Enzyme engineering for eating plastic (paper review)
One of the most common biotech applications I get asked about is "plastic degradation". We all know that huge buildups of trash, a lot of it plastic, cause all sorts of problems all over the world. And we know that the reason plastic holds up so well is exactly the same reason we value it: it's an incredibly resilient material.
Plastics of all kinds—and there are many different kinds of plastic—share the property that they are synthetic polymers, and not much in nature has evolved to eat them. There are dozens of types of plastic in common use, each made from a different monomer with a different chemical linkage. Some plastics, like PET (polyethylene terephthalate), contain chemical linkages like those found in natural systems.
PET monomers are linked with ester bonds, identical to those found in many biological molecules, such as phospholipids, hormones, and neurotransmitters. There are many different kinds of enzyme that make and break ester bonds in nature, and any one of them could potentially be used to break PET plastic down by breaking the ester bonds. The wide variety of functional molecules acted upon by natural enzymes, however, has no evolutionary reason to include plastics.
Another problem with enzymes eating plastic is that enzymes are small Pacman-shaped protein blobs that are maybe 10 nanometers in diameter, whereas things made of plastic like bottles or even microplastics are huge in comparison. How do you get the little Pacman jaws around the bottle to start breaking it down?
A recent research paper by Ana Robles-MartĂn and coauthors, "Sub-micro- and nano-sized polyethylene terephthalate deconstruction with engineered protein nanopores", describes how they created a protein where one end is a pore-forming shape, and the other end is a PET cutting (called a PETase in the jargon of the field). This way, their protein can access nooks and crannies in the macroplastic shapes, allowing tons of copies of this small enzyme to fully degrade a bottle.
Without this, a great deal of physical agitation is required to break down the plastics into small enough chunks that earlier Pacman enzymes could work on, increasing the time and the cost.
I hope we’ll see the idea of linking the enzymatic “scissors” to a protein pore be used to engineer enzymes to degrade other types of plastics in the future, as the general idea of getting the catalytic machinery into physical contact with every bit of the bottle is broadly applicable to all plastics, not just PET (which is great news).