At the rate plastic bottles and the like are entering the world’s oceans, beachgoers may need to start wading long before they reach the water’s edge. And the plastic tide has yet to turn, despite the development of mechanical and chemical recycling technologies. Unless a new kind of recycling technology is developed, natural scenery may disappear beneath waves of human-made polymers, and wildlife may be left bobbing in civilization’s synthetic wake.
Curiously, the best kind of recycling technology may be one that simply negates one of plastic’s greatest advantages: its durability. Such a technology may soon be at hand, now that scientists based at U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and the U.K.’s University of Portsmouth have developed an improved version of a natural plastics-eating enzyme.
Details of the scientists’ work appeared April 17 in the Proceedings of the National Academy of Sciences (PNAS), in an article entitled “Characterization and Engineering of a Plastic-Degrading Aromatic Polyesterase.” This article describes how a plastics-nibbling enzyme from the bacterium Ideonella sakaiensis 201-F6 might be altered to give it a heartier appetite for polyethylene terephthalate (PET), one of the most abundantly produced synthetic polymers.
“[We] have characterized the 3D structure of a newly discovered enzyme that can digest highly crystalline PET, the primary material used in the manufacture of single-use plastic beverage bottles, in some clothing, and in carpets,” wrote the article’s authors. “We engineer this enzyme for improved PET degradation capacity and further demonstrate that it can also degrade an important PET replacement, polyethylene-2,5-furandicarboxylate, providing new opportunities for biobased plastics recycling.”
Initially, the scientists had nothing more in mind than gathering 3D information about the PET-degrading enzyme, the PETase, so that they could better understand how it works. During their study, however, the scientists inadvertently engineered an enzyme that is even better at degrading the plastic than the one that evolved in nature.
The 3D structural work was straightforward enough. The NREL and the University of Portsmouth collaborated closely with a multidisciplinary research team at the Diamond Light Source in the U.K., a large synchrotron that uses intense beams of X rays to probe matter at the atomic scale. Using the Diamond Light Source’s beamline I23, the scientists generated an exquisitely detailed ultra-high-resolution 3D model of the PETase enzyme.
With help from the computational modeling scientists at the University of South Florida and the University of Campinas in Brazil, the NREL/Portsmouth team discovered that while PETase looks very similar to a cutinase, it also 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.
And this is where the unexpected happened.
“By narrowing the binding cleft via mutation of two active-site residues to conserved amino acids in cutinases, we surprisingly observe improved PET degradation, suggesting that PETase is not fully optimized for crystalline PET degradation, despite presumably evolving in a PET-rich environment,” the scientists explained. “Additionally, we show that PETase degrades another semiaromatic polyester, polyethylene-2,5-furandicarboxylate (PEF), which is an emerging, bioderived PET replacement with improved barrier properties.”
I. sakaiensis 201-F6, which was discovered in the soil of a Japanese PET bottle recycling plant more than a year ago, was found to produce the original PETase, but this enzyme doesn’t work fast enough to solve plastic recycling at the industrial scale. The problem is that PET has a highly crystalline structure, one that resists degradation. Although PET can be recycled, most of it is not. PET that is recycled often exhibits inferior material properties as well. In addition, PEF plastics, although bio-based, are not biodegradable, and would still end up as waste in landfills and in the seas.
In the current study, the scientists demonstrated that PETase is active on PET of ∼15% crystallinity. “[While] this observation is encouraging,” the PNAS article’s authors noted, “it is envisaged that its performance would need to be enhanced substantially, perhaps via further active-site cleft engineering similar to ongoing work on thermophilic cutinases and lipases.”
Yes, the improvement is modest, but the discovery that PETase has room for improvement is encouraging. It suggests that additional protein engineering could produce enzymes capable of digesting plastics that would otherwise last for centuries.
“The engineering process is much the same as for enzymes currently being used in bio-washing detergents and in the manufacture of biofuels,” observed John McGeehan, Ph.D., a coauthor of the PNAS paper and director of the Institute of Biological and Biomedical Sciences in the School of Biological Sciences at Portsmouth. “The technology exists, and it's possible that in the coming years, we will see an industrially viable process to turn PET and potentially other substrates like polyethylene furanoate (PEF), polylactic acid (PLA), and polybutylene succinate (PBS), back into their original building blocks so that they can be sustainably recycled.”