Scientists create 'cocktail' of enzymes that can digest plastic up to six times faster, enables endless reuse
The super-enzyme will help reduce reliance on fossil resources such as oil and gas
Plastic pollution represents a global environmental crisis. A few years ago, scientists had engineered an enzyme that can digest some of our most commonly polluting plastics, providing a potential solution to one of the world’s biggest environmental problems. The same team, who re-engineered the plastic-eating enzyme PETase, has now created an enzyme ‘cocktail’ that can digest plastic up to six times faster.
According to experts, PETase breaks down polyethylene terephthalate (PET) back into its building blocks, creating an opportunity to recycle plastic infinitely and reduce plastic pollution and the greenhouse gases driving climate change. The second enzyme MHETase, "found in the same rubbish dwelling bacterium that lives on a diet of plastic bottles," has been combined with PETase to speed up the breakdown of plastic, say authors.
Like PETase, the new combined MHETase-PETase works by digesting PET plastic, returning it to its original building blocks. This allows for plastics to be made and reused endlessly, reducing reliance on fossil resources such as oil and gas.
PET is the most common thermoplastic, used to make single-use drinks bottles, clothing and carpets and it takes hundreds of years to break down in the environment, but PETase can shorten this time to days. The latest discovery implies that another leap forward has been taken towards finding a solution to plastic waste, emphasize the scientists.
In 2016, Japanese researchers discovered a plastic-eating bacterium that had evolved naturally. This initial discovery set up the prospect of a revolution in plastic recycling, creating a potential low-energy solution to tackle plastic waste.
Subsequently, professor John McGeehan from the University of Portsmouth and Dr Gregg Beckham from the US Department of Energy’s National Renewable Energy Laboratory (NREL) solved the crystal structure of PETase — the enzyme that digests PET— and used this 3D information to understand how it works. The experts made a breakthrough when they were examining the structure of this natural enzyme, which is thought to have evolved in a waste recycling center in Japan, allowing a bacterium to degrade plastic as a food source.
They inadvertently engineered an enzyme that was even better at degrading the plastic than the one that evolved in nature. The research team engineered the natural PETase enzyme in the laboratory to be around 20% faster at breaking down PET. The study concluded that the "PETase mutant outperforms the wild-type PETase in degrading PET."
The current report, published in the Proceedings of the National Academy of Sciences (PNAS), has also been co-led by professor McGeehan, Director of the Centre for Enzyme Innovation (CEI) at the University of Portsmouth, and Dr Beckham, a senior research fellow at NREL. They combined PETase and its "partner", a second enzyme called MHETase, to generate much bigger improvements.
The results reveal that simply mixing PETase with MHETase doubled the speed of PET breakdown, and engineering a connection between the two enzymes to create a "super-enzyme" increased this activity by a further three times.
"Gregg and I were chatting about how PETase attacks the surface of the plastics and MHETase chops things up further, so it seemed natural to see if we could use them together, mimicking what happens in nature. Our first experiments showed that they did indeed work better together, so we decided to try to physically link them as two Pac-men joined by a piece of string," writes Professor McGeehan. He adds, "It took a great deal of work on both sides of the Atlantic, but it was worth the effort. We were delighted to see that our new chimeric enzyme is up to three times faster than the naturally evolved separate enzymes, opening new avenues for further improvements."
Professor McGeehan used the Diamond Light Source in Oxfordshire, "a synchrotron that uses intense beams of X-rays 10 billion times brighter than the Sun" to act as a microscope powerful enough to see individual atoms. This allowed the team to solve the 3D structure of the MHETase enzyme, giving them the molecular blueprints to begin engineering a faster enzyme system. "The new research combined structural, computational, biochemical and bioinformatics approaches to reveal molecular insights into its structure and how it functions," the findings state.