Valorization of waste polycaprolactone via microbial upcycling of 6- hydroxyhexanoic acid into adipic acid by metabolically engineered Escherichia coli

This is the authors' abstract. We don't add key points for this paper.

To valorize waste polycaprolactone (PCL), one of the most widely used biodegradable plastics, into a value-added chemical, we upcycled 6-hydroxyhexanoic acid(6-HHA), the sole monomer of PCL, into adipic acid (AA) using a microbial method. Recombinant Escherichia coli strains expressing chnD (6-HHA dehydrogenase) and chnE(6-oxohexanoic acid dehydrogenase) genes from three bacteria were constructed, and all these strains successfully produced AA from 6-HHA. Among these, the E. coli strain harboring ChnDE genes from Acinetobacter strain SE19 (E. coli [pKK-AcChn]) showed the highest AA-producing ability. To increase the AA production titer, we optimized the culture temperature of this strain in flask culture and performed fed-batch fermentation in a 5 L bioreactor. After the fed-batch fermentation, the AA production titer increased to 15.6 g/L. To the best of our knowledge, this is the first report of the upcycle of 6-HHA into AA using a recombinant bacterial strain. Because 6-HHA is a monomer of PCL, our results provide the groundwork for the development of a biocatalytic recycling method for PCL.