1 : 0.5 v/v) containing ethyl oleate or biodiesel as green solvent gave 13.6 g L^-1 PEA (83.1 % conv.) and 11.6 g L^-1 PAA (87.1 % conv.), respectively. Coupled fermentation and biotransformation approach produced 10.4 g L^-1 PEA and 9.2 g L^-1 PAA from glucose or glycerol, respectively. The biosynthesized PAA was converted to natural Et-PA and PE-PA by esterification using lipases with ethanol or 2-phenylethanol derived from sugar, affording 2.7 g L^-1 Et-PA (83.1 % conv.) and 4.6 g L^-1 PE-PA (96.3 % conv.), respectively.

Bioproduction of 2-Phenylethanol through Yeast Fermentation on Synthetic Media and on Agro-Industrial Waste and By-Products: A Review

Due to its pleasant rosy scent, the aromatic alcohol 2-phenylethanol (2-PE) has a huge market demand. Since this valuable compound is used in food, cosmetics and pharmaceuticals, consumers and safety regulations tend to prefer natural methods for its production rather than the synthetic ones. Natural 2-PE can be either produced through the extraction of essential oils from various flowers, including roses, hyacinths and jasmine, or through biotechnological routes. In fact, the rarity of natural 2-PE in flowers has led to the inability to satisfy the large market demand and to a high selling price. Hence, there is a need to develop a more efficient, economic, and environmentally friendly biotechnological approach as an alternative to the conventional industrial one. The most promising method is through microbial fermentation, particularly using yeasts. Numerous yeasts have the ability to produce 2-PE using l-Phe as precursor. Some agro-industrial waste and by-products have the particularity of a high nutritional value, making them suitable media for microbial growth, including the production of 2-PE through yeast fermentation. This review summarizes the biotechnological production of 2-PE through the fermentation of different yeasts on synthetic media and on various agro-industrial waste and by-products.

Recent advances in (chemo)enzymatic cascades for upgrading bio-based resources

Developing (chemo)enzymatic cascades is very attractive for green synthesis, because they streamline multistep synthetic processes. In this Feature Article, we have summarized the recent advances in in vitro or whole-cell cascade reactions with a focus on the use of renewable bio-based resources as starting materials. This includes the synthesis of rare sugars (such as ketoses, L-ribulose, D-tagatose, myo-inositol or aminosugars) from readily available carbohydrate sources (cellulose, hemi-cellulose, starch), in vitro enzyme pathways to convert glucose to various biochemicals, cascades to convert 5-hydroxymethylfurfural and furfural obtained from lignin or xylose into novel precursors for polymer synthesis, the syntheses of phenolic compounds, cascade syntheses of aliphatic and highly reduced chemicals from plant oils and fatty acids, upgrading of glycerol or ethanol as well as cascades to transform natural L-amino acids into high-value (chiral) compounds. In several examples these processes have demonstrated their efficiency with respect to high space-time yields and low E-factors enabling mature green chemistry processes. Also, the strengths and limitations are discussed and an outlook is provided for improving the existing and developing new cascades.

Designing Modular Cell-free Systems for Tunable Biotransformation of l-phenylalanine to Aromatic Compounds

Cell-free systems have been used to synthesize chemicals by reconstitution of in vitro expressed enzymes. However, coexpression of multiple enzymes to reconstitute long enzymatic pathways is often problematic due to resource limitation/competition (e.g., energy) in the one-pot cell-free reactions. To address this limitation, here we aim to design a modular, cell-free platform to construct long biosynthetic pathways for tunable synthesis of value-added aromatic compounds, using (S)-1-phenyl-1,2-ethanediol ((S)-PED) and 2-phenylethanol (2-PE) as models. Initially, all enzymes involved in the biosynthetic pathways were individually expressed by an E. coli-based cell-free protein synthesis (CFPS) system and their catalytic activities were confirmed. Then, three sets of enzymes were coexpressed in three cell-free modules and each with the ability to complete a partial pathway. Finally, the full biosynthetic pathways were reconstituted by mixing two related modules to synthesize (S)-PED and 2-PE, respectively. After optimization, the final conversion rates for (S)-PED and 2-PE reached 100 and 82.5%, respectively, based on the starting substrate of l-phenylalanine. We anticipate that the modular cell-free approach will make a possible efficient and high-yielding biosynthesis of value-added chemicals.

Sensing, Uptake and Catabolism of L-Phenylalanine During 2-Phenylethanol Biosynthesis via the Ehrlich Pathway in Saccharomyces cerevisiae

2-Phenylethanol (2-PE) is an important flavouring ingredient with a persistent rose-like odour, and it has been widely utilized in food, perfume, beverages, and medicine. Due to the potential existence of toxic byproducts in 2-PE resulting from chemical synthesis, the demand for “natural” 2-PE through biotransformation is increasing. L-Phenylalanine (L-Phe) is used as the precursor for the biosynthesis of 2-PE through the Ehrlich pathway by Saccharomyces cerevisiae. The regulation of L-Phe metabolism in S. cerevisiae is complicated and elaborate. We reviewed current progress on the signal transduction pathways of L-Phe sensing, uptake of extracellular L-Phe and 2-PE synthesis from L-Phe through the Ehrlich pathway. Moreover, the anticipated bottlenecks and future research directions for S. cerevisiae biosynthesis of 2-PE are discussed.

Production of (R)-mandelic acid from styrene, L-phenylalanine, glycerol, or glucose via cascade biotransformations

(R)-mandelic acid is an industrially important chemical, especially used for producing antibiotics. Its chemical synthesis often uses highly toxic cyanide to produce its racemic form, followed by kinetic resolution with 50% maximum yield. Here we report a green and sustainable biocatalytic method for producing (R)-mandelic acid from easily available styrene, biobased L-phenylalanine, and renewable feedstocks such as glycerol and glucose, respectively. An epoxidation-hydrolysis-double oxidation artificial enzyme cascade was developed to produce (R)-mandelic acid at 1.52 g/L from styrene with > 99% ee. Incorporation of deamination and decarboxylation into the above cascade enables direct conversion of L-phenylalanine to (R)-mandelic acid at 913 mg/L and > 99% ee. Expressing the five-enzyme cascade in an L-phenylalanine-overproducing E. coli NST74 strain led to the direct synthesis of (R)-mandelic acid from glycerol or glucose, affording 228 or 152 mg/L product via fermentation. Moreover, coupling of E. coli cells expressing L-phenylalanine biosynthesis pathway with E. coli cells expressing the artificial enzyme cascade enabled the production of 760 or 455 mg/L (R)-mandelic acid from glycerol or glucose. These simple, safe, and green methods show great potential in producing (R)-mandelic acid from renewable feedstocks.

Recent advances in artificial enzyme cascades for the production of value-added chemicals

Enzyme cascades are efficient tools to perform multi-step synthesis in one-pot in a green and sustainable manner, enabling non-natural synthesis of valuable chemicals from easily available substrates by artificially combining two or more enzymes. Bioproduction of many high-value chemicals such as chiral and highly functionalised molecules have been achieved by developing new enzyme cascades. This review summarizes recent advances on engineering and application of enzyme cascades to produce high-value chemicals (alcohols, aldehydes, ketones, amines, carboxylic acids, etc) from simple starting materials. While 2-step enzyme cascades are developed for versatile enantioselective synthesis, multi-step enzyme cascades are engineered to functionalise basic chemicals, such as styrenes, cyclic alkanes, and aromatic compounds. New cascade reactions have also been developed for producing valuable chemicals from bio-based substrates, such as ʟ-phenylalanine, and renewable feedstocks such as glucose and glycerol. The challenges in current process and future outlooks in the development of enzyme cascades are also addressed.

Efficient Synthesis of Phenylacetate and 2-Phenylethanol by Modular Cascade Biocatalysis

The green and sustainable synthesis of chemicals from renewable feedstocks by a biotransformation approach has gained increasing attention in recent years. In this work, we developed enzymatic cascades to efficiently convert l-phenylalanine into 2-phenylethanol (2-PE) and phenylacetic acid (PAA), l-tyrosine into tyrosol (p-hydroxyphenylethanol, p-HPE) and p-hydroxyphenylacetic acid (p-HPAA). The enzymatic cascade was cast into an aromatic aldehyde formation module, followed by an aldehyde reduction module, or aldehyde oxidation module, to achieve one-pot biotransformation by using recombinant Escherichia coli. Biotransformation of 50 mM l-Phe produced 6.76 g/L PAA with more than 99 % conversion and 5.95 g/L of 2-PE with 97 % conversion. The bioconversion efficiencies of p-HPAA and p-HPE from l-Tyr reached to 88 and 94 %, respectively. In addition, m-fluoro-phenylalanine was further employed as an unnatural aromatic amino acid substrate to obtain m-fluoro-phenylacetic acid; >96 % conversion was achieved. Our results thus demonstrated high-yielding and potential industrial synthesis of above aromatic compounds by one-pot cascade biocatalysis.

Biochemistry of prenylated-FMN enzymes

The reversible (de)carboxylation of unsaturated carboxylic acids is carried out by the UbiX-UbiD system, ubiquitously present in microbes. The biochemical basis of this challenging reaction has recently been uncovered by the discovery of the UbiD cofactor, prenylated FMN (prFMN). This heavily modified flavin is synthesized by the flavin prenyltransferase UbiX, which catalyzes the non-metal dependent prenyl transfer from dimethylallyl(pyro)phosphate (DMAP(P)) to the flavin N5 and C6 positions, creating a fourth non-aromatic ring. Following prenylation, prFMN undergoes oxidative maturation to form the iminium species required for UbiD activity. prFMN^iminium acts as a prostethic group and is bound via metal ion mediated interactions between UbiD and the prFMN^iminium phosphate moiety. The modified isoalloxazine ring is place adjacent to the E(D)-R-E UbiD signature sequent motif. The fungal ferulic acid decarboxylase Fdc from Aspergillus niger has emerged as a UbiD-model system, and has yielded atomic level insight into the prFMN^iminium mediated (de)carboxylation. A wealth of data now supports a mechanism reliant on reversible 1,3 dipolar cycloaddition between substrate and cofactor for this enzyme. This poses the intriguing question whether a similar mechanism is used by all UbiD enzymes, especially those that act as carboxylases on inherently more difficult substrates such as phenylphosphate or benzene/naphthalene. Indeed, considerable variability in terms of oligomerization, domain motion and active site structure is now reported for the UbiD family.

High-Efficient Production of (S)-1-[3,5-Bis(trifluoromethyl)phenyl]ethanol via Whole-Cell Catalyst in Deep-Eutectic Solvent-Containing Micro-Aerobic Medium System

The ratio of substrate to catalyst (S/C) is a prime target for the application of asymmetric production of enantiomerically enriched intermediates by whole-cell biocatalyst. In the present study, an attractive increase in S/C was achieved in a natural deep-eutectic solvent (NADES) containing reaction system under microaerobic condition for high production of (S)-1-[3,5-bis(trifluoromethyl)phenyl]ethanol ((S)-3,5-BTPE) with Candida tropicalis 104. In PBS buffer (0.2 M, pH 8.0) at 200 rpm and 30 °C, 79.5 g (Dry Cell Weight, DCW)/L C. tropicalis 104 maintained the same yield of 73.7% for the bioreduction of 3,5-bis(trifluoromethyl)acetophenone (BTAP) under an oxygen-deficient environment compared with oxygen-sufficient conditions, while substrate load increased 4.0-fold (from 50 mM to 200 mM). Furthermore, when choline chloride:trehalose (ChCl:T, 1:1 molar ratio) was introduced into the reaction system for its versatility of increasing cell membrane permeability and declining BTAP cytotoxicity to biocatalyst, the yields were further increased to 86.2% under 200 mM BTAP, or 72.9% at 300 mM BTAP. After the optimization of various reaction parameters involved in the bioreduction, and the amount of biocatalyst and maltose co-substrate remained 79.5 g (DCW)/L and 50 g/L, the S/C for the reduction elevated 6.3 times (3.8 mM/g versus 0.6 mM/g). By altering the respiratory pattern of the whole-cell biocatalyst and exploiting the ChCl:T-containing reaction system, the developed strategy exhibits an attractive potential for enhancing catalytic efficiency of whole-cell-mediated reduction, and provides valuable insight for the development of whole-cell catalysis.

One-Pot Conversion of Cinnamaldehyde to 2-Phenylethanol via a Biosynthetic Cascade Reaction

A novel biosynthetic pathway for the production of natural 2-phenylethanol from cinnamaldehyde is reported. An ene-reductase (OYE)-mediated selective hydrogenation of cinnamaldehyde to hydrocinnamaldehyde is followed by a regioselective Baeyer-Villiger oxidation (BVMO) to produce the corresponding formate ester that either spontaneously hydrolyzes to 2-phenylethanol in water or is assisted by a formate dehydrogenase (FDH). This cascade reaction is performed in a one-pot fashion at ambient temperature and pressure. High selectivity and complete conversion were achieved.

Biocatalytic selective functionalisation of alkenes via single-step and one-pot multi-step reactions

Alkenes are excellent starting materials for organic synthesis due to the versatile reactivity of C[double bond, length as m-dash]C bonds and the easy availability of many unfunctionalised alkenes. Direct regio- and/or enantioselective conversion of alkenes into functionalised (chiral) compounds has enormous potential for industrial applications, and thus has attracted the attention of researchers for extensive development using chemo-catalysis over the past few years. On the other hand, many enzymes have also been employed for conversion of alkenes in a highly selective and much greener manner to offer valuable products. Herein, we review recent advances in seven well-known types of biocatalytic conversion of alkenes. Remarkably, recent mechanism-guided directed evolution and enzyme cascades have enabled the development of seven novel types of single-step and one-pot multi-step functionalisation of alkenes, some of which are even unattainable via chemo-catalysis. These new reactions are particularly highlighted in this feature article. Overall, we present an ever-expanding enzyme toolbox for various alkene functionalisations inspiring further research in this fast-developing theme.