Some soil microbes, expert at recycling plants, have started to develop a taste for plastic. A few years back, while messing with one of these highly adapted organisms, researchers created a mutant enzyme by mistake, which can eat 20 percent more plastic than its natural match.
Two years later, the same team has again created something even more remarkable. Merging a newly discovered enzyme with the old variant, they have developed a new super mutant enzyme that breaks down PET in an effective manner.
The massive improvement in efficiency could hint to a possible avenue for future plastic recycling, even though currently, reducing plastic products is still the most efficient way to manage pollution.
Today, plastic waste has basically invaded every part of our planet, and PET, or polyethylene terephthalate, is the most common thermoplastic of them all, mainly used in water bottles and clothing. It takes a few centuries for this type of plastic to decompose fully, but even in the short time this element has existed on Earth, some microbes managed to get to consume it in a few days.
Back in 2016, the first of these organisms was found at a recycling plant in Japan and was called Ideonella sakaiensis. Throughout the years, studies have shown it develops a plastic-degrading enzyme known as PETase in order to break down PET water bottles.
Now, researchers have discovered a second enzyme, which they called MHETase. Together, the two elements create the ideal plastic-consuming pair. While PETase breaks down the surface of plastics, the newly-found enzyme consumes the product even further, until all that’s left are the basic building blocks. This offers the promise of effectively recycling the plastic completely.
“[I]t seemed natural to see if we could use them together, mimicking what happens in nature,” explains structural biologist John McGeehan, who has been part of the research at the University of Portsmouth.
The Perfect Mutant Enzyme Hybrid
Simply combining PETase with the new MHETase was sufficient to double the breakdown of PET plastic. However, when the researchers physically connected them, they worked even better.
With the powerful Diamond Light Source synchrotron in the U.K. as a source of powerful X-ray beams, McGeehan and his colleagues showed off the structure of the new enzyme via X-ray crystallography, which then enabled them to carefully attach the two, creating the perfect duo.
“It took a great deal of work on both sides of the Atlantic, but it was worth the effort,” says McGeehan. “[W]e 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.”
In nature, it is common for microbe-secreted enzymes to work together, breaking down cellulose, chitin, and other string cell structures.
“Given that natural microbial systems evolved over millions of years to optimally degrade recalcitrant polymers, perhaps it is thus not surprising, in hindsight, that a soil bacterium such as I. sakaiensis evolved the ability to utilize [..] a two-enzyme system,” the authors write.
The team concludes the paper, saying: “Going forward, the design of multienzyme systems for depolymerization of mixed polymer wastes is a promising and fruitful area for continued investigation.”
The study was published in the journal Proceedings of the National Academy of Sciences.