Plastic waste is a huge pollutant of terrestrial and marine environments. Photo/Credit: Magnus Larsson/iStock
The invention of plastic some 100 years ago revolutionised many aspects of our day-to-day lives. From the buildings that we live in, to the cars that we drive, to the medical devices that keep us healthy: Modern life would be nigh on unimaginable without this ubiquitous, stable and versatile polymer. No Nobel Prize was explicitly awarded for the invention of plastic, but Hermann Staudinger, dubbed the ‘Father of Polymer Science’ made huge contributions to our understanding of polymer structures which paved the way for the development of plastic. He was awarded the 1953 Nobel Prize in Chemistry. The 1963 Nobel Prize in Chemistry was shared by Karl Ziegler and Giulio Natta for advances in polymer synthesis and in 2000, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa shared the Nobel Prize in Chemistry for discovering that polymers can conduct electricity.
The Dark Side of Plastic
For all its advantages, it is the dark side of plastic that is now increasingly to the fore. Plastic’s high stability means that the material is highly resistant to degradation and persists for long periods of time in both terrestrial and marine environments where it wreaks havoc on natural ecosystems. About eight million tonnes of plastic alone make it into our oceans each year. Of course, one strategy to deal with the problem of plastic’s long life is to recycle it. However, the efficacy of plastic recycling remains controversial: As of two years ago, it was estimated that less than 10 percent of all the plastic ever produced has actually been repurposed and rather than solving their plastic waste problem at home, some countries simply ship it out to other countries who have to deal with it. A recent Greenpeace investigation found mountains of plastic waste from Britain in Malaysia. Obviously, this is no good, but what if there was a better way of making plastic disappear, quite literally?
Generally speaking, plastic is degraded in the environment in a step-wise manner: First, ultraviolet light from the sun provides the energy that allows oxygen atoms to be incorporated into the structure of the plastic; this acts to destabilise the polymers, which gradually break down into smaller and smaller pieces that can finally be metabolized by microorganisms using enzymes. However, this whole process is inefficient and can take up to 50 years. It turns out that nature may also have evolved a better and faster solution.
A Molecular Scissor to Degrade Plastic
In 2016, Japanese researchers isolated bacteria from outside a bottle-recycling facility, i.e. bacteria that were living in the midst of plastic waste. Amazingly, they found that a species which they described for the first time and which they termed Ideonella sakaiensis, had evolved to use polyethylene terephthalate (PET), a molecule which is one of the main polymers used in plastic packaging such as bottles, as a food source and to break it down into benign side-products. Further studies from researchers based in the UK, US and Brazil two years later showed that the enzyme used by this bacterium to degrade plastic could be optimized to work even faster in breaking down PET. Further, a very recent study showed that an enzyme originally isolated from compost could act as a ‘molecular scissors’ that degrades PET into more simple molecules. Through targeted protein mutation, the scientists could engineer a protein that degraded 90 percent or more of PET within 10 hours – a rate that is 100 hundred times higher than what had been previously achieved using plastic-degrading enzymes. If that was not impressive enough, the scientists’ results suggest that it may be possible to use their enzyme to convert already used plastic to produce new plastic products, in this way closing the loop of plastic production and recycling and they are now attempting to establish an industrial-scale set-up using their enzyme.
Proteins evolve on diverse timescales, and the activity of some proteins change only very slowly. The fact that scientists have already succeeded in improving the activity of the plastic-degrading enzymes from Ideonella sakaiensis and from compost suggests that these proteins could be tweaked further to become yet more efficient. Directed evolution is an approach to protein engineering in which scientists take charge of the evolutionary process to repeatedly alter (mutate) and then screen proteins (a kind of natural selection) to identify new variants that possess the attributes that are desired. Frances H. Arnold was awarded one half of the 2018 Nobel Prize in Chemistry for her seminal research in this area and gave a talk about her work during the Online Science Days 2020. Directed evolution has already contributed to environmental protection by increasing the efficiency of enzymatic reactions in industrial settings. It is exciting to speculate that this approach could also contribute to translating the promise of plastic-degrading enzymes into practice.