Hydrolysis of the Urethane Bond Catalyzed by <i>Pseudomonas</i> sp. MIS38 Lipase: A QM/MM Mechanistic Insight

Plastic accumulation has become a major global concern due to the lack of efficient and environmentally friendly strategies to manage the end-of-life of these materials. Among the most used families of plastics is polyurethane (PU), which is valued for its versatility and low production cost. A promising strategy to address the end-of-life challenges of PU is employing efficient PU-degrading enzymes. Notably, an extracellular lipase from the I.3 family originating from Pseudomonas sp. MIS38 has shown significant promise in this regard. In this study, we investigated the enzyme’s capability to hydrolyze a PU fragment. Employing QM/MM computational methodologies, we studied the hydrolysis mechanism of MIS38 lipase and found it to follow the prototypical serine esterase mechanism involving acylation and deacylation stages. The rate-limiting step occurred between the formation of the acyl-enzyme intermediate and the formation of the second tetrahedral intermediate. The Gibbs activation barrier for this step was 19.67 kcal·mol–1, confirming the lipase’s potential to biodegrade PU efficiently. More importantly, we observed that the enzyme preferentially cleaved the C–N bond instead of the C–O bond. This preference was due to the arrangement of the active site and the substrate, which made C–N the more favorable cleavable site. Furthermore, we find that the C–O group is not a suitable cleavable bond due to steric hindrances. This study suggests that the mechanism of urethane bond hydrolysis is more complex than currently assumed because bond cleavage is context-dependent and may differ depending on the enzyme and the substrate.