UK Researchers have determined the structure of a recently discovered enzyme that digests polyethylene terephthalate (PET), one of the most common plastics. This allowed them to engineer a mutant enzyme even better at breaking down PET.
In 2016, a group from Kyoto University isolated a new species of bacteria that uses PET plastics as its food source from a rubbish dump. The new study, by University of Portsmouth researchers, builds on this work by isolating the enzyme responsible – PETase – and solving its crystal structure. The team could then work out how the enzyme interacts with plastic molecules and breaks them down. Using that information, they were able to improve the enzyme’s properties by making two changes in its active site.
‘Although the improvement is modest, this unanticipated discovery suggests there is room to further improve these enzymes, moving us closer to a recycling solution for the ever-growing mountain of discarded plastics,’ comments John McGeehan of the University of Portsmouth, who collaborated with the US Department of Energy’s National Renewable Energy Laboratory. The researchers are now attempting to improve the enzyme further to allow it to be used in large-scale industrial recycling processes.
The team worked with researchers at the Diamond Light Source in Harwell – the UK’s national synchrotron facility – to generate an ultra-high-resolution 3D model of the enzyme (PNAS, 2018; doi: https://doi.org/10.1073/pnas.1718804115).
They found that the structure had features in common with other enzymes called cutinases and lipases, which belong to a family of hydrolases. PETase shared a fold, called the α/β-hydrolase fold, but had a more open ‘binding cleft’ in the active site. The team narrowed the cleft by mutating two portions of the active site to make it more like the cutinases.
However, they were surprised to observe that the mutated version was better at breaking down PET. This suggests that ‘PETase is not fully optimised for crystalline PET degradation, despite presumably evolving in a PET-rich environment,’ they write.
Significantly, the enzyme could also degrade polyethylene furandicarboxylate (PEF), a sugar-based substitute for PET plastics that is being hailed as a replacement for glass beer bottles.
‘This study represents an important step towards developing a bio-recycling method to deal with waste plastics,’ comments Mark Lorch, professor of chemistry at the University of Hull, UK. ‘However, there is still some way to go. The team feels that the enzyme can be made much more efficient still, so further engineering is in order. This represents just part of the solution.
The bacteria and its enzymes only deal with one class of plastic. So it may be that a cocktail of enzymes – many of which are yet to be discovered or engineered – is required to deal with plastic waste.’