Asymmetric alkene reduction by yeast old yellow enzymes and by a novelZymomonas mobilis reductase

Design of a Self-Sufficient Whole-Cell Cascade for the Production of (R)-Citronellal from Geraniol

(R)-Citronellal is a key chiral precursor of high-value chemicals, such as the best-selling flavor compound (-)-menthol; however, the conventional synthesis suffers from low yield and unsatisfactory enantioselectivity. In this study, we developed a highly atom-efficient hydrogen-borrowing cascade for the synthesis of (R)-citronellal from geraniol using alcohol dehydrogenase from Escherichia coli K12 (AdhP) and ene-reductase from Saccharomyces cerevisiae YJM1341 (OYE2p). The key rate-limiting enzyme, AdhP, was subjected to structure-guided semirational engineering, and the triple mutant AdhP260T/284A/268P (M3) was obtained that demonstrated a 1.28-fold improvement in catalytic efficiency (kcat/Km) toward geraniol. After optimization of the reaction conditions, the hydrogen-borrowing cascade system achieved the conversion of 23.14 g/L geraniol into (R)-citronellal at a conversion rate of 98.23% with 96.7% ee. This work represents an alternative approach for the biosynthesis of (R)-citronellal without sacrificing a cosubstrate or additional enzymes.

BioLindlar Catalyst: Ene-Reductase-Promoted Selective Bioreduction of Cyanoalkynes to Give (Z)-Cyanoalkenes

The direct synthesis of alkenes from alkynes usually requires the use of transition-metal catalysts. Unfortunately, efficient biocatalytic alternatives for this transformation have yet to be discovered. Herein, the selective bioreduction of electron-deficient alkynes to alkenes catalysed by ene-reductases (EREDs) is described. Alkynes bearing ketone, aldehyde, ester, and nitrile moieties have been effectively reduced with excellent conversions and stereoselectivities, observing clear trends for the E/Z ratios depending on the nature of the electron-withdrawing group. In the case of cyanoalkynes, (Z)-alkenes were obtained as the major product, and the reaction scope was expanded to a wide variety of aromatic substrates (up to >99 % conversion, and Z/E stereoselectivities of up to >99/1). Other alkynes containing aldehyde, ketone, or ester functionalities also proved to be excellent substrates, and interestingly gave the corresponding (E)-alkenes. Preparative biotransformations were performed on a 0.4 mmol scale, producing the desired (Z)-cyanoalkenes with good to excellent isolated yields (63-97 %). This novel reactivity has been rationalised through molecular docking by predicting the binding poses of key molecules in the ERED-pu-0006 active site.

Applications of Ene-Reductases in the Synthesis of Flavors and Fragrances

Flavors and fragrances (F&F) are interesting organic compounds in chemistry. These compounds are widely used in the food, cosmetic, and medical industries. Enzymatic synthesis exhibits several advantages over natural extraction and chemical preparation, including a high yield, stable quality, mildness, and environmental friendliness. To date, many oxidoreductases and hydrolases have been used to biosynthesize F&F. Ene-reductases (ERs) are a class of biocatalysts that can catalyze the asymmetric reduction of α,β-unsaturated compounds and offer superior specificity and selectivity; therefore, ERs have been increasingly considered an ideal alternative to their chemical counterparts. This review summarizes the research progress on the use of ERs in F&F synthesis over the past 20 years, including the achievements of various scholars, the differences and similarities among the findings, and the discussions of future research trends related to ERs. We hope this review can inspire researchers to promote the development of biotechnology in the F&F industry.

Applications of biocatalytic CC bond reductions in the synthesis of flavours and fragrances

Industrial biotechnology and biocatalysis can provide very effective synthetic tools to increase the sustainability of the production of fine chemicals, especially flavour and fragrance (F&F) ingredients, the market demand of which has been constantly increasing in the last years. One of the most important transformations in F&F chemistry is the reduction of CC bonds, typically carried out with metal-catalysed hydrogenations or hydride-based reagents. Its biocatalytic counterpart is a competitive alternative, showcasing a range of advantages such as excellent chemo-, regio- and stereoselectivity, ease of implementation, mild reaction conditions and modest environmental impact. In the present review, the application of biocatalysed alkene reductions (from microbial fermentations with wild-type strains to engineered isolated ene-reductase enzymes) to synthetic processes useful for the F&F industry will be described, highlighting not only the exquisite stereoselectivity achieved, but also the overall improvement when chirality is not involved. Multi-enzymatic cascades involving CC bioreductions are also examined, which allow much greater chemical complexity to be built in one-pot biocatalytic systems.

Identification and characterization of an ene-reductase from Corynebacterium casei

The asymmetric reduction of α, β-unsaturated compounds conjugated with electron-withdrawing group by ene-reductases (ERs) is a valuable method for the synthesis of enantiopure chiral compounds. This study introduced an ER from Corynebacterium casei (CcER) which was heterologously expressed in Escherichia coli BL21(DE3), and the purified recombinant CcER was characterized for its biocatalytic properties. CcER exhibited the highest specific activity at 40 °C and pH 6.5, and showcased appreciable stability below 40 °C over a pH range of 6.0-7.0. The enzyme displayed high resistance to methanol. CcER accepted NADH or NADPH as a cofactor and exhibited a broad substrate spectrum towards α, β-unsaturated compounds. It achieved complete conversion of 2-cyclohexen-1-one and good performance for stereoselective reduction of (R)-carvone (conversion 98 %, diastereoselectivity 96 %). This study highlights the robustness and potential of CcER.

Catalytic Asymmetric Conjugate Reduction

Enantioselective reduction reactions are privileged transformations for the construction of trisubstituted stereogenic centers. While these include established synthetic strategies, such as asymmetric hydrogenation, methods based on the enantioselective addition of hydridic reagents to electrophilic prochiral substrates have also gained importance. In this context, the asymmetric conjugate reduction (ACR) of α,β-unsaturated compounds has become a convenient approach for the synthesis of chiral compounds with trisubstituted stereocenters in α-, β-, or γ-position to electron-withdrawing functional groups. Because such activating groups are diverse and amenable of further derivatizations, ACRs provide a general and powerful synthetic entry towards a variety of valuable chiral building blocks. This Review provides a comprehensive collection of catalytic ACR methods involving transition-metal, organic, and enzymatic catalysis since its first versions dating back to the late 1970s.

Tunable Production of (R)- or (S)-Citronellal from Geraniol via a Bienzymatic Cascade Using a Copper Radical Alcohol Oxidase and Old Yellow Enzyme

Biocatalytic pathways for the synthesis of (-)-menthol, the most sold flavor worldwide, are highly sought-after. To access the key intermediate (R)-citronellal used in current major industrial production routes, we established a one-pot bienzymatic cascade from inexpensive geraniol, overcoming the problematic biocatalytic reduction of the mixture of (E/Z)-isomers in citral by harnessing a copper radical oxidase (CgrAlcOx) and an old yellow enzyme (OYE). The cascade using OYE2 delivered 95.1% conversion to (R)-citronellal with 95.9% ee, a 62 mg scale-up affording high yield and similar optical purity. An alternative OYE, GluER, gave (S)-citronellal from geraniol with 95.3% conversion and 99.2% ee.

"A Study in Yellow": Investigations in the Stereoselectivity of Ene-Reductases

Ene‐reductases from the Old Yellow Enzyme (OYE) superfamily are a well‐known and efficient biocatalytic alternative for the asymmetric reduction of C=C bonds. Considering the broad variety of substituents that can be tolerated, and the excellent stereoselectivities achieved, it is apparent why these enzymes are so appealing for preparative and industrial applications. Different classes of C=C bonds activated by at least one electron‐withdrawing group have been shown to be accepted by these versatile biocatalysts in the last decades, affording a vast range of chiral intermediates employed in the synthesis of pharmaceuticals, agrochemicals, flavours, fragrances and fine chemicals. In order to access both enantiomers of reduced products, stereodivergent pairs of OYEs are desirable, but their natural occurrence is limited. The detailed knowledge of the stereochemical course of the reaction can uncover alternative strategies to orient the selectivity via mutagenesis, evolution, and substrate engineering. An overview of the ongoing studies on OYE‐mediated bioreductions will be provided, with particular focus on stereochemical investigations by deuterium labelling.

Engineering of Yeast Old Yellow Enzyme OYE3 Enables Its Capability Discriminating of (E)-Citral and (Z)-Citral

The importance of yeast old yellow enzymes is increasingly recognized for direct asymmetric reduction of (E/Z)-citral to (R)-citronellal. As one of the most performing old yellow enzymes, the enzyme OYE3 from Saccharomyces cerevisiae S288C exhibited complementary enantioselectivity for the reduction of (E)-citral and (Z)-citral, resulting in lower e.e. value of (R)-citronellal in the reduction of (E/Z)-citral. To develop a novel approach for the direct synthesis of enantio-pure (R)-citronellal from the reduction of (E/Z)-citral, the enzyme OYE3 was firstly modified by semi-rational design to improve its (R)-enantioselectivity. The OYE3 variants W116A and S296F showed strict (R)-enantioselectivity in the reduction of (E)-citral, and significantly reversed the (S)-enantioselectivity in the reduction of (Z)-citral. Next, the double substitution of OYE3 led to the unique variant S296F/W116G, which exhibited strict (R)-enantioselectivity in the reduction of (E)-citral and (E/Z)-citral, but was not active on (Z)-citral. Relying on its capability discriminating (E)-citral and (Z)-citral, a new cascade reaction catalyzed by the OYE3 variant S296F/W116G and glucose dehydrogenase was developed, providing the enantio-pure (R)-citronellal and the retained (Z)-citral after complete reduction of (E)-citral.

Enzymatic strategies for asymmetric synthesis

Enzymes, at the turn of the 21st century, are gaining a momentum. Especially in the field of synthetic organic chemistry, a broad variety of biocatalysts are being applied in an increasing number of processes running at up to industrial scale. In addition to the advantages of employing enzymes under environmentally friendly reaction conditions, synthetic chemists are recognizing the value of enzymes connected to the exquisite selectivity of these natural (or engineered) catalysts. The use of hydrolases in enantioselective protocols paved the way to the application of enzymes in asymmetric synthesis, in particular in the context of biocatalytic (dynamic) kinetic resolutions. After two decades of impressive development, the field is now mature to propose a panel of catalytically diverse enzymes for (i) stereoselective reactions with prochiral compounds, such as double bond reduction and bond forming reactions, (ii) formal enantioselective replacement of one of two enantiotopic groups of prochiral substrates, as well as (iii) atroposelective reactions with noncentrally chiral compounds. In this review, the major enzymatic strategies broadly applicable in the asymmetric synthesis of optically pure chiral compounds are presented, with a focus on the reactions developed within the past decade.

Biocatalytic Reduction Reactions from a Chemist's Perspective

Reductions play a key role in organic synthesis, producing chiral products with new functionalities. Enzymes can catalyse such reactions with exquisite stereo-, regio- and chemoselectivity, leading the way to alternative shorter classical synthetic routes towards not only high-added-value compounds but also bulk chemicals. In this review we describe the synthetic state-of-the-art and potential of enzymes that catalyse reductions, ranging from carbonyl, enone and aromatic reductions to reductive aminations.

Design and application of a bi-functional redox biocatalyst through covalent co-immobilization of ene-reductase and glucose dehydrogenase

An immobilized bi-functional redox biocatalyst was designed for the asymmetric reduction of alkenes by nicotinamide-dependent ene-reductases. The biocatalyst, which consists of co-immobilized ene-reductase and glucose dehydrogenase, was implemented in biotransformations in the presence of glucose as source of reducing equivalents and catalytic amounts of the cofactor. Enzyme co-immobilization employing glutaraldehyde activated Relizyme HA403/M as support material was performed directly from the crude cell-free extract obtained after protein overexpression in E. coli and cell lysis, avoiding enzyme purification steps. The resulting optimum catalyst showed excellent level of activity and stereoselectivity in asymmetric reduction reactions using either OYE3 from Saccharomyces cerevisiae or NCR from Zymomonas mobilis in the presence of organic cosolvents in up to 20 vol%. The bi-functional redox biocatalyst, which demonstrated remarkable reusability over several cycles, was applied in preparative-scale synthesis at 50 mM substrate concentration and provided access to three industrially relevant chiral compounds in high enantiopurity (ee up to 97 %) and in up to 42 % isolated yield. The present method highlights the potential of (co-)immobilization of ene-reductases, notorious for their poor scalability, and complements the few existing methods available for increasing productivity in asymmetric bioreduction reactions.

Biocatalytic Reduction of Activated Cinnamic Acid Derivatives : Asymmetric reduction of C=C double bonds using Johnson Matthey enzymes

The asymmetric reduction of C=C double bonds is a sought-after chemical transformation to obtain chiral molecules used in the synthesis of fine chemicals. Biocatalytic C=C double bond reduction is a particularly interesting transformation complementary to more established chemocatalytic methods. The enzymes capable of catalysing this reaction are called ene-reductases (ENEs). For the reaction to take place, ENEs need an electron withdrawing group (EWG) in conjugation with the double bond. Especially favourable EWGs are carbonyls and nitro groups; other EWGs, such as carboxylic acids, esters or nitriles, often give poor results. In this work, a substrate engineering strategy is proposed whereby a simple transformation of the carboxylic acid into a fluorinated ester or a cyclic imide allows to increase the ability of ENEs to reduce the conjugated double bond. Up to complete conversion of the substrates tested was observed with enzymes ENE-105 and *ENE-69.

Non-Conventional Yeasts as Sources of Ene-Reductases for the Bioreduction of Chalcones

Thirteen Non-Conventional Yeasts (NCYs) have been investigated for their ability to reduce activated C=C bonds of chalcones to obtain the corresponding dihydrochalcones. A possible correlation between bioreducing capacity of the NCYs and the substrate structure was estimated. Generally, whole-cells of the NCYs were able to hydrogenate the C=C double bond occurring in (E)-1,3-diphenylprop-2-en-1-one, while worthy bioconversion yields were obtained when the substrate exhibited the presence of a deactivating electron-withdrawing Cl substituent on the B-ring. On the contrary, no conversion was generally found, with a few exceptions, in the presence of an activating electron-donating substituent OH. The bioreduction aptitude of the NCYs was apparently correlated to the logP value: Compounds characterized by a higher logP exhibited a superior aptitude to be reduced by the NCYs than compounds with a lower logP value.

Bioconversion of essential oil components of Perilla frutescens by Saccharomyces cerevisiae

The essential oil of perilla (Perilla frutescens) contains volatile low molecular weight compounds such as monoterpenes and phenylpropenes. The composition of the essential oil is classified into about ten chemotypes. The biosynthesis of these constituents is strictly controlled genetically. Among the compounds contained in perilla essential oil, the bioconversion of pure compounds such as perillaldehyde, limonene, and citral has been reported, but that of many other components has not. In addition, changes in the volatile components of raw plant material during brewing have also been investigated for wine and beer. In this study, we examined the bioconversion of perilla essential oil components by Saccharomyces cerevisiae during the brewing of liquor with perilla leaves. S. cerevisiae was added to the ethanol-water extract of dried leaves of P. frutescens and P. citriodora for seven essential oil types: perillaldehyde type, piperitenone type, perillene type, perillaketone type, elsholtziaketone type, citral type, and phenylpropanoid type. Volatile compounds in the reaction mixtures were analyzed by solid-phase microextraction (SPME)-GC-MS, revealing bioconversion of perillaldehyde, isoegomaketone, neral, and geranial by S. cerevisiae. Analysis of the conversion products suggests that they were formed by the reduction of C=C bonds and aldehydes, as well as by esterification and dehydration reactions.

Engineering the Enantioselectivity of Yeast Old Yellow Enzyme OYE2y in Asymmetric Reduction of (E/Z)-Citral to (R)-Citronellal

The members of the Old Yellow Enzyme (OYE) family are capable of catalyzing the asymmetric reduction of (E/Z)-citral to (R)-citronellal-a key intermediate in the synthesis of L-menthol. The applications of OYE-mediated biotransformation are usually hampered by its insufficient enantioselectivity and low activity. Here, the (R)-enantioselectivity of Old Yellow Enzyme from Saccharomyces cerevisiae CICC1060 (OYE2y) was enhanced through protein engineering. The single mutations of OYE2y revealed that the sites R330 and P76 could act as the enantioselectivity switch of OYE2y. Site-saturation mutagenesis was conducted to generate all possible replacements for the sites R330 and P76, yielding 17 and five variants with improved (R)-enantioselectivity in the (E/Z)-citral reduction, respectively. Among them, the variants R330H and P76C partly reversed the neral derived enantioselectivity from 32.66% e.e. (S) to 71.92% e.e. (R) and 37.50% e.e. (R), respectively. The docking analysis of OYE2y and its variants revealed that the substitutions R330H and P76C enabled neral to bind with a flipped orientation in the active site and thus reverse the enantioselectivity. Remarkably, the double substitutions of R330H/P76M, P76G/R330H, or P76S/R330H further improved (R)-enantioselectivity to >99% e.e. in the reduction of (E)-citral or (E/Z)-citral. The results demonstrated that it was feasible to alter the enantioselectivity of OYEs through engineering key residue distant from active sites, e.g., R330 in OYE2y.

Enantioselective Reduction of Citral Isomers in NCR Ene Reductase: Analysis of an Active-Site Mutant Library

A deeper understanding of the >99 % S-selective reduction of both isomers of citral catalyzed by NCR ene reductase was achieved by active-site mutational studies and docking simulation. Though structurally similar, the E/Z isomers of citral showed a significantly varying selectivity response to introduced mutations. Although it was possible to invert (E)-citral reduction enantioselectivity to ee 46 % (R) by introducing mutation W66A, for (Z)-citral it remained ≥88 % (S) for all single-residue variants. Residue 66 seems to act as a lever for opposite binding modes. This was underlined by a W66A-based double-mutant library that enhanced the (E)-citral derived enantioselectivity to 63 % (R) and significantly lowered the S selectivity for (Z)-citral to 44 % (S). Formation of (R)-citronellal from an (E/Z)-citral mixture is a desire in industrial (-)-menthol synthesis. Our findings pave the way for a rational enzyme engineering solution.

Stereoselective Synthesis of Functionalized Bicyclic Scaffolds by Passerini 3-Center-2-Component Reactions of Cyclic Ketoacids

We report the use of bifunctional starting materials (ketoacids) in a diastereoselective Passerini three-center-two-component reaction. Study of the reaction scope revealed the required structural features for stereoselectivity in the isocyanide addition. In this system, an interesting isomerization of the primary Passerini product - the α-carboxamido lactone - into an atypical product, an α-hydroxy imide, was found to occur under acidic conditions. Furthermore, enantioenriched Passerini products can be generated from an enantioenriched ketoacid obtained by chemoenzymatic synthesis.

Revealing Additional Stereocomplementary Pairs of Old Yellow Enzymes by Rational Transfer of Engineered Residues

Every year numerous protein engineering and directed evolution studies are published, increasing the knowledge that could be used by protein engineers. Here we test a protein engineering strategy that allows quick access to improved biocatalysts with very little screening effort. Conceptually it is assumed that engineered residues previously identified by rational and random methods induce similar improvements when transferred to family members. In an application to ene-reductases from the Old Yellow Enzyme (OYE) family, the newly created variants were tested with three compounds, revealing more stereocomplementary OYE pairs with potent turnover frequencies (up to 660 h^-1 ) and excellent stereoselectivities (up to >99 %). Although systematic prediction of absolute enantioselectivity of OYE variants remains a challenge, "scaffold sampling" was confirmed as a promising addition to protein engineers' collection of strategies.

Characterization of 12-Oxophytodienoic Acid Reductases from Rose-scented Geranium (Pelargonium graveolens)

Pelargonium graveolens L'Hér, also referred to as rose geranium, is a popular herbal plant with typical rosy fragrance largely based on the blend of monoterpenoid constituents. Among them, citronellol, which is biosynthesized from geraniol via double bond reduction, is the most abundant scent compound. In this study, three 12-oxophytodienoic acid reductases (PgOPR1–3) have been cloned from P. graveolens, as possible candidates for the double-bond reductase involved in citronellol biosynthesis. The bacterially expressed recombinant PgOPRs did not reduce geraniol to citronellol, but stereoselectively converted citral into ( S)-citronellal in the presence of NADPH. Thus, the α,β-unsaturated carbonyl moiety in the substrate is essential for the catalytic activity of PgOPRs, as reported for OPRs from other plants and structurally related yeast old yellow enzymes. PgOPRs promiscuously accepted linear and cyclic α,β-unsaturated carbonyl substrates, including methacrolein, a typical reactive carbonyl compound. The possible biotechnological applications for PgOPRs in plant metabolic engineering, based on their catalytic properties, are discussed herein.

Applications of protein engineering to members of the old yellow enzyme family

In the 20 years since Massey's initial report in 1995, interest in using alkene reductases to prepare chiral intermediates for synthesis has grown rapidly. While native alkene reductases often show very high stereoselectivities toward favorable substrates, these enzymes have somewhat size-restricted active sites that limit their substrate ranges to small alkenes. In addition, most alkene reductases have the same stereoselectivities, which makes it difficult to access the "other" product enantiomers. Protein engineering strategies have been used to address both of these issues and good progress has been made in several cases. This review summarizes published examples through late 2014 and focuses on studies of six enzymes: Saccharomyces pastorianus OYE 1, tomato OPR1, Zymomonas mobilis NCR, Enterobacter cloacae PB2 PETN reductase, Bacillus subtilis YqjM and Pichia stipitis OYE 2.6.

Transcriptome analysis of Zymomonas mobilis ZM4 reveals mechanisms of tolerance and detoxification of phenolic aldehyde inhibitors from lignocellulose pretreatment

BACKGROUND

Phenolic aldehydes generated from lignocellulose pretreatment exhibited severe toxic inhibitions on microbial growth and fermentation. Numerous tolerance studies against furfural, 5-hydroxymethyl-2-furaldehyde (HMF), acetate, and ethanol were reported, but studies on inhibition of phenolic aldehyde inhibitors are rare. For ethanologenic strains, Zymomonas mobilis ZM4 is high in ethanol productivity and genetic manipulation feasibility, but sensitive to phenolic aldehyde inhibitors. Molecular mechanisms of tolerance for Z. mobilis toward phenolic aldehydes are not known.

RESULTS

We took the first insight into genomic response of Z. mobilis ZM4 to the phenolic aldehyde inhibitors derived from lignocellulose pretreatment. The results suggest that the toxicity to cells is caused by the functional group of phenolic aldehyde, similar to furfural and HMF, rather than aromatic groups or phenolic hydroxyl groups. Transcriptome response against 4-hydroxybenzaldehyde, syringaldehyde, and vanillin, representing phenolic groups H, S, and G, respectively, was investigated. The atlas of the important genes responsible for significantly enhanced and repressed genes at the genomic level was illustrated. 272 genes with twofold greater expressions than non-treated controls and 36 gene clusters in response to challenges of these phenolic aldehydes were identified. Several reductases encoded by ZMO1116, ZMO1696, and ZMO1885 were found to play the key roles in reducing phenolic aldehydes into the corresponding phenolic alcohols. Reduction of phenolic aldehydes by overexpression of ZMO1116, ZMO1696, and ZMO1885 in Z. mobilis ZM4 resulted in the increased inhibitor conversion and ethanol productivity, especially for 4-hydroxybenzaldehyde and vanillin. Several transporter genes such as ZMO0282, ZMO0283, ZMO0798, ZMO0799, and ZMO0800 was also displayed significantly increased expressions against the phenolic aldehydes.

CONCLUSIONS

The genes encoding reductases are with potentials on phenolic aldehydes-tolerant genes contributing to the reduction of phenolic aldehydes into the corresponding phenolic alcohols forms for Z. mobilis ZM4. Overexpression of the key genes improved the conversion ratio and ethanol productivity of 4-hydroxybenzaldehyde and vanillin with high toxicity. New knowledge obtained from this research aids understanding the mechanisms of bacterial tolerance and the development of the next-generation biocatalysts for advanced biofuels production.

Nitrile as Activating Group in the Asymmetric Bioreduction of β-Cyanoacrylic Acids Catalyzed by Ene-Reductases

Asymmetric bioreduction of an (E)-β-cyano-2,4-dienoic acid derivative by ene-reductases allowed a shortened access to a precursor of pregabalin [(S)-3-(aminomethyl)-5-methylhexanoic acid] possessing the desired configuration in up to 94% conversion and >99% ee. Deuterium labelling studies showed that the nitrile moiety was the preferred activating/anchor group in the active site of the enzyme over the carboxylic acid or the corresponding methyl ester.

Overcoming co-product inhibition in the nicotinamide independent asymmetric bioreduction of activated C=C-bonds using flavin-dependent ene-reductases

Eleven flavoproteins from the old yellow enzyme family were found to catalyze the disproportionation (“dismutation”) of conjugated enones. Incomplete conversions, which were attributed to enzyme inhibition by the co-product phenol could be circumvented via in situ co-product removal by scavenging the phenol using the polymeric adsorbent MP-carbonate. The optimized system allowed to reduce an alkene activated by ester groups in a “coupled-substrate” approach via nicotinamide-free hydrogen transfer with >90% conversion and complete stereoselectivity.

The structure of glycerol trinitrate reductase NerA from Agrobacterium radiobacter reveals the molecular reason for nitro- and ene-reductase activity in OYE homologues

In recent years, Old Yellow Enzymes (OYEs) and their homologues have found broad application in the efficient asymmetric hydrogenation of activated C=C bonds with high selectivities and yields. Members of this class of enzymes have been found in many different organisms and are rather diverse on the sequence level, with pairwise identities as low as 20 %, but they exhibit significant structural similarities with the adoption of a conserved (αβ)₈-barrel fold. Some OYEs have been shown not only to reduce C=C double bonds, but also to be capable of reducing nitro groups in both saturated and unsaturated substrates. In order to understand this dual activity we determined and analyzed X-ray crystal structures of NerA from Agrobacterium radiobacter, both in its apo form and in complex with 4-hydroxybenzaldehyde and with 1-nitro-2-phenylpropene. These structures, together with spectroscopic studies of substrate binding to several OYEs, indicate that nitro-containing substrates can bind to OYEs in different binding modes, one of which leads to C=C double bond reduction and the other to nitro group reduction.

Chemoenzymatic asymmetric synthesis of pregabalin precursors via asymmetric bioreduction of β-cyanoacrylate esters using ene-reductases

The asymmetric bioreduction of a library of β-cyanoacrylate esters using ene-reductases was studied with the aim to provide a biocatalytic route to precursors for GABA analogues, such as pregabalin. The stereochemical outcome could be controlled by substrate-engineering through size-variation of the ester moiety and by employing stereochemically pure (E)- or (Z)-isomers, which allowed to access both enantiomers of each product in up to quantitative conversion in enantiomerically pure form. In addition, stereoselectivities and conversions could be improved by mutant variants of OPR1, and the utility of the system was demonstrated by preparative-scale applications.

Crystal structure determination and mutagenesis analysis of the ene reductase NCR

The crystal structure of the "ene" nicotinamide-dependent cyclohexenone reductase (NCR) from Zymomonas mobilis (PDB ID: 4A3U) has been determined in complex with acetate ion, FMN, and nicotinamide, to a resolution of 1.95 Å. To study the activity and enantioselectivity of this enzyme in the bioreduction of activated α,β-unsaturated alkenes, the rational design methods site- and loop-directed mutagenesis were applied. Based on a multiple sequence alignment of various members of the Old Yellow Enzyme family, eight single-residue variants were generated and investigated in asymmetric bioreduction. Furthermore, a structural alignment of various ene reductases predicted four surface loop regions that are located near the entrance of the active site. Four NCR loop variants, derived from loop-swapping experiments with OYE1 from Saccharomyces pastorianus, were analysed for bioreduction. The three enzyme variants, P245Q, D337Y and F314Y, displayed increased activity compared to wild-type NCR towards the set of substrates tested. The active-site mutation Y177A demonstrated a clear influence on the enantioselectivity. The loop-swapping variants retained reduction efficiency, but demonstrated decreased enzyme activity compared with the wild-type NCR ene reductase enzyme.

Response surface methodology as optimization strategy for asymmetric bioreduction of (4S)-(+)-carvone by Cryptococcus gastricus

Response surface methodology was applied in optimizing the asymmetric bioreduction of (4S)-(+)-carvone to dihydrocarvone (with low incidence of unsought side reactions) by using whole-cells of Cryptococcus gastricus. A factorial design (2(5)) including five independent variables was performed: X(1)=incubation time; X(2)=pH; X(3)=amount of whole-cells; X(4)=concentration of (4S)-(+)-carvone; X(5)=concentration of cofactor-recycling system. The utilization of glucose and glycerol as cofactor-recycling systems was checked. On the basis of the results of factorial design, three independent variables (X(1), X(3) and X(4)) out of five were further selected for performing a central composite design (CCD). First and second order polynomial equations obtained by CCD were used to select the optimal values of independent variables in order to maximize the bioreduction yield of (4S)-(+)-carvone and, at the same time, to minimize the occurrence of side reactions (i.e. further reduction of dihydrocarvone to dihydrocarveol).

Asymmetric bioreduction of activated carbon-carbon double bonds using Shewanella yellow enzyme (SYE-4) as novel enoate reductase

Shewanella yellow enzyme (SYE-4), a novel recombinant enoate reductase, was screened against a variety of different substrates bearing an activated double bond, such as unsaturated cyclic ketones, diesters, and substituted imides. Dimethyl- and ethyl esters of 2-methylmaleic acid were selectively reduced to (R)-configured succinic acid derivatives and various N-substituted maleimides furnished the desired (R)-products in up to >99% enantiomeric excess. Naturally occurring (+)-carvone was selectively reduced to (-)-cis-dihydrocarvone and (-)-carvone was converted to the diastereomeric product, respectively. Overall SYE-4 proved to be a useful biocatalyst for the selective reduction of activated C = C double bonds and complements the pool of synthetic valuable enoate reductases.

Asymmetric bioreduction of activated alkenes to industrially relevant optically active compounds

Ene-reductases from the ‘Old Yellow Enzyme’ family of flavoproteins catalyze the asymmetric reduction of various α,β-unsaturated compounds at the expense of a nicotinamide cofactor. They have been applied to the synthesis of valuable enantiopure products, including chiral building blocks with broad industrial applications, terpenoids, amino acid derivatives and fragrances. The combination of these highly stereoselective biocatalysts with a cofactor recycling system has allowed the development of cost-effective methods for the generation of optically active molecules, which is strengthened by the availability of stereo-complementary enzyme homologues.