Asymmetric Whole-Cell Bio-Reductions of (R)-Carvone Using Optimized Ene Reductases

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.

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.

Loop 6 and the β-hairpin flap are structural hotspots that determine cofactor specificity in the FMN-dependent family of ene-reductases

Flavin mononucleotide (FMN)-dependent ene-reductases constitute a large family of oxidoreductases that catalyze the enantiospecific reduction of carbon-carbon double bonds. The reducing equivalents required for substrate reduction are obtained from reduced nicotinamide by hydride transfer. Most ene-reductases significantly prefer, or exclusively accept, either NADPH or NADH. Despite their usefulness in biocatalytic applications, the structural determinants for cofactor preference remain elusive. We employed the NADPH-preferring 12-oxophytodienoic acid reductase 3 from Solanum lycopersicum (SlOPR3) as a model enzyme of the ene-reductase family and applied computational and structural methods to investigate the binding specificity of the reducing coenzymes. Initial docking results indicated that the arginine triad R283, R343, and R366 residing on and close to a critical loop at the active site (loop 6) are the main contributors to NADPH binding. In contrast, NADH binds unfavorably in the opposite direction toward the β-hairpin flap within a largely hydrophobic region. Notably, the crystal structures of SlOPR3 in complex with either NADPH4 or NADH4 corroborated these different binding modes. Molecular dynamics simulations confirmed NADH binding near the β-hairpin flap and provided structural explanations for the low binding affinity of NADH to SlOPR3. We postulate that cofactor specificity is determined by the arginine triad/loop 6 and the residue(s) controlling access to a hydrophobic cleft formed by the β-hairpin flap. Thus, NADPH preference depends on a properly positioned arginine triad, whereas granting access to the hydrophobic cleft at the β-hairpin flap favors NADH binding.

Identification of functional genes associated with the biotransformation of limonene to trans-dihydrocarvone in Klebsiella sp. O852

BACKGROUND

Natural dihydrocarvone has been widely used in the food, cosmetics, agrochemicals and pharmaceuticals industries because of its sensory properties and physiological effects. In our previous study, Klebsiella sp. O852 was shown to be capable of converting limonene to trans-dihydrocarvone with high catalytic efficiency. Thus, it was essential to identify and characterize the functional genes involved in limonene biotransformation using genome sequencing and heterologous expression.

RESULTS

The 5.49-Mb draft genome sequence of Klebsiella sp. O852 contained 5218 protein-encoding genes. Seven candidate genes participating in the biotransformation of limonene to trans-dihydrocarvone were identified by genome analysis. Heterologous expression of these genes in Escherichia coli BL21(DE3) indicated that 0852_GM005124 and 0852_GM003417 could hydroxylate limonene in the six position to yield carveol, carvone and trans-dihydrocarvone. 0852_GM002332 and 0852_GM001602 could catalyze the oxidation of carveol to carvone and trans-dihydrocarvone. 0852_GM000709, 0852_GM001600 and 0852_GM000954 had high carvone reductase activity toward the hydrogenation of carvone to trans-dihydrocarvone.

CONCLUSION

The results obtained in the present study suggest that the seven genes described above were responsible for converting limonene to trans-dihydrocarvone. The present study contributes to providing a foundation for the industrial production of trans-dihydrocarvone in microbial chassis cells using synthetic biology strategies. © 2021 Society of Chemical Industry.

Identification of a novel ene reductase from Pichia angusta with potential application in (R)-levodione production

Asymmetric reduction of electronically activated alkenes by ene reductases (ERs) is an attractive approach for the production of enantiopure chiral products. Herein, a novel FMN-binding ene reductase (PaER) from Pichia angusta was heterologously expressed in Escherichia coli BL21(DE3), and the recombinant enzyme was characterized for its biocatalytic properties. PaER displayed optimal activity at 40 °C and pH 7.5, respectively. The purified enzyme was quite stable below 30 °C over a broad pH range of 5.0–10.0. PaER was identified to have a good ability to reduce the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C bond of various α,β-unsaturated compounds in the presence of NADPH. In addition, PaER exhibited a high reduction rate (k_cat = 3.57 s⁻¹) and an excellent stereoselectivity (>99%) for ketoisophorone. Engineered E. coli cells harboring PaER and glucose dehydrogenase (for cofactor regeneration) were employed as biocatalysts for the asymmetric reduction of ketoisophorone. As a result, up to 1000 mM ketoisophorone was completely and enantioselectively converted to (R)-levodione with a >99% ee value in a space–time yield of 460.7 g L⁻¹ d⁻¹. This study provides a great potential biocatalyst for practical synthesis of (R)-levodione.

Asymmetric reduction of electronically activated alkenes by ene reductases (ERs) is an attractive approach for the production of enantiopure chiral products.

Ene-Reductase: A Multifaceted Biocatalyst in Organic Synthesis

Biocatalysis integrate microbiologists, enzymologists, and organic chemists to access the repertoire of pharmaceutical and agrochemicals with high chemoselectivity, regioselectivity, and enantioselectivity. The saturation of carbon-carbon double bonds by biocatalysts challenges the conventional chemical methodology as it bypasses the use of precious metals (in combination with chiral ligands and molecular hydrogen) or organocatalysts. In this line, Ene-reductases (ERs) from the Old Yellow Enzymes (OYEs) family are found to be a prominent asymmetric biocatalyst that is increasingly used in academia and industries towards unparalleled stereoselective trans-hydrogenations of activated C=C bonds. ERs gained prominence as they were used as individual catalysts, multi-enzyme cascades, and in conjugation with chemical reagents (chemoenzymatic approach). Besides, ERs' participation in the photoelectrochemical and radical-mediated process helps to unlock many scopes outside traditional biocatalysis. These up-and-coming methodologies entice the enzymologists and chemists to explore, expand and harness the chemistries displayed by ERs for industrial settings. Herein, we reviewed the last five year's exploration of organic transformations using ERs.

Screening a Strain of Klebsiella sp. O852 and the Optimization of Fermentation Conditions for Trans-Dihydrocarvone Production

Flavors and fragrances have high commercial value in the food, cosmetic, chemical and pharmaceutical industries. It is interesting to investigate the isolation and characterization of new microorganisms with the ability to produce flavor compounds. In this study, a new strain of Klebsiella sp. O852 (accession number CCTCC M2020509) was isolated from decayed navel orange (Citrus sinensis (L.) Osbeck), which was proved to be capable of converting limonene to trans-dihydrocarvone. Besides, the optimization of various reaction parameters to enhance the trans-dihydrocarvone production in shake flask was performed for Klebsiella sp. O852. The results showed that the yield of trans-dihydrocarvone reached up to 1 058 mg/L when Klebsiella sp. O852 was incubated using LB-M medium for 4 h at 36 °C and 150 rpm, and the biotransformation process was monitored for 36 h after adding 1680 mg/L limonene/ethanol (final ethanol concentration of 0.8% (v/v)). The content of trans-dihydrocarvone increased 16 times after optimization. This study provided a basis and reference for producing trans-dihydrocarvone by biotransformation.

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.

Selective ene-reductase immobilization to magnetic nanoparticles through a novel affinity tag

BACKGROUND

Magnetic nanoparticles (MNPs) are becoming more important as carriers, because of their large specific surface area and easy separability. They are increasingly used in enzyme technology, diagnostics, and drug delivery.

MAJOR RESULTS

For the directed and almost irreversible immobilization of proteins on MNPs, we have developed a new selective (His-Arg)₄ peptide-tag, that binds fusion proteins directly from an E. coli cell lysate to non-functionalized, low-cost MNPs. Using the immobilization of an ene-reductase as an example, we could demonstrate that the fusion with this tag increases thermostability without reducing overall activity (ER w/o tag: t_1/2  = 3.7 h, (HR)₄ -ER: t_1/2  = 9.9 h). Immobilization by adsorption in Tris buffer resulted in very high enzyme loads with approx. 380 mg g^-1 and 67% residual activity. The immobilization on the MNPs allowed a fast concentration, buffer exchange, and reuse. While about 50% of the activity was lost after the first reuse, we were able to show that the activity did not decrease further and was stable for another nine cycles.

CONCLUSION

According to our studies, our tag highly works for any kind of immobilization on MNPs and holds the potential for enzyme immobilizations as well as for drug delivery and sensors.

Asymmetric Reduction of (R)-Carvone through a Thermostable and Organic-Solvent-Tolerant Ene-Reductase

Ene‐reductases allow regio‐ and stereoselective reduction of activated C=C double bonds at the expense of nicotinamide adenine dinucleotide cofactors [NAD(P)H]. Biological NAD(P)H can be replaced by synthetic mimics to facilitate enzyme screening and process optimization. The ene‐reductase FOYE‐1, originating from an acidophilic iron oxidizer, has been described as a promising candidate and is now being explored for applied biocatalysis. Biological and synthetic nicotinamide cofactors were evaluated to fuel FOYE‐1 to produce valuable compounds. A maximum activity of (319.7±3.2) U mg⁻¹ with NADPH or of (206.7±3.4) U mg⁻¹ with 1‐benzyl‐1,4‐dihydronicotinamide (BNAH) for the reduction of N‐methylmaleimide was observed at 30 °C. Notably, BNAH was found to be a promising reductant but exhibits poor solubility in water. Different organic solvents were therefore assayed: FOYE‐1 showed excellent performance in most systems with up to 20 vol% solvent and at temperatures up to 40 °C. Purification and application strategies were evaluated on a small scale to optimize the process. Finally, a 200 mL biotransformation of 750 mg (R)‐carvone afforded 495 mg of (2R,5R)‐dihydrocarvone (>95 % ee), demonstrating the simplicity of handling and application of FOYE‐1.