Scientists in the UK have genetically modified it Escherichia coli Convert plastic waste into vanillin. “Instead of just recycling plastic waste into more plastic, our system shows for the first time that you can use plastic as a raw material for microbial cells and turn it into something of greater value and more industrial use,” says Stephen Wallace of Edinburgh University. Biotransformation “not only replaces a current chemical process, but actually achieves something that is not possible with modern synthetic methods”.

Polyethylene terephthalate (PET) is one of the most common types of plastic. Most existing recycling technologies break down PET into its substituent monomers ethylene glycol and terephthalic acid and then use them in second generation plastic materials. Wallace and Joanna Sadler, also from the University of Edinburgh, want to process these monomers into alternative products.

A picture showing the proposed enzymatic pathway

The research duo used genetic engineering to create a strain of E.. coli which converts terephthalic acid into vanillin. Vanillin is the molecule that is responsible for the characteristic smell and taste of vanilla. It is used in food and cosmetics and is an industrially important platform chemical. Traditionally it is extracted from the vanilla plant, but global demand far outweighs supply from natural sources. Sadler isn’t sure if vanillin, obtained directly from waste, would meet regulatory standards for food consumption, but he believes it would be suitable for cosmetics or other uses as a bulk chemical.

Your modified E. coli produces enzymes to convert terephthalic acid into vanillin through an oxidation, a methylation and two reduction steps. These enzymes were introduced into the cell on plasmids, which Wallace describes as “circular pieces of DNA that are practically a guide to the chemistry this cell is supposed to perform”. Adding such a large number of non-native enzymes to it E. coli turned out to be a great challenge. “When you start introducing new enzymes, you put a lot of stress on the cell and at some point it will be dissatisfied with the amount of work you are asking it to do. This poses various biological hurdles, ”explains Sadler. By carefully optimizing the reaction conditions and media, they finally found a sweet spot where each of the enzymes could play its role in the biocatalytic conversion of terephthalic acid to vanillin.

To collect the trash

Unfortunately, getting the cell to do the transformation was only half the battle. “Whole-cell biocatalysis has many advantages, but a major challenge is getting the substrate physically into the cell so that the chemistry can take place,” says Sadler. To overcome this, they added small amounts of alcohol, which essentially drills holes in the cell membrane to increase its permeability. Another complexity was that vanillin itself is toxic to their new variety of E. coli. To alleviate these toxicity problems, they removed the product in situ from the reaction mixture by extracting it into oleyl alcohol. Under optimized conditions, the team converted a plastic garbage bottle that had been picked from a street in Edinburgh directly into vanillin in a one-pot process.

“This is a very promising and up-to-date study that shows that terephthalic acid can be integrated into a biotechnological process,” comments Wolfgang Streit, an expert on microbial plastic degradation at the University of Hamburg, Germany. “You outline a clear strategy for the biological upcycling of PET waste into a single value-adding small molecule. Since vanillin has the potential to serve as a chemical platform compound, it makes a significant contribution to the emerging field of microbiological plastics recycling. ‘ And Zeynep Cetecioglu, researcher in the field of microbial waste recycling at the KTH Royal Institute of Technology, Sweden, describes the work as “a pretty important step not only for creating biorefinery hubs in cities, but also for a more sustainable society”. “Plastic waste is a big problem for the whole world and there is a need to find a solution to remove or dispose of it. This study went one step further and successfully turned the compound from plastic waste into a high quality chemical. ‘

Aside from the useful conversion, Wallace says her research “completely changes the perception of plastic waste as a problematic end product into a usable raw material for modern industrial biotechnology”. Wallace and Sadler plan to further optimize and scale the reaction, as well as develop alternative ways to make other valuable molecules from PET waste. “For me this is just the beginning,” says Sadler. “I think we are in a really exciting place now that we realize we can do all sorts of things with plastic waste.”


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