Yeast fermentation for the upcycling of PET monomers

Plastic has become an indispensable material in many fields, with production increasing every year; however, most of the plastic waste is still incinerated or landfilled, and only 10% of the new plastic is recycled even once. Considering that plastic is derived from petroleum-based resources, this creates a worrying loss of resources and a cascade of environmental issues. Among all plastic, polyethylene terephthalate (PET) is the most produced polyester worldwide (56 Mt/year). This work focuses on the upcycling of PET monomers – terephthalic acid (TPA) and ethylene glycol (EG) – by yeast fermentation for the production of industrially relevant organic acids (protocatechuic acid, cis,cis-muconic acid, 3-carboxy-cis,cis-muconic acid, glycolic acid). Thanks to synthetic biology (EASY-MISE toolkit) and metabolic engineering, different Saccharomyces cerevisiae strains were created for the bioconversion of TPA and the use of EG as a carbon source. For the bioconversion of TPA, several genes from Ideonella sakaiensis were introduced; TPA toxicity on the new strains was assessed in microwell plates. Since EG assimilation has never been described in yeast, different medium combinations were tested for the assimilation of EG; in parallel, two synthetic metabolic pathways were introduced in S. cerevisiae for a more efficient assimilation. In silico constraint-based models are currently being developed to optimize growth and production on TPA and EG, and to shed light on the yeast native metabolism of the latter. Our research shows promising results in the biodegradation and upcycling of PET monomers by yeast fermentation, and it will become even more interesting thanks to the recent advances in enzymatic PET hydrolysis.