Engineered compost enzyme degrades PU shoe foam

The modified enzyme degraded around 1.4% of the polyurethane in pieces of a commercial shoe sole over three days. Although the conversion remains far below that required for industrial recycling, the researchers said the experiment demonstrated that enzymatic degradation of real PU foam is possible without first chemically or mechanically treating the material.

The work was conducted by researchers at Aarhus University in Denmark and the University of Porto in Portugal and published in the journal Chem Catalysis.
The enzyme, named CCPUR1, is produced by Chelatococcus composti, a bacterium isolated from PU-enriched compost in Denmark. Researchers had previously identified it during a wider search for naturally occurring enzymes capable of breaking bonds found in polyurethane and polyamide.

That study examined bacteria collected from a landfill in Kenya, a tropical zoo, compost and the digestive systems of wax-moth larvae. Fluorescent molecules designed to mimic chemical bonds in PU and nylon were used to identify microorganisms with potentially useful enzymes.

Fluorescence-activated cell sorting enabled the researchers to screen bacteria at rates of up to 10,000 cells/s. They identified 29 plastic-degrading bacterial species and subsequently isolated ten enzymes with activity against polyurethane and two active against nylon.

CCPUR1 emerged as the most promising candidate. It has a comparatively wide binding cleft capable of accommodating large polymer molecules and a measured melting temperature of 68.3°C. The enzyme also showed greater resistance to heat and solvents than the other candidates.

For the latest study, the researchers used molecular-dynamics simulations to examine how CCPUR1 interacted with PU-like molecules. They then modified individual amino acids in the enzyme in an attempt to improve the fit between its active site and the polymer.

One of the resulting variants was able to depolymerize approximately 1.4% of the untreated shoe foam during the three-day experiment.

“This is an early demonstration that shows that we can degrade polyurethane with enzymes without processing it beforehand,” said co-first author Pedro Paiva of the University of Porto.

Enzymatic recycling could potentially operate at lower temperatures and pressures than many chemical recycling processes. The selectivity of enzymes could also allow different polymers to be separated sequentially from mixed-material waste.

However, the researchers acknowledged that the degradation rate must be increased substantially before the technology could be used commercially. Enzymes might ultimately be combined with mechanical or chemical processes that make the PU more accessible.

The Chem Catalysis paper is titled “Molecular-dynamics-guided engineering of a thermostable polyurethane hydrolase for hydrolysis of commercial polyurethane shoe foam”.

Photo: an AI-generated image of a running shoe biodegrading in compost prompted by PU-Magazine

Chem Catalysis