Newly discovered enzyme breaks down PET plastic in record time

Research results from Leipzig could make biological recycling more economically attractive

Plastic bottles, fruit bowls, foils: these lightweight PET plastic packaging become a problem if not recycled. Scientists from the University of Leipzig have now discovered a highly efficient enzyme that breaks down PET in record time. With the enzyme PHL7, which the researchers found on a Leipzig compost heap, biological PET recycling could be possible much faster than previously thought. The results have now been published in the scientific journal ChemSusChem and have been chosen as the cover story.

In nature, enzymes are used by bacteria, for example, to decompose parts of plants. It has long been known that some enzymes, so-called polyester-splitting hydrolases, can also break down PET. The enzyme LCC, for example, which was discovered in Japan in 2012, is considered to be a particularly effective “plastic decomposer”. The team led by young scientist Dr. Christian Sonnendecker from the University of Leipzig as part of the EU-funded third-party projects MIPLACE and ENZYCLE. They found what they were looking for at the southern cemetery in Leipzig: The researchers took samples of leaf compost there and found in one sample the blueprint of an enzyme that decomposed PET at record speed in the laboratory.

The researchers from the Institute for Analytical Chemistry had found and examined seven different enzymes. The seventh candidate, entitled PHL7, achieved results that were well above average in the laboratory: In the tests, the researchers placed PET in containers with an aqueous solution that contained either PHL7 or LCC, i.e. the previous leader in PET decomposition. Then they measured the amount of plastic that was broken down over a given period of time and compared the values.

PHL7 twice as active as the previous leader

The result: PHL16 decomposed 7 percent of the PET within 90 hours, while LCC managed to degrade just 45 percent in the same time. "Our enzyme is therefore twice as active as the gold standard among polyester-splitting hydrolases," explains Sonnendecker. A plastic bowl in which grapes are sold in the supermarket, for example, could be decomposed with PHL7 in less than 24 hours. The researchers discovered that a single building block of the enzyme is responsible for the above-average activity: PHL7 carries a leucine at the point where other known polyester-splitting hydrolases contain a phenylalanine residue.

Biological PET recycling has a number of advantages compared to traditional recycling methods. These rely primarily on thermal processes in which the plastic waste is melted at high temperatures. This process takes a lot of energy and the quality of the plastic decreases with each recycling cycle. Enzymes, on the other hand, only need an aqueous environment and a temperature of 65 to 70 degrees Celsius for their work. Another plus point: They break down the PET into its components terephthalic acid and ethylene glycol, from which new PET can then be produced again - a closed cycle is created. So far, however, biological PET recycling has only been tested at a pilot plant in France.

Environmentally friendly method of reusing plastic

"The enzyme discovered in Leipzig can make an important contribution to establishing alternative energy-saving plastic recycling processes," says Prof. Dr. Wolfgang Zimmermann, who played a key role in building up the research area on enzyme-based technologies at the University of Leipzig. “Due to the enormous problems caused by the global pollution of the environment with plastic waste, environmentally friendly methods of reusing plastic in a sustainable circular economy are becoming increasingly important. The biocatalyst now being developed in Leipzig has proven to be highly effective in the rapid decomposition of used PET food packaging and is suitable for use in an environmentally friendly recycling process in which new plastic can be produced from the decomposition products.”

Biological recycling should become cheaper

The researchers from Leipzig hope that the newly discovered enzyme PHL7 will also advance biological recycling in practice and are looking for industrial partners. They are convinced that the higher speed can significantly reduce the costs for recycling. In the next two to three years, a prototype is to be created that will allow the economic advantages of their rapid biological recycling process to be quantified more precisely.

The scientists at the Institute for Analytical Chemistry in the working group of Prof. Dr. Jörg Matysik also want to clarify the structure and function of the enzymes using NMR spectroscopy. In addition, they are working on a new pre-treatment method that is intended to solve a problem in biological recycling: PET decomposition by enzymes has so far only worked for so-called amorphous PET, which is used for fruit packaging, for example, but not for plastic bottles made from so-called stretched PET exist.

Title of the original publication in ChemSusChem: 
"Low Carbon Footprint Recycling of Post-Consumer PET Plastic with a Metagenomic Polyester Hydrolase", doi.org/10.1002/cssc.202101062

Source: Press release from the University of Leipzig from May 16.05.2022, XNUMX