Heterologous production of raspberry ketone in the wine yeast Saccharomyces cerevisiae via pathway engineering and synthetic enzyme fusion

Background: Raspberry ketone is the primary aroma compound found in raspberries and naturally derived raspberry ketone is a valuable flavoring agent. The economic incentives for the production of raspberry ketone, combined with the very poor yields from plant tissue, therefore make this compound an excellent target for heterologous production in synthetically engineered microbial strains. Methods: A de novo pathway for the production of raspberry ketone was assembled using four heterologous genes, encoding phenylalanine/tyrosine ammonia lyase, cinnamate-4-hydroxlase, coumarate-CoA ligase and benzalacetone synthase, in an industrial strain of Saccharomyces cerevisiae. Synthetic protein fusions were also explored as a means of increasing yields of the final product. Results: The highest raspberry ketone concentration achieved in minimal media exceeded 7.5 mg/L when strains were fed with 3 mM p-coumaric acid; or 2.8 mg/L for complete de novo synthesis, both of which utilized a coumarate-CoA ligase, benzalacetone synthase synthetic fusion protein that increased yields over fivefold compared to the native enzymes. In addition, this strain was shown to be able to produce significant amounts of raspberry ketone in wine, with a raspberry ketone titer of 3.5 mg/L achieved after aerobic fermentation of Chardonnay juice or 0.68 mg/L under anaerobic winemaking conditions. Conclusions: We have shown that it is possible to produce sensorially-relevant quantities of raspberry ketone in an industrial heterologous host. This paves the way for further pathway optimization to provide an economical alternative to raspberry ketone derived from plant sources.