Biologist Hal Alper on the self-producing enzyme who thrives on PET products – it is able to break apart plastic polymers and give back virgin material without any loss of integrity
Enzymatic biodegradation of PET
Polyester waste recycling through enzymatic degradation is possible. We spoke to Professor Hal Alper, professor in the McKetta Department of Chemical Engineering at UT Austin, on the discovery of an enzyme variant whose potential is to break down PET – Poly(ethylene terephthalate)–, the most common type of plastic found in waterways and soil that typically take centuries to degrade, in just a matter of hours.
Experimental results from studies at the U.S. Department of Energy’s Advanced Photon Source (APS) have been published in Nature and demonstrate a circular carbon economy for PET is theoretically attainable using rapid enzymatic depolymerization followed by repolymerization or conversion/valorization into other products.
«We are interested in the ability to make new chemicals, the ability to make sustainable and renewable solutions. On the other side, looking for new sustainable and renewable feedstocks for basic chemical material production. That is the area of waste valorisation: the ability to take waste (that is around or created) and be able to make it into something else. First we are trying to understand how to break down plastics and ultimately be able to utilize it as a starting point to make sustainable chemicals».
A response to the proliferation of plastic waste
Enzymatic depolymerization of PET was first scientifically reported in 2005. After that, a total of 159 single or several predicted mutations were generated, yet, among this set, only four mutations resulted in the highest improvements, both single and in combination, and were selected for further assembly and analysis.
According to professor Alper, this enzyme is a result of a new function evolved by nature itself in response to the proliferation of plastic waste. «As we have begun to have plastic waste in our waterways and soil, microorganisms have begun to start to see how to utilize some of this plastic. It’s a slow process, and it does not happen for all the types of plastic».
Materials like PET do make an exception: in 2016, scientists in Japan unearthed and isolated an organism that was able to grow on plastic bottles, a scientific revelation that led to the discovery of enzymes that self-produced and are able to break apart plastic polymers.
The engineering and testing phases
To enhance the activity of the enzyme, the researchers used material machine-learning and AI intelligence approaches to supercharge the properties of the microorganism to make it work at much faster rates. The result is a new enhanced enzyme that can chew plastic polymers apart and break it back to its starting point.
«You can think about this as beads on a string», explained Alper; «it chops across and releases its individual beads. From that point we can build it up again to create the same exact type of virgin PET plastic, or maybe in the future, we can use those beads to solve some other type of approach, making a new chemical or a new type of fuel». The most notable PET-eating variants (called ThermoPETase17 and DuraPETase22) were created through rational protein engineering and computational redesign strategies, respectively.
As specified in the Nature scientific report published by researchers Hongyuan Lu, Daniel J. Diaz, Natalie J. Czarnecki, Congzhi Zhu, Wantae Kim, Raghav Shroff, Daniel J. Acosta, Bradley R. Alexander, Hannah O. Cole, Yan Zhang, Nathaniel A. Lynd, Andrew D. Ellington and Hal S. Alper, they used a three-dimensional selfdimensional self-supervised, convolutional neural network called MutCompute24 to identify stabilizing mutations.
This algorithm learns the local chemical microenvironments of amino acids on the basis of training over 19,000 sequence-balanced protein structures from the Protein Data Bank and can readily predict positions within a protein in which wild-type amino acids are not optimized for their local environments.
Depolymerizing untreated portions of a commercial water bottle
«Right now our enzyme is able to work in a controlled type of environment and does not require any special environmental conditions for it to be working. Our mutant and scaffold combination contains five mutations compared to wild-type PETase and shows superior PET-hydrolytic activity relative to both wild-type and engineered alternatives between 30 and 50 °C and a range of pH levels. We demonstrate that untreated, postconsumer-PET from 51 different thermoformed products can all be almost completely degraded by FAST-PETase in 1 week. FAST-PETase can also depolymerize untreated, amorphous portions of a commercial water bottle and an entire thermally pretreatedpre-treated water bottle at 50ºC».
Moreover, the testing of these enzymes on raw, untreated pc-PET, where 51 samples of post-consumer plastic products used in the packaging of food, beverages, medications, office supplies, household goods and cosmetics were collected, showed the same results. Despite their heterogeneity across crystallinity, molecular weight, thickness and additives, all PET products were fully degraded in 1 week and in as little as 24 hours.
The presence of polyester in fashion
It was proven that this type of enzyme can degrade down the PET component that is typically used in combination treatments for mixed fabric, providing a potential route for recovering PET monomers from commercial polyester products and reducing the leaching of microfibers into the environment.
«This process would allow to separate the PET component from the mixed fabric and get some initial value added to that component and take whatever is left from that mixture and go to the next step of the process, whatever is going to be another enzyme that can break down the next component or some other chemical process that can break down the rest of other components that are in there». A closed-cycle PET reconstitution by first depolymerizing tinted postconsumer plastic waste and subsequently recovering monomers and repolymerizing into virgin PET.
The faults in the traditionalthe traditional recycling processes
In most recycling processes, recycling is done by melting, having only a limited number of times to recycle the product before its integrity starts to break down. That’s why a lot of most recycled products have only a certain percentage of post-consumer product in it, because you always have to put fresh material to make sure you’re getting the right property traits.
«In contrast, when you’re using enzymatic degradation, you’re going back to the original starting point. You’re essentially creating a closed loop cycle when you can create, use, deconstruct, recreate and use once again. And you get to keep that property traits each and every time, because you are regenerating in virgin PET product, and not having that loss of integrity that happens with traditional recycling processes.
The creation of a closed loop process
Since this enzyme is able to complete a circular process of breaking down the plastic into smaller parts and then chemically putting it back together, it is allowing the creation of a closed loop process from which biology can enable biodegradation.
Although biodegradation has become a loaded word in terms of what people is associating it with, according to the testing made by Alper and his associates, the researchers are still working trying to find organism to work together with this enzyme that would enable to consider true biodegradability and leave no carbon footprint behind. The real problem lies in the presence of theratholyc acid, a substance released from this process that wouldn’t be safe to have it released around waterways and soil freely. The goal would be to have it consumed and made it into a more benign state.
«We demonstrated a closed-loop PET recycling process, and our results demonstrate a viable route for enzymatic plastic recycling at the industrial scale. This enzyme in biology is great for being very specific. It can specifically break down PET and PET related polymers. We’re trying to identify other enzymes that could work on other types of plastics».
Hal Alper
A professor in the McKetta Department of Chemical Engineering at UT Austin. His team of biologists discovered an enzyme able to decompose PET plastic.
