Introduction of PET
What is plastic pollution? Or, what will happen if plastics don’t get recycled? Since plastics can’t be dissolved in water, as the plastic drains into the ocean along with the rain, the marine animals face the risk of swallowing a giant plastic bag, leading to environmental hazards. Among the six types of commonly used plastic, PET (polyethylene terephthalate) plastic stands out as a major environmental threat. In order to tackle the plastic pollution crisis, scientists have uncovered a potential ally in nature: bacteria with remarkable abilities to break down plastic, offering hope in the battle against plastic pollution.
The Dynamic Duo: PETase and MHETase
Looking closer into the molecular structure of PET plastics, although PET plastic exhibits resistance to natural degradation, it is not impossible to be deconstructed. The PET plastic molecules are composed of resilient polymer chains, which includes two major monomers - terephthalic acid and ethylene glycol (an acid and an alcohol). While these monomers can undergo natural decomposition individually, the challenge arises when they form a resilient bond known as an ester bond, linking an acid and an alcohol. However, scientists have discovered microbes with evolutionary ability to split the eastern bond through hydrolysis (chemical reaction with water), and they are identified as esterase enzymes. Therefore, given the prevalence of esterase enzymes and the molecular structure of PET plastics, scientists have been working on discovering methods to incorporate enzymes into depolymerizations of the plastic molecules.
Surprisingly, it was not until recently that scientists isolated an enzyme from a soil bacterium - Ideonella Sakaiensis. The bacterium develops “a two-enzyme system for polyethylene terephthalate (PET) deconstruction”. In the first step, the enzyme PETase acts as a “cutinase-like serine hydrolase that attacks the PET polymer”, which converts the polymer into soluble intermediate molecules MHET. Then the soluble MHET product is further hydrolyzed by MHETase, another enzyme, to produce TPA and EG. After a series of chemical reactions, the PET plastic is no longer a stubborn molecule, but pieces of monomers that could dissolve in nature. To enhance efficiency, scientists have ingeniously combined these enzymes to create a super enzyme, increasing the breakdown efficiency by six times.
Limitations:
Despite these promising advancements, the war against plastic pollution is far from won. The super enzyme's speed is yet to reach commercial viability, prompting scientists to explore more potent solutions. As we navigate through this environmental challenge, it is crucial for humanity to take charge and ensure proper recycling practices. By managing plastic waste responsibly, we can significantly reduce the burden on scientists and contribute to a cleaner, healthier planet.
Sources:
Dutfield, S. (2022, March 23). Plastic-eating bacteria: Genetic Engineering and Environmental impact. LiveScience. https://www.livescience.com/plastic-eating-bacteria
Genetically modified bacteria break down plastics in saltwater. NSF. (n.d.). https://new.nsf.gov/news/genetically-modified-bacteria-break-down-plastics
Characterization and engineering of a two-enzyme system for ... - PNAS. (n.d.). https://www.pnas.org/doi/pdf/10.1073/pnas.2006753117
For more in depth understanding:
Knott BC, Erickson E, Allen MD, Gado JE, Graham R, Kearns FL, Pardo I, Topuzlu E, Anderson JJ, Austin HP, Dominick G, Johnson CW, Rorrer NA, Szostkiewicz CJ, Copié V, Payne CM, Woodcock HL, Donohoe BS, Beckham GT, McGeehan JE. Characterization and engineering of a two-enzyme system for plastics depolymerization. Proc Natl Acad Sci U S A. 2020 Oct 13;117(41):25476-25485. doi: 10.1073/pnas.2006753117. Epub 2020 Sep 28. PMID: 32989159; PMCID: PMC7568301.