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

Enantioselective synthesis of (R)-citronellal from geraniol with an immobilised copper alcohol oxidase and ene reductase

(R)-Citronellal is one of the key chiral intermediates in the synthesis of the isomer (-)-menthol, one of the most commercialised terpenoid flavours worldwide. Enzymatic approaches could represent a less energy-demanding alternative for its synthesis, such as a previously reported bienzymatic cascade starting from inexpensive, commercially available geraniol. A copper radical oxidase (CgrAlcOx) followed by a flavin-dependent ene reductase (OYE2) were used to obtain (R)-citronellal. Here, we used a metal-affinity immobilisation strategy on the His-tagged enzymes for the cascade and studied enzyme recovery and reusability as well as increased solvent tolerance. After screening a panel of resins for enzyme immobilisation and water-immiscible co-solvents, we successfully obtained 95% conversion to (R)-citronellal with 96.9% enantiomeric excess (ee) in a concurrent cascade after 7 h of reaction time, starting from 10 mM of geraniol.

Ene-Reductases-Catalyzed Non-Natural Reactions

Flavin-dependent ene-reductases (EREDs), members of the Old Yellow Enzyme (OYE) superfamily, are highly efficient biocatalysts primarily known for catalyzing the asymmetric reduction of activated alkenes. Beyond this native function, the chemical versatility of the flavin cofactor and the sophisticated architecture of their protein structures enable EREDs to exhibit catalytic multifunctionality. The catalytic promiscuity not only highlights the adaptability of these enzymes but also expands their potential to broaden the scope of enzyme-catalyzed reactions in organic synthesis. Given the inherent challenges associated with discovering novel enzyme activities, such catalytic promiscuity of EREDs offers a promising pathway for expanding their applications. This mini-review provides a comprehensive overview of the catalytic multifunctionality of flavin-dependent EREDs, with a particular focus on their "non-natural" functionalities in organic synthesis. This review is primarily divided into two main sections: hydride-dependent reactions and hydride-independent reactions. By highlighting these unconventional biocatalytic pathways, we aim to inspire further exploration into the untapped potential of EREDs and their role in advancing synthetic chemistry.

His126 Substitution Shifts Perakine Reductase Activity from Carbonyl Reduction to Ene Reduction

Perakine reductase (PR) is an aldo-keto reductase (AKR) carbonyl reductase. His126 is one of the AKR catalytic tetrads in PR. Substitution of His126 with any other amino acids except Gln switched PR to an ene reductase. Molecular simulation suggested that the substrate-binding pose and the properties of the 126 residue determined the chemoselectivity. Given the strict conservation of His126 in AKR superfamily carbonyl reductases, modification of the corresponding site is a feasible strategy to design AKR-derived novel ene reductases.

Isolation and structural determination of natural products bearing tetrahydrogenated isoprenoid side-chains at their ω-termini

Hydrogenated isoprenoids are found in a range of biologically important natural products, such as isoprenoid quinones, chlorophyll, vitamin E, and dolichol. In this study, a new method was developed for determining the chirality of the tetrahydrogenated isoprenoid (THI) structures of two natural products, namely heptaprenylcycli-14E,18E-diene and (22R,5E,9E,13E,17E)-6,10,14,18,22,26-hexamethylheptacosa-5,9,13,17-tetraen-2-one, for which the chiral carbon centres have never been elucidated. Our research group previously isolated the former sesquarterpene from Mycobacterium chlorophenolicum, while the latter was isolated as a new polyprenyl acetone from Conexibacter woesei in the current study. To determine their chiralities, these two THI-containing terpenoids were subjected to ozonolysis to produce (R)-6,10-dimethyl-2-undecanones. The optically active (S)-6,10-dimethyl-2-undecanone was prepared as an authentic chiral sample by the reduction of (S,E)-6,10-dimethylundeca-3,9-dien-2-one. Chiral high-performance liquid chromatographic analysis of the ozonolysis products and of authentic (S)- and racemic 6,10-dimethyl-2-undecanone samples unambiguously assigned the THI chiral carbon centres of both compounds as R. This new method could be a useful tool for determining the chiralities of other THIs. It is also expected to contribute to our understanding of the biosynthetic mechanisms and biological roles of THIs.

Computational Design of Burkholderia cepacia Lipase Mutants that Show Enhanced Stereoselectivity in the Production of l-Menthol

l-Menthol, valued for its aroma, cooling properties, and biological activity, is often produced as a mixture with d-menthol, which negatively impacts taste and odor. To improve the purity of industrial l-menthol production, we engineered Burkholderia cepacia lipase (BCL) to improve its stereoselectivity. While wild-type BCL achieves only 98% enantiomeric excess, we used molecular dynamics simulations of BCL bound to menthol acetate, combined with computational rational design method (MSPER), to identify key mutation sites. We experimentally confirmed that five mutations that can effectively improve the enantiomeric excess. In particular, Q88A increased enantiomeric excess to 99.4% and productivity from 14.5% to 49.9%. Q88G exhibited the highest productivity at 54.0% with 99.3% optical purity. These BCL mutants offer a greener, more efficient approach to high-purity l-menthol production, contributing to more sustainable chemical processes.

Design, Synthesis, and Biological Activity Evaluation of 5-Aryl-cyclopenta[c]pyridine Derivatives

Taking the natural product cerbinal as the lead compound, 30 novel 5-aryl-cyclopenta[c]pyridine derivatives were designed and synthesized based on the previous bioactivity studies of the cyclopenta[c]pyridines. The modification of the position-5 of compound 2 was achieved by amination, bromination, and cross coupling using cerbinal as the raw material. The results of the bioactivity tests demonstrated that partial compounds exhibited superior activity against plant viruses compared to compound 2. Compounds 4g and 4k showed higher anti-TMV activity levels than commercial varieties of ribavirin at concentrations of 500 and 100 μg/mL. In particular, compound 4k, which contained a m-methoxyphenyl substitution, displayed the most potent anti-TMV activity in vivo (inactivation effect 51.1 ± 1.9%, curative effect 50.7 ± 3.6%, protection effect 53.8 ± 2.8% at 500 μg/mL) The toxicological experiments also revealed that compound 4k exhibited low toxicity to zebrafish. Additionally, molecular docking results indicated that access to the benzene ring enhanced the binding affinity of these derivatives for TMV receptor proteins. Furthermore, studies on the insecticidal and fungicidal activities of these derivatives showed that most of the compounds exhibited good larvicidal efficacy against Plutella xylostella and broad-spectrum fungicidal activities. Notably, compound 4i (3,4,5-trifluorophenyl) displayed an outstanding inhibition ratio of 91.9% against Sclerotinia sclerotiorum, 75% against Botrytis cinerea, and 62.5% against Phytophthora infestans at a concentration of 50 μg/mL. These results suggest that 5-aryl-cyclopenta[c]pyridine derivatives could serve as promising candidate agents for antiviral, insecticidal, and fungicidal applications in agricultural production.