Bioconversion to Raspberry Ketone is Achieved by Several Non-related Plant Cell Cultures

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.

Raspberry Ketone Accumulation in Nicotiana benthamiana and Saccharomyces cerevisiae by Expression of Fused Pathway Genes

Raspberry ketone has generated interest in recent years both as a flavor agent and as a health promoting supplement. Raspberry ketone can be synthesized chemically, but the value of a natural nonsynthetic product is among the most valuable flavor compounds on the market. Coumaroyl-coenzyme A (CoA) is the direct precursor for raspberry ketone but also an essential precursor for flavonoid and lignin biosynthesis in plants and therefore highly regulated. The synthetic fusion of 4-coumaric acid ligase (4CL) and benzalacetone synthase (BAS) enables the channeling of coumaroyl-CoA from the ligase to the synthase, proving to be a powerful tool in the production of raspberry ketone in both N. benthamiana and S. cerevisiae. To the best of our knowledge, the key pathway genes for raspberry ketone formation are transiently expressed in N. benthamiana for the first time in this study, producing over 30 μg/g of the compound. Our raspberry ketone producing yeast strains yielded up to 60 mg/L, which is the highest ever reported in yeast.

Biosynthesis of 4-hydroxybenzylideneacetone by Whole-Cell Escherichia coli

4-Hydroxy benzylideneacetone (4-HBA) is an organic synthesis intermediate and can be used as a precursor for the synthesis of raspberry ketone. Herein, 2-deoxy-D-ribose 5-phosphate aldolase (DERA) was overexpressed in E. coli BL21 (DE3) as an attractive catalyst for enzymatic aldol reactions. The aldol reaction between 4-hydroxybenzaldehyde (4-HBD) and acetone to biosynthesize 4-HBA was catalyzed by whole-cell E. coli BL21 (DE3) (pRSF-Deoc). The yield and 4-HBA concentration were 92.8% and 111.35 mM, respectively, when using 120 mM 4-HBD and acetone as substrates. When the concentration of 4-HBD was increased to 480 mM, 376.4 mM 4-HBA was obtained by a fed-batch strategy with a yield of 78.4%, which was about a 28% improvement compared to the one-time addition strategy. E. coli BL21 (DE3) (pRSF-Deoc) cells were further immobilized with K-carrageenan, and the immobilized cells still maintained a residual activity of above 90% after 10 repeated uses. Our study provides a promising method of biosynthesizing 4-HBA.

Potential metabolic activities of raspberry ketone

Novel food and food compounds interventions have attracted a lot of attention nowadays for the prevention and treatment of metabolic diseases. Raspberry ketone (RK) is aromatic compound found within red fruits and berries, has been used as an over-the-counter product for weight loss. However, actually, the effect of RK on weight loss is still controversial, and the mechanism is largely unknown. Besides, in vivo and in vitro studies have demonstrated the beneficial effect of RK on the development of other metabolic diseases. In this review, we comprehensively highlighted the synthesis, bioavailability, and metabolism of RK, and summarized the progress made in our understanding of the potential biological activities of RK, including antiobesity, antidiabetes, cardioprotection, and hepatoprotection, as well as their underlying mechanisms. This paper provides a critical overview about the current findings and proposes the future studies in the area of RK on human health. PRACTICAL APPLICATIONS: Raspberry ketone (RK) has been used for weight control for years, but this effect is controversial considering food intake. Additionally, RK is beneficial for T2DM, liver and heart injury. The underlying mechanisms of the protective effect of RK including accelerating fatty acid oxidation, balancing serum glucose level, anti-inflammation, antioxidant process, and so on. In this context, we provide a comprehensive analysis of the benefits of RK against many metabolic diseases and discuss the underlying molecular mechanisms. We hope our work will be helpful for further researches on RK and improve its public recognition.

Production of raspberry ketone by redirecting the metabolic flux to the phenylpropanoid pathway in tobacco plants

Raspberry ketone is one of the characteristic flavors of raspberry fruits, and it is an important and expensive ingredient in the flavor and fragrance industries. It is present at low levels in plant tissues, and its occurrence is limited to a few taxa. In this context, the stable production of nature-identical raspberry ketone using heterologous synthesis in plants hosts has recently garnered the attention of plant biochemists. In this study, we demonstrate the rational switching of the metabolic flow from anthocyanin pigments to volatile phenylbutanoid production via the phenylpropanoid pathway. This shift led to the efficient and stable production of raspberry ketone and its glycosides via heterologous expression of the biosynthetic enzymes benzalacetone synthase (BAS) and raspberry ketone/zingerone synthase 1 (RZS1) in the transgenic tobacco (Nicotiana tabacum ‘Petit Havana SR-1’). Additionally, we achieved improved product titers by activating the phenylpropanoid pathway with the transcriptional factor, production of anthocyanin pigment 1 (PAP1), from Arabidopsis thaliana. We further demonstrated another metabolic-flow switching by RNA interference (RNAi)-mediated silencing of chalcone synthase (CHS) to increase pathway-intermediate p-coumaroyl-CoA in transgenic tobacco for raspberry-ketone production. The redirection of metabolic flux resulted in transgenic lines producing 0.45 μg/g of raspberry ketone and 4.5 μg/g, on the fresh weight basis, of its glycosides in the flowers. These results suggest that the intracellular enforcement of endogenous substrate supply is an important factor while engineering the phenylpropanoid pathway. This strategy might be useful for the production of other phenylpropanoids/polyketides that are produced via the pathway-intermediate p-coumaroyl-CoA, in tobacco plants.

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Overexpression of BAS and RZS1 enabled de novo production of raspberry ketone in transgenic tobacco.

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Intracellular enforcement of endogenous p-coumaroyl-CoA supply improved raspberry-ketone production.

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The metabolic flow switching can produce raspberry ketone and its glycosides with a titer of 0.45 and 4.5 μg/gFW, respectively.

Construction of synthetic pathways for raspberry ketone production in engineered Escherichia coli

Raspberry ketone is an important ingredient in the flavor and fragrance industries. Due to its low content in fruits and vegetables, the production of natural raspberry ketone using heterologous synthesis in microbial strains is recently attracting increased attention. In this work, a heterologous pathway to produce raspberry ketone from p-coumaric acid, including 4-coumarate: CoA ligase (4CL), benzalacetone synthase (BAS), and raspberry ketone/zingerone synthase (RZS1) from plants, was successfully assembled in Escherichia coli. When the RZS1 gene was introduced into E. coli and co-expressed with two other genes, the intermediate 4-hydroxybenzylidene acetone in the pathway was almost completely transformed into a raspberry ketone. Substituting TB medium for M9 medium increased raspberry ketone titers by 3-4 times. Furthermore, the heterologous pathway was partitioned into two modules; module one produced p-coumaroyl-CoA from p-coumaric acid by 4CL, and module two produced raspberry ketone from coumaroyl-CoA by the action of BAS and RZS1. Optimizing the balanced expression of the two modules, it was shown that moderate expression of module one and high expression of module two was the best combination to enhance raspberry ketone production. The engineered strain CZ-8 reached 90.97 mg/l of raspberry ketone, which was 12 times higher than previously reported. In addition, the preferred approach of the heterologous pathway was related to the heterologous genes from different sources; for example, 4CL from Arabidopsis thaliana seemed to be more suitable for raspberry ketone production than that from Petroselinum crispum. This work paves an alternative way for future economic production of natural raspberry ketone.