Functional Characterization and Structural Insights Into Stereoselectivity of Pulegone Reductase in Menthol Biosynthesis

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.

Discovery and Engineering of a Bacterial (+)-Pulegone Reductase for Efficient (-)-Menthol Biosynthesis

The biosynthesis of valuable plant-derived monoterpene (-)-menthol from readily available feedstocks (e. g., (-)-limonene) is of great significance because of the high market demand for this product. However, biotransforming (+)-pulegone into (-)-menthone, the (-)-menthol precursor, through (+)-pulegone reductase (PGR) catalysis is inefficient because of the poor protein expression or catalytic efficiency (kcat/Km) of plant origin PGRs. In this study, a novel bacterial PGR from Pseudomonas resinovorans (PrPGR) was identified, and the most successful variant, PrPGRM2-1 (A50 V/G53 W), was obtained, showing respective 20-fold and 204-fold improvements in specific activity and catalytic efficiency. PrPGRM2-1 was employed to bioreduce (+)-pulegone, resulting in 4.4-fold and 35-fold enhancements in (-)-menthone titers compared with the bioreductions catalyzed by wild-type (WT) PrPGR and MpPGR, respectively. Furthermore, a whole-cell biocatalyst containing PrPGRM2-1, MpMMR, and BstFDH was constructed and achieved the highest (-)-menthol titer reported to date without externally supplemented NADPH/NADP+. Overall, this study details an efficient PGR with high catalytic efficiency that possesses great potential for (-)-menthol biosynthesis.

DFT Studies of the Activity and Reactivity of Limonene in Comparison with Selected Monoterpenes

Nowadays, the effective processing of natural monoterpenes that constitute renewable biomass found in post-production waste into products that are starting materials for the synthesis of valuable compounds is a way to ensure independence from non-renewable fossil fuels and can contribute to reducing global carbon dioxide emissions. The presented research aims to determine, based on DFT calculations, the activity and reactivity of limonene, an organic substrate used in previous preparative analyses, in comparison to selected monoterpenes such as cymene, pinene, thymol, and menthol. The influence of the solvent model was also checked, and the bonds most susceptible to reaction were determined in the examined compounds. With regard to EHOMO, it was found that limonene reacts more easily than cymene or menthol but with more difficultly than thymol and pienene. The analysis of the global chemical reactivity descriptors "locates" the reactivity of limonene in the middle of the studied monoterpenes. It was observed that, among the tested compounds, the most reactive compound is thymol, while the least reactive is menthol. The demonstrated results can be a reference point for experimental work carried out using the discussed compounds, to focus research on those with the highest reactivity.

Mentha piperita: Essential Oil and Extracts, Their Biological Activities, and Perspectives on the Development of New Medicinal and Cosmetic Products

This review aims to analyze Mentha piperita L. as a potential raw material for the development of new health-promoting products (nutraceuticals, cosmetics, and pharmaceutical products). A lot of scientific publications were retrieved from the Scopus, PubMed, and Google Scholar databases which enable the study and generalization of the extraction procedures, key biologically active compounds of essential oil and extracts, biological properties, and therapeutic potential of M. piperita, along with perspectives on the development of its dosage forms, including combinations of synthetic active substances and herbal preparations of M. piperita. The results of this review indicate that M. piperita is a source rich in phytoconstituents of different chemical nature and can be regarded as a source of active substances to enhance health and to develop medicinal products for complementary therapy of various conditions, especially those related with oxidant stress, inflammation, and moderate infections. Essential oil has a broad spectrum of activities. Depending on the test and concentration, this essential oil has both anti- and prooxidant properties. Gram-positive bacteria are more sensitive to the essential oil of M. piperita than Gram-negative ones. This review also considered some facets of the standardization of essential oil and extracts of M. piperita. Among the identified phenolics of extracts were caffeic acid, rosmarinic acid, eriocitrin, luteolin derivates (luteolin-7-O-rutinoside, luteolin-7-O-glucoronide), and hesperidin. The concentration of these phenolics depends on the solvent used. This review also considered the relationships between the chemical component and biological activity. The results showed that the essential oil and extracts reduced inflammation in vitro by inhibiting the production of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6), and in vivo by reducing the paw edema induced using carrageenan injection in rats. Therefore, herbal preparations of M. piperita are promising medicinal and cosmetic preparations for their usage in skincare and oral cavity care products with antimicrobial, anti-inflammatory, and wound-healing properties. This plant can also be regarded as a platform for the development of antibacterial preparations and combined anti-inflammatory and cardioprotective medicinal products (synthetic active substances plus herbal preparations). This review could be considered for the justification of the composition of some medicinal products during their pharmaceutical development for writing a registration dossier in the format of Common Technical Document.

The revealing of a novel double bond reductase related to perilla ketone biosynthesis in Perilla frutescens

BACKGROUND

Perilla frutescens is widely used as both a medicine and a food worldwide. Its volatile oils are its active ingredients, and, based on the different volatile constituents, P. frutescens can be divided into several chemotypes, with perilla ketone (PK) being the most common. However, the key genes involved in PK biosynthesis have not yet been identified.

RESULTS

In this study, metabolite constituents and transcriptomic data were compared in leaves of different levels. The variation in PK levels was the opposite of that of isoegoma ketone and egoma ketone in leaves at different levels. Based on transcriptome data, eight candidate genes were identified and successfully expressed in a prokaryotic system. Sequence analysis revealed them to be double bond reductases (PfDBRs), which are members of the NADPH-dependent, medium-chain dehydrogenase/reductase (MDR) superfamily. They catalyze the conversion of isoegoma ketone and egoma ketone into PK in in vitro enzymatic assays. PfDBRs also showed activity on pulegone, 3-nonen-2-one, and 4-hydroxybenzalacetone. In addition, several genes and transcription factors were predicted to be associated with monoterpenoid biosynthesis, and their expression profiles were positively correlated with variations in PK abundance, suggesting their potential functions in PK biosynthesis.

CONCLUSIONS

The eight candidate genes encoding a novel double bond reductase related to perilla ketone biosynthesis were identified in P. frutescens, which carries similar sequences and molecular features as the MpPR and NtPR from Nepeta tenuifolia and Mentha piperita, respectively. These findings not only reveal the pivotal roles of PfDBR in exploring and interpreting PK biological pathway but also contribute to facilitating future studies on this DBR protein family.

A chromosome-level genome assembly reveals that a bipartite gene cluster formed via an inverted duplication controls monoterpenoid biosynthesis in Schizonepeta tenuifolia

Biosynthetic gene clusters (BGCs) are regions of a genome where genes involved in a biosynthetic pathway are in proximity. The origin and evolution of plant BGCs as well as their role in specialized metabolism remain largely unclear. In this study, we have assembled a chromosome-scale genome of Japanese catnip (Schizonepeta tenuifolia) and discovered a BGC that contains multiple copies of genes involved in four adjacent steps in the biosynthesis of p-menthane monoterpenoids. This BGC has an unprecedented bipartite structure, with mirrored biosynthetic regions separated by 260 kilobases. This bipartite BGC includes identical copies of a gene encoding an old yellow enzyme, a type of flavin-dependent reductase. In vitro assays and virus-induced gene silencing revealed that this gene encodes the missing isopiperitenone reductase. This enzyme evolved from a completely different enzyme family to isopiperitenone reductase from closely related Mentha spp., indicating convergent evolution of this pathway step. Phylogenomic analysis revealed that this bipartite BGC has emerged uniquely in the S. tenuifolia lineage and through insertion of pathway genes into a region rich in monoterpene synthases. The cluster gained its bipartite structure via an inverted duplication. The discovered bipartite BGC for p-menthane biosynthesis in S. tenuifolia has similarities to the recently described duplicated p-menthane biosynthesis gene pairs in the Mentha longifolia genome, providing an example of the convergent evolution of gene order. This work expands our understanding of plant BGCs with respect to both form and evolution, and highlights the power of BGCs for gene discovery in plant biosynthetic pathways.

Secondary Metabolites with Biomedical Applications from Plants of the Sarraceniaceae Family

Carnivorous plants have fascinated researchers and hobbyists for centuries because of their mode of nutrition which is unlike that of other plants. They are able to produce bioactive compounds used to attract, capture and digest prey but also as a defense mechanism against microorganisms and free radicals. The main purpose of this review is to provide an overview of the secondary metabolites with significant biological activity found in the Sarraceniaceae family. The review also underlines the necessity of future studies for the biochemical characterization of the less investigated species. Darlingtonia, Heliamphora and Sarracenia plants are rich in compounds with potential pharmaceutical and medical uses. These belong to several classes such as flavonoids, with flavonol glycosides being the most abundant, monoterpenes, triterpenes, sesquiterpenes, fatty acids, alkaloids and others. Some of them are well characterized in terms of chemical properties and biological activity and have widespread commercial applications. The review also discusses biological activity of whole extracts and commercially available products derived from Sarraceniaceae plants. In conclusion, this review underscores that Sarraceniaceae species contain numerous substances with the potential to advance health. Future perspectives should focus on the discovery of new molecules and increasing the production of known compounds using biotechnological methods.

Comparison of Pulegone and Estragole Chemotypes Provides New Insight Into Volatile Oil Biosynthesis of Agastache rugosa

The aerial parts of Agastache rugosa are rich in essential oils containing monoterpenoids, phenylpropanoids, and aromatic compounds. These are used as herbs, perfume plants, and ornamental plants. Based on the difference in the constituents of the essential oil, A. rugosa is divided into pulegone and estragole chemotypes, but the mechanism of key metabolite biosynthesis in these two A. rugosa chemotypes remains unclear. In this study, we compared the morphological differences, metabolite constituents, and transcriptomic data between the two chemotypes of A. rugosa. Monoterpenoid was the main compound in the pulegone chemotype, and phenylpropanoid was the main compound in the estragole chemotype; however, limonene was detected in both chemotypes. Furthermore, 46 genes related to pulegone and estragole biosynthesis were identified. Limonene synthase, limonene-3-hydroxylase, and isopiperitenol dehydrogenase were upregulated in the pulegone chemotype, while phenylalanine ammonia-lyase, 4-coumarate: CoA ligase, CYP73A, coumaroyl-aldehyde dehydrogenase, and eugenol synthase were downregulated in the pulegone chemotype. We identified chavicol methyl transferase and limonene-3-hydroxylase in A. rugosa. This work not only provides the difference in morphology and metabolites in pulegone and estragole chemotypes, but also offers a molecular mechanism of volatile oil biosynthesis, which could be a basis for specialized metabolites in specialized chemotypes.