Could a combination of microwave irradiation and enzymes help to solve the growing plastic waste problem?

ScienceViews 28. feb 2023 4 min Associate Professor Per-Olof Syrén, Professor, Head of Division Minna Hakkarainen Written by Per-Olof Syrén

Humans urgently need to enable a transition from a linear to a circular materials economy. Each year, 3 million tonnes of plastic debris enters our oceans from rivers – endangering animals ingesting it. Enzymes are key to breaking down and reusing the synthetic polymers, but making them fit the many different chemistries, forms and shapes of plastics is a challenge. A combination of new ways of engineering plastic-degrading enzymes and pretreating the plastics with microwaves might provide value to plastic waste and help prevent the formation of plastic debris.

The total accumulated amount of waste associated with human activities worldwide has now exceeded the living biomass on Earth. This waste includes more than 8 billion tonnes of solid plastic waste so far, and each year about 3 million tonnes of plastic debris enters our oceans from rivers.

Manufacturing of plastics is a large application area for crude oil (about 350 million tonnes per year), and resulting in about 2% of the total CO2 emissions (850 million tonnes of CO2 emitted per year).

Currently implemented material production is not sustainable, with immense consequences for our environment and ecosystems. When deposited in nature, solid plastic waste breaks down slowly (the half-life could be centuries), resulting in the formation of microscopic and nanoscopic particles, endangering life forms – especially in oceans across the planet.

More than 3 billion tonnes of non-recyclable waste

In addition, during their lifetime, human-made materials and plastics have indispensable functions for our everyday life and are commonly the most environmentally friendly and hygienic option, saving energy, enabling conservation of food and beverages and serving an indispensable role in healthcare, green energy production and engineering.

We urgently need to enable a transition from a linear to a circular materials economy. With our project EazyPlast, we seek to contribute to reaching this goal by using innovative biotechnologies.

Complex plastic blends and composites commonly found in today’s waste streams pose hurdles for recycling. Sorting plastic materials according to the type is a tough task – both for ordinary people and recycling companies using such methods as infrared spectroscopy – and chemical and/or mechanical recycling typically only works well for pure plastic materials.

Since general and efficient methods for mild chemical recycling of plastics and composites are lacking, only 10% of all post-consumer synthetic polymers can be upcycled with currently available technologies. For example, we recently showed how mixed synthetic plastic fibres in municipal textile waste will contribute to more than 3 billion tonnes of non-recyclable waste by 2030.

Only works for a few polymer types

Biocatalytic upgrading of plastic waste into high-added-value compounds holds great promise in tackling the plastic problem by using mild green chemistries.

Specific enzymes benefitting from high specificity can break down – depolymerise – specific polymer types selectively, even from complex blends found in post-consumer plastic waste streams.

This, in turn, greatly facilitates downstream processing during recycling.

In addition, the mild reaction conditions associated with biocatalysis ensure that intact plastic monomers can be retrieved and reused, as opposed to, for example, thermomechanical recycling, which relies on harsh conditions that can damage constituent building blocks and lead to non-selective reactions.

Nevertheless, enzymatic degradation of plastics typically only works for a few polymer types and at rates that are usually incompatible with industrial applications.

Inducing a twist

The budding field of biocatalytic plastic recycling was sparked by key discoveries of polyesterase enzymes, which are capable of breaking down PET – an industrially important polyester plastic type – by the process of hydrolysis.

Subsequent protein engineering has successfully generated stable polyesterase enzymes that remain active over extended periods of time and at elevated temperatures, achieving high depolymerisation activity of even sometimes bulky plastic chains.

But engineering enzymes that can adopt to several discrete conformations – so that enzyme shape matches that of the polymer – remains an unmet challenge.

For example, for PET, the backbone building block ethylene glycol moiety can adopt two different conformations: trans and gauche. Whereas trans results in a more linear polymer chain, gauche induces a twist. How an enzyme is capable of recognising these very different bulky substrate conformations remained unknown – at least until our EazyPlast project was launched.

The importance of accumulating

The question we wanted to answer was: Is it possible to change the enzyme so it can adapt to different plastic substrates at the same time?

We recently showed how the original wild-type plastic-degrading PET enzymes are indeed flexible. They can adopt many different structures and conformations, each with a different affinity to discrete plastic conformers.

However, most of these conformations were found to be unproductive, having the essential catalytic amino acids residing in an unfavourable geometric arrangement. So flexibility led to inactivity.

What do these findings tell us about possible evolutionary drivers underpinning the emergence of enzymatic plastic degradation activity? Our findings underscore the importance of accumulating mutations in an inherently dynamic scaffold to drive the emergence of plastic substrate acceptance.

Aligned with this hypothesis, we showed how even single-point mutations on the surface of the enzyme are enough to significantly alter the protein motion of the enzymes. This favours certain protein structures above others, resulting in changed specificity for certain substrates. These small changes can be key to tuning the enzymes to other plastic types.

Interestingly, one variant found in our project was found to prefer the trans-form of PET, a conformation especially occurring in crystalline parts of the plastic material.

Need for pretreatment

A second remaining challenge in biocatalytic breakdown of plastics is gaining accessibility to scissile (breakable) bonds in the material.

The importance of pretreating plastic to break strong secondary interactions and to open up the material structure is widely recognised in the field. Methods commonly employed mostly focus on reduction of size by grinding, amorphisation using ultracold liquid nitrogen and harsh chemical pretreatment.

Our results show how microwave irradiation can transform post-consumer PET built up from high-molecular-weight chains with mixed conformations (a mixture of trans and gauche in amorphous regions and trans in crystalline parts) into a more uniform material with lower-molecular-weight chains showing almost exclusively trans – being more ready for breakdown by enzymes. Pretreatment might thus have an important role in making plastics more accessible even with respect to the molecular conformation.

Protecting the sea from plastics by endowing waste with value

Combining this pretreatment method with a trans-selective enzyme developed in our project resulted in a higher biocatalytic rate. Thus, our work emphasises once more the importance of matching the enzyme and substrate conformations – and this is a special challenge in plastics. Both accessibility and shape need to be considered when developing enzyme-based plastic recycling methods.

This project takes the first steps in implementing these potential solutions towards a future circular materials economy founded on enzymes. It is encouraging to see that even small changes in the protein sequence can have dramatic consequences for the conformational landscape displayed by PETase and consequently its activity and selectivity.

Biocatalysis holds great promise in addressing the plastic problem and will greatly benefit from discovery of enzymes capable of attacking otherwise non-hydrolysable bonds in, for example, polyolefins.

These findings also open up possible avenues for bioremediation applications in which, for instance, the plastic-degrading PETase put into cyanobacteria – naturally living in the sea – either by recombinant technology or horizontal gene transfer followed by natural evolution – could help clear plastics from the sea. Enzyme engineering is required for a more efficient process, however, and the importance of pretreatment needs to be considered.

A 2021 review before the onset of this project, “Biocatalysis in the recycling landscape for synthetic polymers and plastics towards circular textiles” was published in ChemSusChem. The EazyPlast project has been funded by the Novo Nordisk Foundation and has resulted in two articles so far: “Whole-cell mediated carboxylation of 2-furoic acid towards the production of renewable platform chemicals and biomaterials” published in ChemCatChem (2022) and “Conformational selection in biocatalytic plastic degradation by PETase” published in ACS Catalysis (2021). A third study on using microwave technology in concert with enzyme engineering for plastic recycling has been submitted.

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