MATERIALS / RECYCLING PETplanet Insider Vol. 23 No. 10/22 www.petpla.net 15 Enzyme discovered in human saliva microbes could be used to decompose PET-based plastics New PET hydrolase Researchers in Thailand have discovered that an enzyme found in human saliva has the capacity to decompose certain forms of PET. Chayasith Uttamapinant, from the Vidyasirimedhi Institute of Science and Technology (Vistec) in Rayong, Thailand, and Worawan Bhanthumnavin, from Chulalongkorn University, Bangkok, Thailand, and their teams and colleagues, investigated whether humans, who consume vast amounts of food and beverages that have been packaged in PET, may have evolved the ability to digest microplastics. Landfill sites and harbours have already been found to be promising sites for finding bacteria that have adapted to consume or make use of plastics. The candidate bacteria have evolved enzymes, known as PET hydrolases, which can break PET down into smaller molecules. The teams did not discover any PET-digesting ability in the human gut but they did find a new hydrolase, which they named MG8, while searching a public metagenome database that contains samples from seawater and human saliva. The teams were able to attribute the likely source of the enzyme to gram-negative bacteria that may reside in human saliva. These bacteria are similar to strains found near the “Pacific trash vortex”, which have also evolved to produce PET hydrolases. Their paper, published in the journal Angewandte Chemie, reports that this newly discovered hydrolase performs better than many other known bacterial PET hydrolases. It can be produced using biotechnological methods and could be put to use in plastic recycling or for functionalising plastics. In their quest for enough material to perform their experiments, the teams modified a bacterium that can be cultured in laboratories to produce the enzyme. They easily recovered an active form of the MG8 enzyme from a denatured form, which can be isolated in large amounts. This discovery indicates a high level of potential for scaling up in the future. The team discovered that MG8 can not only decompose PET with ease but, with a small modification, it can also bind to it, highly effectively. They achieved this by modifying the protein sequence, replacing one of the naturally occurring amino acids (serine) at the active site with an unnatural amino acid: DAP (diaminopimelic acid). DAP is a characteristic of cell walls of some bacteria; it is often found in the peptide linkages of NAM-NAG (amino sugars present in the peptidoglycan layer of bacteria) chains that make up the cell wall of gram-negative bacteria. The modified enzyme was found to immediately adhere to PET powder. The report paper suggests that, in this form, it could be used as a vehicle for functionalising PET surfaces, increasing its versatility in medical devices and enhancing the versatility of rPET. While it looks promising, MG8 is still at an early stage of development. Consumer-grade PET plastics with high crystallinity cannot be decomposed by it yet, for example. Further research will be necessary to reach the stage where a whole plastic water bottle can be dissolved in a simple solution containing the enzyme. Dr Chayasith Uttamapinant is a lecturer in the School of Biomolecular Science & Engineering, Vidyasirimedhi Institute of Science and Technology (VISTEC), Rayong, Thailand. The Uttamapinant group develops chemical biology technologies to study proteoforms and local translation, bioremediation tools, and open-source molecular diagnostic platforms. Dr Worawan Bhanthumnavin is an Associate Professor at the Chemistry Department of Chulalongkorn University, Bangkok, Thailand. Her lab has broad interests in synthetic methodological development for bioactive compounds, fluorescent probes and sensors, as well as biological applications of these synthetic molecules. The research paper was published in Angewandte Chemie. The full article can be found at: Discovery and Genetic Code Expansion of a Polyethylene Terephthalate (PET) Hydrolase from the Human Saliva Metagenome for the Degradation and Bio-Functionalization of PET. https://onlinelibrary.wiley.com Some additional information was found and included in this article by Ruari McCallion, PETplanet Insider Editor-at-large. The hydrolase can be produced using biotechnological methods and could be put to use in plastic recycling or for functionalising plastics (source: Wiley-VCH, ‘Angewandte Chemie’). MATERIALS
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