Metabolic Engineering of a Serotonin Overproducing <i>Saccharomyces cerevisiae</i> Strain

The EU Green Deal prioritises the transformation of the chemical industry to a more environmentally sustainable model. This involves using microorganisms, such as Saccharomyces cerevisiae, to produce molecules more sustainably through biotechnological approaches. In this study, we demonstrate an example of serotonin production using S. cerevisiae as a cell factory, along with its optimisation and upscaling. To achieve this, we introduced two heterologous genes, the combination of tryptophan decarboxylase from Clostridium sporogenes (CsTDC) and tryptamine 5-hydroxylase from Oryza sativa (OsT5H), to complete the serotonin biosynthetic pathway using L-tryptophan (L-TRP) as a precursor. By modifying ARO4 to a feedback-resistant version (ARO4*), the flux of the shikimate pathway was significantly increased and serotonin production was achieved at levels up to 120 mg/L directly from the glucose source. After a medium optimisation, a final concentration of 80 g/L glucose and 300 mg/L of nitrogen resulted in better conditions for increasing serotonin titres. Using this medium in a 1 L bioreactor fermentation resulted in approximately 250 mg/L of serotonin. A targeted metabolomic study of the bioreactor growth medium identified potential bottlenecks in the serotonin-overproducing strain and future targets for increasing its titre. We have constructed a strain of S. cerevisiae that represents the first steps towards feasible industrial production of serotonin using a sustainable and environmentally friendly approach, paving the way for the development of similar biotechnological strategies in the future.