Physiologic and metabolic characterization of Saccharomyces cerevisiae reveals limitations in the synthesis of the triterpene squalene

Heterologous synthesis of triterpenoids in Saccharomyces cerevisiae from its native metabolite squalene has been reported to offer an alternative to chemical synthesis and extraction from plant material if productivities can be increased.Here, we physiologically characterized a squalene overproducing S. cerevisiae CEN.PK strain to elucidate the effect of cultivation conditions on the production of this central triterpenoid precursor. The maximum achievable squalene concentration was substantially influenced by nutritional conditions, medium composition and cultivation mode. Batch growth on glucose resulted in minimal squalene accumulation, while squalene only significantly accumulated during ethanol consumption (up to 59 mg/gCDW), probably due to increased acetyl-CoA supply on this carbon source. Likewise, low squalene concentrations were observed in glucose-limited chemostat cultivations and improved up to 8-fold upon increasing the ethanol fraction in the feed. In those experiments, a constant, growth-rate-independent specific squalene accumulation rate (2.2 mg/gCDW/h) was recorded resulting in a maximal squalene loading of 30 mg/gCDW at low dilution rates with longer residence times. Coenzyme A availability was identified as possible bottleneck as increased vitamin concentrations, including the Coenzyme A precursor pantothenate, improved squalene titers in batch and chemostat cultivations. This analysis demonstrates that thorough physiologic characterization of production strains is valuable for the identification of bottlenecks already in early stages of strain development and for guiding further optimization efforts.