In many people’s minds plastic is the world’s latest, greatest evil. For despite all the good that it has done helping to create the modern world (preserving food, protecting goods, making carpets, clothes, and countless other products), plastic has one major flaw: it takes 800 years to degrade. In a world filled with all the advantages of plastic, the disadvantages will be around for a very, very long time.
Much of this plastic is in the form of PET (polyethylene terephthalate), and while some is burnt, and some is dumped in landfill, National Geographic reports that, “In 2010, eight million tons of plastic trash ended up in the ocean.” Adding that this amount, “… is on target to increase tenfold in the next decade.”
But in 2016, a cleaner path for disposing of PET was opened when Shosuke Yoshida and his team discovered Ideonella sakaiensis, the world’s first (and currently only) known plastic eating bacterium. As the team reported in The International Journal of Systematic and Evolutionary Microbiology, “We screened for microorganisms that were able to utilize PET film as the major carbon source for growth by using PET-debris-contaminated environmental samples such as sediment, soil, waste water and sludge as targets. We succeeded in isolating a microbial consortium (named No. 46) that degraded and grew on PET from the samples collected at a PET-bottle recycling site in Sakai city, Japan.”
They quickly realised that they had found a new route to disposing of plastic waste as the bacterium not only fed on PET but produced harmless waste from it. As the journal Science reported at the time, “When grown on PET, this strain [of bacteria] produces two enzymes capable of hydrolyzing PET. Both enzymes are required to convert PET efficiently into its two environmentally benign monomers, terephthalic acid and ethylene glycol.” One of these enzymes the research team called PETase.
Due to its crystallinity PET is very hard to break down, but PETase was able to attack the polymer and break it down into small units of mono(2-hydroxyethyl) terephthalic acid, or MHET. These are then converted into terephthalic acid which is ‘eaten’ by the bacterium.
Rodrigo Leandro Silveira, a postdoctoral fellow at the University of Campinas's Chemistry Institute (IQ-UNICAMP), explained the process in an interview with the online scientific journal Phys.org. Stating that, “PETase does the hardest part, which is breaking down the crystal structure and depolymerizing PET into MHET," said the FAPESP-funded researcher. "The work done by the second enzyme, which converts the MHET into terephthalic acid, is much simpler. For this reason, research has focused on PETase.”
His team hoped to better understand how PETase works so that it could be modified and improved. “In our research project, we focused on finding out what gives PETase the capacity to do something other enzymes can't do very efficiently. We began by characterizing the 3-D structure of this protein. Obtaining the 3-D structure means discovering the x, y and z coordinates of each of the thousands of atoms that comprise the macromolecule. Our British colleagues did this using a well-known and widely used technique called X-ray diffraction.”
Further analysis with computer simulations enabled the team to better understand how PETase binds and interacts with PET.
Then, by using molecular biology procedures, the researchers developed mutations of PETase hoping to, “… find out which components gave it this unique property of degrading PET.” However, the experiments failed to yield the desired results, and instead produced an even more effective PETase, one that was able to break down PET at a much faster rate.
The journal Phys.org further notes how, “More computer simulations were required to understand why the mutant PETase was better than the original PETase. Modeling and simulations clearly showed that the alterations produced in the original PETase facilitated the enzyme's binding to the substrate. This binding depends both on geometry, with two molecules fitting together like key and keyhole, and on thermodynamic factors. The elegant way to describe this is that the modified PETase has ‘greater affinity’ for the substrate.”
"Serendipity often plays a significant role in fundamental scientific research and this discovery here is no exception," says structural biologist John McGeehan from the University of Portsmouth in the UK. "This unanticipated discovery suggests that there is room to further improve these enzymes, moving us closer to a recycling solution for the ever-growing mountain of discarded plastics."
As Silveira observes, “We characterized the three-dimensional structure of the enzyme that can digest PET plastic, and engineered it to boost its degradation capacity.” Adding that, “We also demonstrated that it also acts on polyethylene-2,5-furandicarboxylate (PEF), a PET substitute made from renewable raw materials.”
What certainly is amazing is that a bacterium that lives by breaking down PET can only have come into existence since the time that PET was first made in the 1940’s, becomming widely used in the 1970's. This means that the bacterium has evolved in just a few short decades.
A Research and Markets report states that the PET global market will have grown annually by 7.3% from 2014 to 2019. Its market value is expected to exceed $47 billion by 2019.
In a world now full of plastic, nature is adapting to mankind’s influence on the environment. As a publication in the Proceedings of the National Academy of Sciences notes, “In response to the accumulation of plastics in the biosphere, it is becoming increasingly recognized that microbes are adapting and evolving enzymes to partially degrade man-made plastics as carbon and energy sources. These evolutionary footholds offer promising starting points for industrial biotechnology and synthetic biology to help address the looming environmental threat posed by man-made synthetic plastics.”
Simply put, microbes may hold the key to stop mankind from drowning in its own plastic waste. With technological assistance we could soon have a practical solution to disposing of waste plastic that doesn’t involve, burning, burying, or dumping it at sea. But we still have a tremendous amount to learn.
As the online scientific journal Phys.org observes, “In terms of obtaining an enzyme that can digest tons of plastic waste, the study was a great success, but why PETase is PETase remains a mystery.”
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Photo credit: Phys.org, MotherJonesCork, C&EN, TJUSLSChina, ScienceAlert, & Adproo.
