359 million tons of plastics are produced every year in the world, of which 150-200 million tons are piled up in waste dumps or exposed to nature.
And polyethylene terephthalate (PET) plastic is the most abundant, with a global annual output of nearly 70 million tons - it is the plastic bottle that we may touch every day.
Recycling of plastic bottles plays an important role in environmental protection.
This is what nature's latest cover article focuses on.
However, the current recycling technology will either destroy the mechanical properties of pet, or the enzymatic hydrolysis efficiency is too low.
Recently, a team from the University of Toulouse in France proposed a new enzyme: it can hydrolyze 90% of plastic bottles of pet in 10 hours. Relevant papers were published in nature under the title of "an engineered pet depolymerase to break down and recycle plastic bots". On the same day, science magazine reported that "a tiger step forward." mutant enzyme could vastly improve recycling of plastic bots ".
This is higher than the efficiency of all previous PET hydrolases. More importantly, the monomer produced after hydrolysis has the same characteristics as the monomer in petrochemical materials.
This greatly promotes the reuse of plastic bottles.
To hydrolyze pet, understand it first.
Pet can be solidified into two forms: one is the tightly packed crystal form, the other is the relatively loose and disordered form.
These two pet forms are found in most plastic bottles. Manufacturers will adjust their proportions according to the material characteristics of plastic bottles.
However, the structure of crystalline pet is too stable to be digested by the most effective enzyme.
You may ask, high temperature does not make crystalline pet loose and disordered?
But the enzyme has "high temperature intolerance". When the temperature rises, the enzyme will be inactivated.
So how to cross the "two mountains" and make enzyme hydrolyze pet better?
First, there are structural problems.
The researchers looked at the structure of cutinase, an α / β hydrolase, and conducted chemical simulations to find out where pet interacted with the enzyme: they found that pet matched the "grooves" on the enzyme surface (including where pet was cut).
To improve the fit between pet and the groove, the researchers created a large number of mutant versions of the enzyme, which changed each amino acid inside the groove through different combinations.
Next, the enzyme is not resistant to high temperature.
After studying the related enzymes, there seems to be a clue: many enzymes are stable by interacting with a metal ion, which fixes the two parts of the enzyme together.
So, starting with the original version of the enzyme, researchers engineered two amino acids to form a chemical bond between the two parts. This version of the enzyme is more stable at high temperatures.
Finally, the researchers created two versions of the enzyme through a series of improvements, and carried out experiments on chopped plastic bottles.
Not only high hydrolysis efficiency, but also very cheap
The efficiency of the improved hydrolase is very high.
The hydrolase proposed by predecessors can digest half of pet in 20 hours. In this work, the digestion rate can reach 85% in only 15 hours.
Moreover, by optimizing the conditions, they were able to decompose pet to 90% in 10 hours.
Although some crystalline pet remains, the efficiency is already very high: 863kg of raw materials can be extracted from 1000kg of pet waste.
In other words, the redesigned enzyme is more efficient than our body's ability to break down amylase.
The researchers then used these materials to make new PET products to industrial standards.
The pressure resistance of these new products is only 5% lower than that of PET produced with standard chemical materials, and the appearance is only 10% lower, which conforms to the PET production standard.
Of course, cost is also an important consideration.
What is the cost of using recycled PET compared with petrochemical raw materials?
The researchers estimate that if the cost of making the protein is $25 per kilogram, the cost of the process would be 4% of the cost of making pet from the protein.
Although it may not be as cheap as petrochemicals, it will be relatively unaffected by future price shocks and more sustainable.
One More Thing
The enzyme cannot recover other major types of plastics, such as polyethylene and polystyrene, whose bonds are more difficult to break. But if successful, it can help society solve one of the most challenging plastic problems we face.
