New PET hydrolase

Human saliva may contain an enzyme that can break down plastic polyethylene terephthalate (PET). The researchers found the promising enzyme, a hydrolase, in a database containing human metagenome samples. As reported in the newspaper. applied Chemistry, this newly discovered hydrolase performs better than many other known bacterial PET hydrolases. It can be produced using biotechnological methods and could be used in plastic recycling or to functionalize plastics, the authors add.

Landfills and ports are known to be particularly promising places to find bacteria that have adapted to consume or use plastics. 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 discovered the first enzyme to break down PET from a rather more surprising source: genome of microbial communities in human saliva.

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

First they needed enough material to run 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, capable of breaking down PET, from a denatured form that can be isolated in large quantities. The researchers stress that this shows great promise for future scaling.

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

Despite the promise of MG8 in recycling and functionalizing plastics, the team acknowledges that MG8, like other PET hydrolases, still needs some work. At the moment, consumer PET plastics with high crystallinity cannot be broken down by this hydrolase. Therefore, more research will be needed to get to the stage where an entire plastic water bottle can be dissolved in a simple solution containing the enzyme.