Meet PETase!

In honor of Plastic Free July, allow me to introduce PETase!

PETase is an enzyme that eats plastic. Specifically, it dines on polyethylene terephthalate, the kind of plastic that most “disposable” water bottles are made of.

Three dimensional structure model of PETase. A cartoon plastic water bottle is shown going in, and several small shapes are coming out.
from PDB 5XJH

PETase was recently discovered (in 2016) in species of bacteria called Ideonella sakaiensis. It’s believed that this enzyme evolved from one that breaks down waxy coatings on plants in response to an abundance of yummy delicious plastic near a recycling facility. Plant waxes and PET have similar types of chemical linkages between their parts, so they can both be broken down with a similar strategy.

Left panel shows the chemical structure of PET, with scissors "cutting" a bond after an ester group (a carbon with a single bond to one oxygen and a double bond to another oxygen).
Right panel shows the chemical structure of a plant wax, and again scissors are "cutting" after an ester group.

As soon as PETase was discovered, scientists knew that it could become a useful tool to clean up the overwhelming amount of plastic littering our planet. They got to work learning everything they could about PETase’s sequence of amino acids (protein pieces) and its structure, down to each atom. Armed with this information, they started engineering their own versions of PETase to create one that had a “larger appetite.”

Cartoon man walking represents the native PETase. Cartoon man running represents a "remodeled" PETase that is the product of protein engineering.

PETase digests PET into molecules called MHET. Now it may be handy for us that PET is broken down, but bacteria have no reason to eat something unless they can use it. Unfortunately, MHET is not all that useful for a bacteria to do much of anything with. Luckily for the bacteria, PETase does not work alone! PETase has a co-worker: MHETase.

As its name implies, MHETase breaks down MHET and the resulting molecules are ethylene glycol and terephthalic acid. These are molecules that bacteria can use. In fact, with PETase and MHETase working together, these incredible bacteria can fully rely on plastic water bottles as their only source of carbon!

Scientists have found that linking their hungry PETase with MHETase makes a “super-enzyme” that eats up plastic even faster.

Diagram showing the breakdown of PET, represented as a chain of alternating X's and O's (XOXOXO). PETase breaks down XOXOXO into single XO pieces (called MHET). MHETase breaks down those XO's into single X's and O's: ethylene glycol molecules and terephthalic acid molecules.
PETase: PDB 5XJH; MHETase: PDB 6QGA

It is likely that, given some more time, the bacteria Ideonella sakaiensis and others will continue to evolve to consume human trash as their food. But if you’ve ever seen the Great Pacific Garbage Patch or even the mounds of plastic your own household throws away in just a week, you know that getting a handle on our plastic waste is an urgent matter. There is a lot of work to be done, but thanks to these scientists (and don’t forget the ones who laid their groundwork, developed their tools, and trained them in research) we are making progress.

Austin, H. P., Allen, M. D., Donohoe, B. S., Rorrer, N. A., Kearns, F. L., Silveira, R. L., Pollard, B. C., Dominick, G., Duman, R., Omari, K. El, Mykhaylyk, V., Wagner, A., Michener, W. E., Amore, A., Skaf, M. S., Crowley, M. F., Thorne, A. W., Johnson, C. W., Woodcock, H. L., … Beckham, G. T. (2018). Characterization and engineering of a plastic-degrading aromatic polyesterase. Proceedings of the National Academy of Sciences, 115(19), E4350–E4357. https://doi.org/10.1073/PNAS.1718804115

Knott, B. C., Erickson, E., Allen, M. D., Gado, J. E., Graham, R., Kearns, F. L., Pardo, I., Topuzlu, E., Anderson, J. J., Austin, H. P., Dominick, G., Johnson, C. W., Rorrer, N. A., Szostkiewicz, C. J., Copié, V., Payne, C. M., Woodcock, H. L., Donohoe, B. S., Beckham, G. T., & McGeehan, J. E. (2020). Characterization and engineering of a two-enzyme system for plastics depolymerization. Proceedings of the National Academy of Sciences, 117(41), 25476–25485. https://doi.org/10.1073/PNAS.2006753117

Yoshida, S., Hiraga, K., Takehana, T., Taniguchi, I., Yamaji, H., Maeda, Y., Toyohara, K., Miyamoto, K., Kimura, Y., & Oda, K. (2016). A bacterium that degrades and assimilates poly(ethylene terephthalate). Science, 351(6278), 1196–1199. https://doi.org/10.1126/SCIENCE.AAD6359

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