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Enzyme in human salivary microbes decomposes PET-based plastics

Enzyme in salivary microbes decomposes PET-based plastic

Credit: Wiley

Human saliva may contain an enzyme that can break down the plastic polyethylene terephthalate (PET). Researchers found the promising enzyme, a hydrolase, in a database of human metagenome samples. As they report in the news International edition of Angewandte Chemie, this newly discovered hydrolase outperforms many other known bacterial PET hydrolases. It can be produced using biotechnological methods and can be used to recycle plastics or to functionalize plastics, the authors add.

Landfills and harbors are known to be particularly promising sites for finding bacteria that have adapted to consume or make use of plastic. These bacteria have developed enzymes, known as PET hydrolases, that can break down PET into smaller molecules. Chayasith Uttamapinant of the Vidyasirimedhi Institute of Science and Technology (VISTEC) in Rayong, Thailand, and Worawan Bhanthumnavin of Chulalongkorn University, Bangkok, Thailand, and colleagues have now discovered the first enzyme that breaks down PET from a more surprising source: the genome of microbial communities in human saliva.

The researchers believe that because humans consume massive amounts of food packaged with PET, microbes in the saliva or gastrointestinal tract may have evolved to digest microplastics. The team discovered the new hydrolase, which they named MG8, while searching a public metagenome database of seawater and human saliva samples, and were able to attribute the likely source of the enzyme to Gram-negative bacteria that may be in human saliva. These bacteria are similar to strains found near the “Pacific trash vortex,” which have also evolved to produce PET hydrolases.

They first needed enough material to conduct their experiments, so they modified a bacterium that can be grown in labs to produce the enzyme. They easily recovered an active form of the enzyme, which can break down PET, from a denatured form that can be isolated in large quantities. The researchers emphasize that this holds great promise for scaling up in the future.

Aside from the prospects for recycling scalability, the team envisions another use for MG8. They discovered that the PET can not only break down easily, but with a small adjustment can also bind to it very effectively. To achieve this, they modified the protein sequence by replacing one of the naturally occurring amino acids (serine) in the active site with an unnatural amino acid, DAP. The modified enzyme immediately adhered to PET powder. This can be used as a means to functionalize PET surfaces, increase the versatility of PET in medical devices, for example, and increase the versatility of recycled PET.

Despite MG8’s promise in plastics recycling and functionalization, the team recognizes that MG8, like other PET hydrolases, still needs some work. For the time being, consumer grade PET plastics with high crystallinity cannot be degraded with this hydrolase. Therefore, further research will be needed to reach the stage where an entire plastic water bottle can be dissolved in a simple solution containing the enzyme.

Enzyme breaks down PET plastic in record time

More information:
Bhumrapee Eiamthong et al, Discovery and genetic code extension of a polyethylene terephthalate (PET) hydrolase of the human salivary metagenome for the degradation and biofunctionalization of PET, International edition of Angewandte Chemie (2022). DOI: 10.1002/anie.202203061

Quote: Enzyme in human salivary microbes decomposes PET-based plastic (2022, June 29) retrieved June 29, 2022 from https://phys.org/news/2022-06-enzyme-human-salivari-microbes-decomposes.html

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