Biocatalysis and Metabolic Engineering

Improving Nature’s Tools for Digesting Plastics

Floating empty plastic water bottles

Enzymes found in nature can break down certain plastics, but not well enough to support industrial recycling and stem the scourge of plastic waste. Building on what nature has provided, researchers at Rensselaer have improved the efficiency of a leaf and branch compost cutinase that breaks down polyethylene terephthalate (PET), the plastic used in clear and colored plastic water bottles and many other products. Researchers believe the enzyme can be further refined, offering a promising candidate to fuel limitless recycling of PET and possibly other plastics such as cellulose acetate.

In work published in the journal Biochemistry, the researchers used yeast cells to express the leaf and branch compost cutinase (LCC) modified by the addition of sugar molecules — or glycans — in two locations. The “glycosylated” modified enzyme retained at least half of its activity after 48 hours at 75 degrees Celsius, versus a previously reported half-life of 40 minutes for the unmodified enzyme at 70 degrees Celsius.

“We need plastics and other materials that retain good performance and, after use, can then be broken down by safe and mild processes to their original building blocks for reuse,” says Richard Gross, Constellation Professor of Biocatalysis and Metabolic Engineering and lead author of the research. “The goal should be zero waste and to do that, we have to build reuse into the design of a wide range of polymers and materials. This is an encouraging step toward that goal.”

With existing technologies, a plastic bottle isn’t so much recycled as down-cycled. After a single use, a high percentage of PET bottles go directly to landfills or are reused as other plastics such as PET fibers and fleece for clothes, carpets, bags, furniture, and packing materials. Eventually, down-cycled PET makes its way to landfills or other undesirable environments such as oceans and lakes, a fate many consumers are unaware of as they toss their water bottles in a recycling bin.

Breaking PET down into its building blocks would enable the limitless reuse more commonly associated with other recyclable materials such as glass and metal. Some naturally occurring enzymes can break down PET, but not within the constraints of time and temperature required by an industrial recycling process. Many enzymes lose their activity at higher temperatures, and eventually denature. An enzyme suitable for industrial recycling must be able to operate at optimal temperature for breaking down PET, which is about 75 degree Celsius, and it must retain its activity long enough to do its job cost-effectively at that temperature.

“This cutinase is an excellent candidate for commercialization, but this work will also help us redesign other cutinases to break down other polymers, and that’s a much larger end game,” says Gross.