A 2023 experiment conducted by Valenzuela-Ortega et al. reveals the first renewable pathway for upcycling post-consumer plastic, using E. coli as a vessel. Specifically, this pathway converts terephthalic acid, which is found in many post-consumer plastic materials, into adipic acid, which is used as a precursor material in cosmetic and pharmaceutical industries. Currently, 80% of post-consumer plastic waste consists of single-use plastics, and industrial adipic acid production releases nitrous oxide, which significantly contributes to our planet’s climate change crisis. Creating a biosynthetic pathway to upcycle terephthalic acid to adipic acid thus eliminates the extra waste accumulated from existing industrial processes.
Engineers in the Valenzuela-Ortega lab had previously converted terephthalic acid into vanillin (the component that gives vanilla its flavor), and now sought to expand this type of bio-upcycling in a way that could be more applicable to a circular materials economy. To engineer this new pathway in E. coli, they introduced eight genes and six enzymes. Genes were inserted into a “blank” plasmid, which was then taken up by E.coli and replicated to produce the necessary proteins. After trial and error, they were able to isolate two pathways: one that converts terephthalic acid to a biosynthetic intermediate, PCA, and another that converts PCA into the desired product, adipic acid. After efforts to combine these two pathways, researchers found the most efficient conversion was maintained by leaving them separate. One main challenge was finding a way to increase the rate of BcER enzyme activity, which was achieved by immersing the bacteria in calcium alginate hydrogel. Leaving the E. coli in the hydrogel medium ultimately yielded a 79% conversion rate from terephthalic acid into adipic acid.
New genes were transformed into E. coli bacteria’s genome, which can produce necessary enzymes to break down terephthalic acid in many plastics.
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This method was effective in converting the terephthalic acid from post-consumer plastic bottles and hot stamping foils into adipic acid. Thus, biosynthetic upcycling with E. coli can be applicable for a variety of plastic wastes in the manufacturing and recycling sector. Future areas of research include discovering ways to maintain this upcycling on a larger scale and to make the pathway applicable for different types of waste molecules produced by industrial plants.
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