Abstract
BackgroundThe phenylbutanoid 4-(4-hydroxyphenyl)butan-2-one, commonly known as raspberry ketone, is responsible for the typical scent and flavor of ripe raspberries. Chemical production of nature-identical raspberry ketone is well established as this compound is frequently used to flavor food, beverages and perfumes. However, high demand for natural raspberry ketone, but low natural abundance in raspberries, render raspberry ketone one of the most expensive natural flavoring components.ResultsIn this study, Corynebacterium glutamicum was engineered for the microbial synthesis of the character impact compound raspberry ketone from supplemented p-coumaric acid. In this context, the NADPH-dependent curcumin/dihydrocurcumin reductase CurA from Escherichia coli was employed to catalyze the final step of raspberry ketone synthesis as it provides a hitherto unknown benzalacetone reductase activity. In combination with a 4-coumarate: CoA ligase from parsley (Petroselinum crispum) and a monofunctional benzalacetone synthase from Chinese rhubarb (Rheum palmatum), CurA constitutes the synthetic pathway for raspberry ketone synthesis in C. glutamicum. The resulting strain accumulated up to 99.8 mg/L (0.61 mM) raspberry ketone. In addition, supplementation of other phenylpropanoids allowed for the synthesis of two other naturally-occurring and flavoring phenylbutanoids, zingerone (70 mg/L, 0.36 mM) and benzylacetone (10.5 mg/L, 0.07 mM).ConclusionThe aromatic product portfolio of C. glutamicum was extended towards the synthesis of the flavoring phenylbutanoids raspberry ketone, zingerone and benzylacetone. Key to success was the identification of CurA from E. coli having a benzalacetone reductase activity. We believe, that the constructed C. glutamicum strain represents a versatile platform for the production of natural flavoring phenylbutanoids at larger scale.
Highlights
The phenylbutanoid character impact compound raspberry ketone (4-(4-hydroxyphenyl)butan-2-one, RK) defines the typical scent and taste of raspberries
Previous studies utilizing either E. coli or S. cerevisiae for microbial RK synthesis demonstrated that both hosts provide an endogenous benzalacetone reductase (BAR) activity [1, 13]
Supernatants from the C. glutamicum microbioreactor cultivations performed in the context of the pHBA cytotoxicity experiments, were analyzed by high-performance liquid chromatography (HPLC) for the presence of potentially accumulating RK
Summary
The phenylbutanoid character impact compound raspberry ketone (4-(4-hydroxyphenyl)butan-2-one, RK) defines the typical scent and taste of raspberries. It is utilized by food and beverage industries to flavor beverages and foods, e.g. pudding, yogurt or sweets [1, 2]. Different strategies can be followed to obtain RK, e.g. extraction from natural plant material or chemical synthesis. Prerequisite for establishing a microbial RK production process is the functional introduction of the natural biosynthesis pathway from the plant into a heterologous host. The phenylbutanoid 4-(4-hydroxyphenyl)butan-2-one, commonly known as raspberry ketone, is responsible for the typical scent and flavor of ripe raspberries. Chemical production of nature-identical raspberry ketone is well established as this compound is frequently used to flavor food, beverages and perfumes. High demand for natural raspberry ketone, but low natural abundance in raspberries, render raspberry ketone one of the most expensive natural flavoring components
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