Eight million metric tons of plastic waste, including polyethylene terephthalate (PET) bottles, enter the oceans each year, creating huge manmade islands of garbage. Experts estimate that by 2050, there will be as much waste plastic in the ocean by mass as there are fish. A bacterium, Ideonella sakaiensis 201-F6, can degrade plastic bottles but it doesn’t work fast enough to solve plastic recycling at the industrial scale.
An improved variant of the enzyme was discovered that can break down the bottles. PETase is an enzyme that digests PET. Experiments were conducted by taking samples of PET from soda bottles. After 96 hours via electron microscopy, it was evident that the PETase degraded PET. The experiments originally were conducted to determine how the enzyme evolved from breaking down cutin — the waxy substance on the surface of plants — with cutinase, to degrading synthetic PET with PETase.
PETase looks very similar to a cutinase, but it has some unusual surface features and a much more open active site. These differences indicated that PETase must have evolved in a PET-containing environment to enable the enzyme to degrade PET. To test that hypothesis, the researchers mutated the PETase active site to make it more like a cutinase. It was found that the PETase mutant outperforms the wild-type PETase in degrading PET. Understanding how PET binds in the PETase catalytic site using computational tools helped illuminate the reasons for the improved performance.
Another significant aspect of the research was the discovery that PETase can also degrade polyethylene furandicarboxylate (PEF), a bio-based substitute for PET plastics. The enhanced oxygen barrier properties of PEF could lead to its widespread use in bottles that could ultimately find their way into the environment, adding to the pollution problem.