Will the depolymerization of plastics through the new enzymes be the alternative to mechanical and chemical recycling?

Today the production of plastic waste continues to exceed the capacity of their mechanical recycling, so much so that integrative solutions are being studied to reduce this gap.

In addition to the countless avenues that could open up chemical recycling, biological engineering is making huge strides in identifying the correct enzymes that can degrade plastic.

Through a study by a team of American scientists, aimed at identifying a modified enzyme, combinations of amino acids that could degrade PET in a shorter time were studied. faster than in the past.

The organism has two enzymes that hydrolyze the polymer first into mono- (2-hydroxyethyl) terephthalate and then into ethylene glycol and terephthalic acid to be used as a source of power.

One enzyme in particular, PETase, has become the target of protein engineering efforts to make it stable at higher temperatures and increase its catalytic activity.

A team around Hal Alper of the University of Texas at Austin in the United States has created a PETase capable of degrading 51 different PET products, including containers and whole plastic bottles.

In the construction of the study they used an algorithm that used 19,000 proteins of similar size and, for each PETase amino acid, the program studied their adaptation the environment they lived in compared to other proteins.

An amino acid that doesn't fit well can be a source of instability and the algorithm suggests a different amino acid instead.

Millions of combinations were then verified and, at the end of the analysis work, the researchers focused on three solutions that seemed to be the most promising ones.

By further intervening with direct modifications, the scientists created a highly active enzyme on PET that worked quickly and at lower temperatures than in the past.

At 50 °C, the enzyme is almost twice as active in hydrolyzing a small sample of a PET food container compared to another PETase engineered at 70 ° C.

The enzyme even depolymerized an entire plastic cake tray in 48 hours, and the team showed it can create a new plastic item from degraded waste.

It is important to emphasize that the tests were performed not on amorphous PET samples specially made in the laboratory, but on PET packaging purchased directly from supermarkets.

This brings the tests performed even closer to the context in which they should operate, that is, in the context of recycling or depolymerization of plastics.

It remains to be seen whether enzymatic depolymerization will eventually be used for large-scale recycling. In fact, most of the PET in the world is recycled not by depolymerization, but by melting and remodeling, but its properties deteriorate with each cycle.

As we said there are some methods of chemical depolymerization, but they involve a very high energy consumption and, in view of the circularity of the products, the appearance of environmental impact that recycling entails must be taken into consideration, especially when renewable energies are not available.

The great advantage of enzymes is that they can be much more specific than chemical catalysts and, therefore, it may be easier, in theory, to degrade a waste stream.

Scientists do not hide, however, that the study of enzymes that depolymerize PET, however complicated and lengthy, could be even simpler than their applications on polyolefins or on mixed plastics.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.