Enzymatic recycling: synthetic biology as a way to achieve a sustainable consumption model

Recycling data obtained in 2015 indicated that only 9% of the plastic that we throw away in the garbage is effectively recycled, while 12% of total plastic ends up being incinerated and 79% accumulated in a garbage dump [1]. Recently, synthetic biology allows for the production of engineered enzymes that can degrade plastic in a more efficient, time-saver and environment friendly way. Actual recycling procedures are very inefficient, as only 30% of total plastic has a useful life again, and commonly as a kind of plastic with worse properties than the original [2]. Recent research on plastic degrading enzymes, which are proteins capable of cutting the plastic chemical bonds, could increase plastic recycling [3-5]. Engineered enzymes can recycle ~90% of the plastic with high quality and performance. In the present there are several laboratories trying to scale up these reactions in order to open a plastic processing plant the next year. 

One of the most commonly used plastic in the world is poly (ethylene terephthalate) (PET), and one of its main disadvantages consists of the highest temperatures needed to melt the different types of PET present in the recycling facilities. As a result, a grey or black mass with very poor quality is produced, and many companies don't want it for their products. Melted plastic is usually transformed into plastic carpets or fibers that will end up buried or burned. Of course, one can imagine that this is not recycling at all.

PET bottles in a garbage dump. Thanks to Recycling Today (www.recyclingtoday.com).

But here is when science, specifically synthetic biology, come to save the day. Several researchers have searched for enzymes capable of breaking the chemical bonds that forms the PET or another commonly used plastics. In 2012, researchers from Osaka University found an enzyme called leaf-branch compost cutinase (LLC) which breaks the chemical bonds between the two components of PET: terephthalate acid and ethylene-glycol. But the degrading reaction is not ideal, as LLC slowly separates the PET chemical bonds and the enzyme is eventually degraded after a few days working at 65º C, the ideal temperature for the PET to soften. Therefore, the enzyme degrades at a temperature that makes it easier to meet PET chemical bonds, and this is a problem.

In a recent article published in Nature [6], scientists from the University of Toulouse have improved this enzyme, thus solving the temperature problem and enhancing its plastic degradation capacity. They analyzed the crystal structure of the enzyme to characterize the active site where the PET chemical bonds are degraded, and then tried by different ways to improve the enzyme performance and its resistance to the temperatures at which the PET is soften. This, which seems simple, is a process of trial and error looking for different proteins that, by chance, would work better at high temperatures. In the end they managed to isolate a mutant enzyme that is 10,000 times more efficient at breaking PET bonds than the original enzyme. Furthermore, this enhanced version of the enzyme is capable of resist temperatures around 72º C without degrading, enough temperature to allows the PET to be melted. Therefore, the enzyme can easily reach the PET chemical bonds. 

The new enzyme really works. A small reactor was produced to test the recycling activity of the engineered enzyme, and the results were astonishing. Only 3 mg of the engineered enzyme broke up 90% of 200 grams of PET in about 10 hours. The leftover terephthalate acid and ethylene-glycol was used to produce new plastic bottles with the same properties, strength, durability and aspect than the original ones. One of the most surprising finding was the fact that the enzyme can break the chemical bonds while ignoring any dye or color applied to the plastic. Thus, the recycle plastic paste have the same characteristics as the original material.  

A brief view of the identification and production of PET degrading enzymes. Copyright: The Biology Notes (thebiologynotes.com).

Authors proposed a model in which the purified terephthalate acid monomers will be used to synthetize new PET for being ultimately used to produce bottles, closing the loop of the circular economy. With an urgent global need to address the issue of plastic disposal, the enzymatic processing of PET waste may help to meet such goals. Many governments, national and international agencies and manufacturers which are committed to sustainability goals should reinforce this circular economy strategy to solve one of our environmental problems. Much investment and further scale-up testing will undoubtedly be required to see how profitable the enzyme production process is compared to recycling performance and the cost savings from both waste storage and the generation of new plastics. Nevertheless, synthetic biology techniques now allow for the production of larger amounts of enzymes in large bioreactors, which can be used to progressively change the actual recycling methodologies. We have to bear in mind that investments in more efficient processes will result in a long-term benefit that is much higher than the initial costs.

References:

1. Geyer, R., J.R. Jambeck, and K.L. Law, Production, use, and fate of all plastics ever made. Sci Adv, 2017. 3(7): p. e1700782.

2. Ragaert, K., L. Delva, and K. Van Geem, Mechanical and chemical recycling of solid plastic waste. Waste Manag, 2017. 69: p. 24-58.

3. Wei, R. and W. Zimmermann, Microbial enzymes for the recycling of recalcitrant petroleum-based plastics: how far are we? Microb Biotechnol, 2017. 10(6): p. 1308-1322.

4. Yoshida, S., et al., A bacterium that degrades and assimilates poly(ethylene terephthalate). Science, 2016. 351(6278): p. 1196-9.

5. Brueckner, T., et al., Enzymatic and chemical hydrolysis of poly(ethylene terephthalate) fabrics. 2008. 46(19): p. 6435-6443.

6. Tournier, V., et al., An engineered PET depolymerase to break down and recycle plastic bottles. Nature, 2020. 580(7802): p. 216-219.