A Solution to Plastic Waste Might Come from Enzymes
New avenues for a more sustainable solution to the increasing global plastic waste problem.
Since the invention of plastics in 1907, our world quickly became dependent on these versatile materials. Plastics are present in appliances, electronics, furniture, and are often used as packaging materials for food (just take a closer look at your refrigerator.)
Although plastics have made our lives much easier, today we are faced with a global crisis primarily due to the poor management of plastic waste.
By 2018, estimates suggested that of 8.3 billion US tons of plastic manufactured worldwide, 6.9 billion US tons of plastic waste were generated. Of this plastic waste, only 21 percent was recycled or incinerated, while 79 percent accumulated in landfills and in our natural environment.
The effect of this overwhelming amount of plastic waste is already showing. Seabirds and turtles can inadvertently swallow plastic debris or even mistake it for food. Nearly invisible plastics known as microplastics have already been detected on the ocean floor, soils, sediments, and freshwater, largely impacting the food chain.
Although legal mechanisms such as plastic bag bans are a promising way to reduce plastic waste, their sole implementation would not be enough. Plastics are highly resistant to degradation, taking as long as 1,000 years. Finding a way to decompose what is already out there in our landfills is critical.
One possible way to do this is through the use of enzymes.
Plastic-eating enzymes
Enzymes are substances produced by living organisms that help speed up chemical reaction or transformation that otherwise occurs very slowly.
To our luck, scientists have found enzymes in fungi and bacterial organisms that can degrade one of the most abundant plastic worldwide — poly(ethylene terephthalate) or PET — also the raw material used to make water and soft-drink bottles.
Thermobifida fusca, Fusarium solani, Humicola insolens, and an unknown bacterial source in leaf and branch compost, are some examples of organisms known to produce PET-degrading enzymes.
So, how do these enzymes work?
PET is a polymer composed of long chains of repetitive interlinked units. Each unit corresponds to a terephthalate (TPA) moiety, and they are connected through ethylene glycol bridges. This connection between TPA and ethylene glycol bridges form highly stable links known as ester bonds. PET-degrading enzymes act as molecular scissors by cutting these links off, therefore forming smaller units known as TPA and MHET, a TPA derivative.
However, the industrial application of these enzymes to treat plastic waste is limited by how efficient and how fast they can break down plastic. In addition, these enzymes are expected to perform well at high temperatures. Only at temperatures above 50 °C PET chains become more flexible and can be degraded much faster. However, at these temperatures, enzymes can aggregate and become inactivated.
Only four years ago, the discovery of a new bacterial organism gained a lot of attention. In Science, researchers announced the finding of a new organism isolated from a PET-recycling facility in Japan. Named as Ideonella sakaiensis 201-F6 (Is 201-F6), this organism was able to use PET as a primary nutritional source.
To find the gene in Is 201-F6 that was likely producing a PET-eating enzyme, scientists used a gene template obtained from a fungal enzyme known to decompose PET. Upon finding this gene, they observed that the enzyme produced was able to chew not only PET films but also commercial bottle-derived PET in 18 hours at 30 °C. But there was one caveat, the observed degradation was only partial.
Further studies then found that the enzyme, which these scientists named PETase, had poor water solubility, low efficiency at high temperatures, and became unstable outside the bacterial cell. Therefore, the potential use of this enzyme to treat plastic waste at industrial level was not feasible.
However, only a few months ago, a new study published in Nature reported the discovery of an engineered enzyme from leaf and branch compost (LCC) with high potential for industrial plastic waste treatment.
This engineered enzyme was designed from a native PET-degrading enzyme previously found in LCC — known as LCC cutinase. To do this, researchers used snapshots of cutinase molecular structure and combined them with computational tools to create mutants (meaning different versions or variants) of cutinase. Broadly speaking, each amino acid in the enzyme-PET interface was exchanged one by one with one of the other 19 possible amino acids available in nature — a process known as site-selective saturation mutagenesis or protein engineering.
209 cutinase mutants were isolated and their ability to degrade PET was compared based on the original version. Of these, 25 were identified to have modest to high activity and were further optimized to improve their stability at high temperatures.
After several rounds of optimization, researchers found the best LCC cutinase mutants, and labeled as WCCG and ICCG. While 90 percent decomposition was obtained in 10.5 hours for WCCG, 9.3 hours were required for the ICCG variant.
“After two years of the work of 20 scientists, we obtained this incredible enzyme (meaning the ICCG cutinase variant) able to deconstruct 90 percent of PET in less than 10 hours” — said Alain Marty, the team leader behind this project.
What is more surprising is that this LCC cutinase variant was able to decompose almost any form of PET waste as the researchers mentioned in an interview. They did not see any inhibition of the enzyme by colorants, pigments, isophthalic acid (common additive for PET products), carbon black, titanium dioxide, other polymers. These are all non-PET components often present in plastic waste.
Re-purposing PET decomposition products
Research has found that half of manufactured plastic becomes trash within a year. Of this plastic waste, more than 70 percent enters the landfill or escapes the collection system, while only 14 percent is recycled.
So, can this small recycling percentage be increased? Carbios and their partners at Toulouse Biotechnology Institute (TBI) — authors of the Nature publication — also looked into this pressing question. With an already optimized enzymatic PET degradation protocol, recycling was just one more thing to test.
In their study, researchers showed that they could take TPA produced from their enzymatic PET degradation process, purified it, and seemingly re-used it to make new PET bottles. What is even more surprising is that these virgin plastic bottles had similar mechanical and physical properties as commercial PET bottles.
Yet, the team foresees a small caveat. Recycled PET would be more expensive than virgin PET because of the extra costs of TPA purification and plastic priming (such as grinding and heating), which is required before enzyme addition.
Regardless, Carbios is set to launch a demonstration plant by 2021, opening new avenues for a more sustainable solution to the increasing global plastic waste problem.
