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

Application of protein engineering to ene-reductase for the synthesis of chiral compounds through asymmetric reaction

Ene-reductase (ER) has been widely applied for asymmetrical synthesis of chiral intermediates due to its substrate promiscuity, photoexcited reactivity, and excellent property with producing two chiral centers at a time. Natural ERs often exhibit the same stereoselectivity, and they need to be engineered for opposite configuration of chiral compounds. The hydrogenation process toward activated alkenes by ERs is composed of reductive half reaction and oxidative half reaction, which are dependent upon two cofactors NAD(P)H and flavin mononucleotide. The catalytic activity of ERs will be affected by the size of the substrate, the activating strength of the electron-withdrawing groups, redox potential of cofactors, and the loop flexibility around catalytic cavity. Currently, protein engineering to ERs has been successfully employed to enhance various catalytic properties, including photoexcited asymmetric synthesis. This review summarizes the approaches to reverse the stereoselectivity and enhance catalytic activity of ERs and new applications of the engineered ERs in photobiocatalytic asymmetric synthesis, besides the discussion with the existing molecular mechanisms of mutants regarding the improved catalytic performance.

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.

Asymmetric α-benzylation of cyclic ketones enabled by concurrent chemical aldol condensation and biocatalytic reduction

Chemoenzymatic cascade catalysis has emerged as a revolutionary tool for streamlining traditional retrosynthetic disconnections, creating new possibilities for the asymmetric synthesis of valuable chiral compounds. Here we construct a one-pot concurrent chemoenzymatic cascade by integrating organobismuth-catalyzed aldol condensation with ene-reductase (ER)-catalyzed enantioselective reduction, enabling the formal asymmetric α-benzylation of cyclic ketones. To achieve this, we develop a pair of enantiocomplementary ERs capable of reducing α-arylidene cyclic ketones, lactams, and lactones. Our engineered mutants exhibit significantly higher activity, up to 37-fold, and broader substrate specificity compared to the parent enzyme. The key to success is due to the well-tuned hydride attack distance/angle and, more importantly, to the synergistic proton-delivery triade of Tyr28-Tyr69-Tyr169. Molecular docking and density functional theory (DFT) studies provide important insights into the bioreduction mechanisms. Furthermore, we demonstrate the synthetic utility of the best mutants in the asymmetric synthesis of several key chiral synthons.

Asymmetric Synthesis of Both Enantiomers of Dimethyl 2-Methylsuccinate by the Ene-Reductase-Catalyzed Reduction at High Substrate Concentration

Chiral dimethyl 2-methylsuccinate (1) is a very important building block for the manufacturing of many active pharmaceutical ingredients and fine chemicals. The asymmetric reduction of C=C double bond of dimethyl citraconate (2), dimethyl mesaconate (3) or dimethyl itaconate (4) by ene-reductases (ERs) represents an attractive straightforward approach, but lack of high-performance ERs, especially (S)-selective ones, has limited implementing this method to prepare the optically pure dimethyl 2-methylsuccinate. Herein, three ERs (Bac-OYE1 from Bacillus sp., SeER from Saccharomyces eubayanus and AfER from Aspergillus flavus) with high substrate tolerance and stereoselectivity towards 2, 3 and 4 have been identified. Up to 500 mM of 3 was converted to (S)-dimethyl 2-methylsuccinate ((S)-1) by SeER in high yields (80%) and enantioselectivity (98% ee), and 700 mM of 2 and 400 mM of 4 were converted to (R)-1 by Bac-OYE1 and AfER, respectively, in high yields (86% and 77%) with excellent enantioselectivity (99% ee). The reductions of diethyl citraconate (5), diethyl mesaconate (6) and diethyl itaconate (7) were also tested with the three ERs. Although up to 500 mM of 5 was completely converted to (R)-diethyl 2-methylsuccinate ((R)-8) by Bac-OYE1 with excellent enantioselectivity (99% ee), the alcohol moiety of the esters had a great effect on the activity and enantioselectivity of ERs. This work provides an efficient methodology for the enantiocomplementary production of optically pure dimethyl 2-methylsuccinate from dimethyl itaconate and its isomers at high titer.

Enantioselective ene-reduction of E-2-cyano-3-(furan-2-yl) acrylamide by marine and terrestrial fungi and absolute configuration of (R)-2-cyano-3-(furan-2-yl) propanamide determined by calculations of electronic circular dichroism (ECD) spectra

This work reports the green organic chemistry synthesis of E-2-cyano-3(furan-2-yl) acrylamide under microwave radiation (55 W), as well as the use of filamentous marine and terrestrial-derived fungi, in the first ene-reduction of 2-cyano-3-(furan-2-yl) acrylamide to (R)-2-cyano-3-(furan-2-yl)propanamide. The fungal strains screened included Penicillium citrinum CBMAI 1186, Trichoderma sp. CBMAI 932 and Aspergillus sydowii CBMAI 935, and the filamentous terrestrial fungi Aspergillus sp. FPZSP 146 and Aspergillus sp. FPZSP 152. A compound with an uncommon CN-bearing stereogenic center at the α-C position was obtained by enantioselective reactions mediated in the presence of the microorganisms yielding the (R)-2-cyano-3-(furan-2-yl) propanamide 3a. Its isolated yield and e.e. ranged from 86% to 98% and 39% to 99%, respectively. The absolute configuration of the biotransformation products was determined by time-dependent density functional theory (TD-DFT) calculations of electronic circular dichroism (ECD) spectra. Finally, the tautomerization of 2-cyano-3-(furan-2-yl) propanamide 3a to form an achiral ketenimine was observed and investigated in presence of protic solvents.

C3 and C6 Modification-Specific OYE Biotransformations of Synthetic Carvones and Sequential BVMO Chemoenzymatic Synthesis of Chiral Caprolactones

The scope for biocatalytic modification of non-native carvone derivatives for speciality intermediates has hitherto been limited. Additionally, caprolactones are important feedstocks with diverse applications in the polymer industry and new non-native terpenone-derived biocatalytic caprolactone syntheses are thus of potential value for industrial biocatalytic materials applications. Biocatalytic reduction of synthetic analogues of R-(-)-carvone with additional substituents at C3 or C6, or both C3 and C6, using three types of OYEs (OYE2, PETNR and OYE3) shows significant impact of both regio-substitution and the substrate diastereomer. Bioreduction of (-)-carvone derivatives substituted with a Me and/or OH group at C6 is highly dependent on the diastereomer of the substrate. Derivatives bearing C6 substituents larger than methyl moieties are not substrates. Computer docking studies of PETNR with both (6S)-Me and (6R)-Me substituted (-)-carvone provides a model consistent with the outcomes of bioconversion. The products of bioreduction were efficiently biotransformed by the Baeyer-Villiger monooxygenase (BVase) CHMO_Phi1 to afford novel trisubstituted lactones with complete regioselectivity to provide a new biocatalytic entry to these chiral caprolactones. This provides both new non-native polymerization feedstock chemicals, but also with enhanced efficiency and selectivity over native (+)-dihydrocarvone Baeyer-Villigerase expansion. Optimum enzymatic reactions were scaled up to 60-100 mg, demonstrating the utility for preparative biocatalytic synthesis of both new synthetic scaffold-modified dihydrocarvones and efficient biocatalytic entry to new chiral caprolactones, which are potential single-isomer chiral polymer feedstocks.

Plant-Mediated Biotransformations of S(+)- and R(–)-Carvones

The enzymatic system of vegetables is well known as an efficient biocatalyst in the stereoselective reduction of ketones. Therefore, we decided to use the comminuted material of several plants including five vegetables (Apium graveolens L., Beta vulgaris L., Daucus carota L., Petroselinum crispum L., and Solanum tuberosum L.) and three fruits (Malus pumila L. “Golden” and “Kortland” as well as Pyrus communis L. “Konferencja”) to obtain enantiomerically pure carveol, which is commercially unavailable. Unexpectedly, all of the used biocatalysts not only reduced the carbonyl group of (4R)-(–)-carvone and (4S)-(+)-carvone, but also reduced the double bond in the cyclohexene ring. The results revealed that (4R)-(–)-carvone was transformed into (1R, 4R)- and (1S, 4R)-dihydrocarvones, and (1R,2R,4R)-dihydrocarveol. Although the enzymatic system of the potato transformed the substrate almost completely, the %de was not the highest. Potato yielded 92%; however, when carrot was used as the biocatalyst, it was possible to obtain 17% of (1R, 4R)-(+)-dihydrocarvone with 100% diastereomeric excess. In turn, the (4S)-(+)-carvone was transformed, using the biocatalysts, into (1R, 4S)- and (1S, 4S)-dihydrocarvones and dihydrocarveols. Complete substrate conversion was observed in biotransformation when potato was used. In the experiments using apple, (1R, 4S)-dihydrocarvone with 100% diastereomeric excess was obtained. Using NMR spectroscopy, we confirmed both diastereoisomers of 4(R)-1,2-dihydrocarveols, which were unseparated in the GC condition. Finally, we proved the high usefulness of vegetables for the biotransformation of both enantiomers of carvone as well as dihydrocarvone.

Whole Cells as Biocatalysts in Organic Transformations

Currently, the power and usefulness of biocatalysis in organic synthesis is undeniable, mainly due to the very high enantiomeric excess reached using enzymes, in an attempt to emulate natural processes. However, the use of isolated enzymes has some significant drawbacks, the most important of which is cost. The use of whole cells has emerged as a useful strategy with several advantages over isolated enzymes; for this reason, modern research in this field is increasing, and various reports have been published recently. This review surveys the most recent developments in the enantioselective reduction of carbon-carbon double bonds and prochiral ketones and the oxidation of prochiral sulfides using whole cells as biocatalytic systems.

Endophytic biocatalysts with enoate reductase activity isolated from Mentha pulegium

The biotransformation of (4R)-(-)-carvone by Mentha pulegium (pennyroyal) leaves and its endophytic bacteria was performed in order to search for novel biocatalysts with enoate reductase activity. The obtained results clearly indicated that endophytes play an important role in the biotransformation of (4R)-(-)-carvone with pennyroyal plant tissues. The best activity was associated to the endophytic bacteria Pseudomonas proteolytica FM18Mci1 and Bacillus sp. FM18civ1. Enoate reductase activity for the reduction of (4R)-(-)-carvone and (4S)-(+)-carvone as model substrates was evaluated for each strain. Finally, both isolated strains were evaluated for the kinetic resolution of racemic carvone. The two bacteria gave (1R, 4R) or (1R, 4S)-dihydrocarvone as major products. P. proteolytica FM18Mci1 had preference for the 4S-(-)-carvone, reaching a conversion 95% in 24 h. In contrast, Bacillus sp. FM18civ1 had preference for (4R)-(-)-carvone. The results obtained in the kinetic resolution of carvone indicated that the Bacillus strain could be useful for resolving a racemic mixture of carvone.

Highly effective, regiospecific reduction of chalcone by cyanobacteria leads to the formation of dihydrochalcone: two steps towards natural sweetness

BACKGROUND

Chalcones are the biogenetic precursors of all known flavonoids, which play an essential role in various metabolic processes in photosynthesizing organisms. The use of whole cyanobacteria cells in a two-step, light-catalysed regioselective bio-reduction of chalcone, leading to the formation of the corresponding dihydrochalcone, is reported. The prokaryotic microalgae cyanobacteria are known to produce phenolic compounds, including flavonoids, as natural components of cells. It seems logical that organisms producing such compounds possess a suitable "enzymatic apparatus" to carry out their biotransformation. Therefore, determination of the ability of whole cells of selected cyanobacteria to carry out biocatalytic transformations of chalcone, the biogenetic precursor of all known flavonoids, was the aim of our study.

RESULTS

Chalcone was found to be converted to dihydrochalcone by all examined cyanobacterial strains; however, the effectiveness of this process depends on the strain with biotransformation yields ranging from 3% to >99%. The most effective biocatalysts are Anabaena laxa, Aphanizomenon klebahnii, Nodularia moravica, Synechocystis aquatilis (>99% yield) and Merismopedia glauca (92% yield). The strains Anabaena sp. and Chroococcus minutus transformed chalcone in more than one way, forming a few products; however, dihydrochalcone was the dominant product. The course of biotransformation shed light on the pathway of chalcone conversion, indicating that the process proceeds through the intermediate cis-chalcone. The scaled-up process, conducted on a preparative scale and by using a mini-pilot photobioreactor, fully confirmed the high effectiveness of this bioconversion. Moreover, in the case of the mini-pilot photobioreactor batch cultures, the optimization of culturing conditions allowed the shortening of the process conducted by A. klebahnii by 50% (from 8 to 4 days), maintaining its >99% yield.

CONCLUSIONS

This is the first report related to the use of whole cells of halophilic and freshwater cyanobacteria strains in a two-step, light-catalysed regioselective bio-reduction of chalcone, leading to the formation of the corresponding dihydrochalcone. The total bioconversion of chalcone in analytical, preparative, and mini-pilot scales of this process creates the possibility of its use in the food industry for the production of natural sweeteners.

Alkene hydrogenation activity of enoate reductases for an environmentally benign biosynthesis of adipic acid

Adipic acid, a precursor for Nylon-6,6 polymer, is one of the most important commodity chemicals, which is currently produced from petroleum. The biosynthesis of adipic acid from glucose still remains challenging due to the absence of biocatalysts required for the hydrogenation of unsaturated six-carbon dicarboxylic acids to adipic acid. Here, we demonstrate the first enzymatic hydrogenation of 2-hexenedioic acid and muconic acid to adipic acid using enoate reductases (ERs). ERs can hydrogenate 2-hexenedioic acid and muconic acid producing adipic acid with a high conversion rate and yield in vivo and in vitro. Purified ERs exhibit a broad substrate spectrum including aromatic and aliphatic 2-enoates and a significant oxygen tolerance. The discovery of the hydrogenation activity of ERs contributes to an understanding of the catalytic mechanism of these poorly characterized enzymes and enables the environmentally benign biosynthesis of adipic acid and other chemicals from renewable resources.

Mimicking nature: synthetic nicotinamide cofactors for C═C bioreduction using enoate reductases

A series of synthetic nicotinamide cofactors were synthesized to replace natural nicotinamide cofactors and promote enoate reductase (ER) catalyzed reactions without compromising the activity or stereoselectivity of the bioreduction process. Conversions and enantioselectivities of >99% were obtained for C═C bioreductions, and the process was successfully upscaled. Furthermore, high chemoselectivity was observed when employing these nicotinamide cofactor mimics (mNADs) with crude extracts in ER-catalyzed reactions.