In a study published in Bioresource Technology Journal, scientists from the universities of Portsmouth and Manchester report that a specially engineered enzyme can significantly speed up the breakdown of PET - the plastic used in water bottles, food packaging and polyester clothing - when it is processed at high concentrations similar to those used in industry.
PET, short for poly (ethylene terephthalate), is cheap, durable, and widely used. But those same qualities mean it builds up in vast quantities once thrown away.
Polyester textiles are notoriously difficult to recycle. Their fibres are tightly packed and highly ordered into a structure created during manufacturing, which makes them resistant to biological breakdown.
Enzymes are natural proteins that can speed up chemical reactions. The team combined two different components into one fusion enzyme. The first was a heat-tolerant cutinase; a natural enzyme that normally breaks down a protective polyester found on plant surfaces called cutin. The second was a binding module designed to help the enzyme to attach more tightly to plastic.
The two components were carefully matched, so they work best at the same temperature and are suited to the same kinds of plastic structure. The aim was to make the enzyme stick to PET and ensure it could continue breaking it down efficiently under realistic recycling conditions.
While the modified enzyme did attach more strongly to highly crystalline PET - the tough, tightly packed form found in many plastics - did not automatically lead to faster breakdown. In fact, when the plastic structure remained highly ordered, there was limited gain.
The real progress came when the plastic was less crystalline and as a result more accessible to the enzyme. Under controlled conditions that mimic industrial recycling - including carefully managed pH and plastic concentrations of 20 per cent by weight - the fused enzyme broke down less-ordered PET much more quickly.
The biggest improvement was seen in a pre-consumer polyester textile that had been specially treated to make it less crystalline and finely ground. In that case, the amount of useful breakdown products doubled.
“By matching the enzyme with the right binding module and preparing the plastic in the right way, we can overcome a major bottleneck in plastic recycling,” said Professor Andrew Pickford, Director of the University of Portsmouth’s Centre for Enzyme Innovation (CEI). “This isn’t just about helping the enzyme stick to the surface - it’s about making sure the chemical reaction can run efficiently at the high plastic concentrations used in industry.”
The findings also help explain why earlier studies of similar enzyme combinations have produced mixed results. If an enzyme binds too tightly to the surface, it can slow the reaction - a well-established concept in chemistry known as the Sabatier principle.
The study suggests that enzyme-based recycling of PET - a promising but technically challenging solution - could become more practical at scale but success, depends on getting three factors right: the enzyme, any helper module that guides it to the plastic, and the structure of the material itself.