Updated April 20, 2018: Researchers in the United States and England have engineered an enzyme that could degrade scrap PET bottles and turn them into the original raw materials.
The breakthrough is the latest in a series of tantalizing research results hinting that certain enzymes and microbes might pave a way to degrade mountains of plastics scrap.
Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory and the University of Portsmouth say they have tweaked a bacterium's enzyme to improve its ability to degrade PET. The natural enzyme is in a bacterium called Ideonella sakaiensis, which researchers recently found was degrading PET in a Japanese waste recycling center.
Millions of tons of PET bottles are made each year. Collecting and recycling them is widespread, but they still pose a big litter problem. PET can last hundreds of years in a natural environment. Researchers say the enzyme could degrade PET in a matter of days.
“This represents huge potential for recycling,” said one of the researchers, H. Lee Woodcock, associate professor at the University of South Florida, in an April 18 phone interview.
“I envision a many fold improvement in the enzyme,” Woodcock told Plastics News.
Woodcock and his group at USF helped figure out the structure and degrading activity of the enzyme. In general, his group focuses on modeling chemical systems, especially biochemical systems.
“We can all play a significant part in dealing with the plastic problem, but the scientific community who ultimately created these ‘wonder materials' must now use all the technology at their disposal to develop real solutions,” explained one of the lead researchers, John McGeehan, in an April 16 news release. McGeehan is director of the Institute of Biological and Biomedical Sciences in the School of Biological Sciences at University of Portsmouth in England.
McGeehan and colleagues were examining the structure of the natural, PET-degrading enzyme when they found they could increase the degradation rate by manipulating the chemical structure. The improvement was modest, but the scientists believe bigger improvements are possible by modifying the protein portion of the enzyme.
“The potential of this to turn into a recycling solution, the use of an enzyme like PETase and hopefully a much-improved mutant of PETase in the future, could enable the circular economy for PET plastic,” said Gregg Beckham, senior research fellow and group leader at the National Renewable Energy Laboratory of the U.S. Department of Energy. Beckham's group cloned the enzyme and tested its ability to degrade PET.
Researchers found the natural and improved enzymes first act by slipping into the folds of the PET long-chain molecule. Degradation begins there when the polymer chain is cut in two by the enzyme. Degradation then proceeds to the termini of the severed polymer chain.
Enzyme-treated PET degrades into its monomers terephthalic acid and ethylene glycol and related chemicals, researchers discovered. Theoretically, the monomers could be reused to make PET, thus offering a closed-loop system of manufacturing and recycling.
The U.S./United Kingdom authors think such enzyme degradation could be suitable to a broad range of polyesters other than PET. They found the enzyme works on poly(ethylene furanoate), a relatively new bio derived polymer attracting interest for its gas barrier properties that could make it attractive for beer containers. With more tweaking, the enzyme possibly could degrade biopolymers such as polylactic acid and polyhydroxyalkanoates. In the future, enzymes might be found to degrade other plastics.
Independent of the U.S. DOE and U.K. work, researchers at the Korea Advanced Institute of Science and Technology were studying the natural enzyme's structure and they, too, claimed they found a way to improve its PET degradation potential. The U.S. and U.K. results were published in the peer-reviewed Proceedings of the National Academy of Sciences journal and then announced in a University of Portsmouth news release. The South Korean research was published in the peer-reviewed Nature Communications and in a subsequent news release from KAIST.
Special analytical techniques by the U.S. and U.K. researchers were key to figuring out the enzyme's 3D structure and thus its mechanism to degrade the PET polymer chain. Advanced X-ray crystallography allowed researchers to deduce the enzyme's structure. The technique can have a resolution of less than an Angstrom unit, putting it into the microscopic world of nanotechnology where it can discern individual atoms.
