Urethanases for the Enzymatic Hydrolysis of Low Molecular Weight Carbamates and the Recycling of Polyurethanes

Abstract

Enzymatic degradation and recycling can reduce the environmental impact of plastics. Despite decades of research, no enzymes for the efficient hydrolysis of polyurethanes have been reported. Whereas the hydrolysis of the ester bonds in polyester-polyurethanes by cutinases is known, the urethane bonds in polyether-polyurethanes have remained inaccessible to biocatalytic hydrolysis. Here we report the discovery of urethanases from a metagenome library constructed from soil that had been exposed to polyurethane waste for many years. We then demonstrate the use of a urethanase in a chemoenzymatic process for polyurethane foam recycling. The urethanase hydrolyses low molecular weight dicarbamates resulting from chemical glycolysis of polyether-polyurethane foam, making this strategy broadly applicable to diverse polyether-polyurethane wastes.

From green to circular chemistry paved by biocatalysis

“…In this regard, Branson et al developed a recycling procedure consisting of glycolysis followed by enzymatic hydrolysis, allowing both the polyether polyols and the aromatic diamines to be recovered from polyether-polyurethane foams. 128 The process consists of the glycolysis of the polymer at 200 °C using an excess of diethylene glycol (DEG) containing 1% (w/w) of tin( ii )-2-ethylhexanoate as a catalyst. The glycolysis is followed by the enzymatic hydrolysis of the resulting low molecular weight dicarbamate by a metagenome-derived urethanase releasing the glycol (DEG), carbon dioxide, and the aromatic diamine (TDA) and opening this strategy to broadly diverse polyether-polyurethane wastes.…”

Microbial Enzyme Biotechnology to Reach Plastic Waste Circularity: Current Status, Problems and Perspectives

“…A laccase-mediated (EC 1.10.3.2) system previously reported to degrade PE caused a reduction of a few percentage points in weight of lab synthesized polyether-type PURs in 18 days [ 164 ]. Urethanases (EC 3.5.1.75), which are enzymes capable of specifically hydrolyzing the urethane bond between polyols and isocyanate units, were recently discovered [ 165 ]; although their use for full depolymerization of PURs within a few days is very promising, these enzymes were used and demonstrated to be effective only on the urethane bonds in low-molecular-weight dicarbamate aromatic diamines obtained after the chemical depolymerization (by glycolysis) of non-urethane bonds in a polyether PUR foam. Therefore, this process is bio-based only in the last step.…”

“…Amines such as toluene diamine (TDA) can be recovered and used to directly synthesize polyamides or toluene isocyanate (TDI) to subsequently make virgin PURs (bio-based closed-loop recycling). Although the full depolymerization of a polyether-PUR foam was demonstrated with a chemoenzymatic approach [ 165 ], its application on a real waste PUR material and the use of the degradation products (polyether-polyols, diethylene glycol, and aromatic diamines) for the resynthesis of a waste-like PUR material was never attempted.…”

“…A synergy with promising methods of abiotic secondary and tertiary PUR depolymerization approaches could allow for depolymerization of recalcitrant PUR waste, but no method has been tested on real PUR waste to date [ 49 , 165 , 168 , 194 , 195 ].…”

Validated High-Throughput Screening System for Directed Evolution of Nylon-Depolymerizing Enzymes

“…PA and PUR are chemically related through their nitrogen-containing backbones and liberate primary amines upon hydrolysis. To date, most reported PUR-degrading enzymes (urethanases, amidases, and esterases) only hydrolyze soluble carbamates or polyester-PUR but not polycarbamates. − Considering PA, only two enzymes are known to significantly degrade the polymer, i.e., PA 6 and PA 6,6, the white rot fungus manganese peroxidase (MnP) and the N-terminal threonine hydrolase NylC. − Among depolymerases, NylC represents the only well-characterized hydrolase showing activity on short oligomers and polymeric PA 6 and −6,6. , Moreover, NylC has already been rationally engineered to greatly improve its thermal stability . However, reported enzymes are not suitable for the efficient recycling of polymeric plastic waste due to their low activity .…”