Identification of Urethanases for Biocatalytic Recycling of Toluene Diisocyanate- and Methylene Diphenyl Diisocyanate-Based Polyurethanes

Abstract

In this study, the three urethanases TflABH, MthABH, and OspAmd, originating from two distinct enzyme superfamilies, are identified and characterized with respect to their potential in polyurethane (PUR) degradation. The substrate scope included five industrially relevant toluene diisocyanate- and methylene diphenyl diisocyanate-based carbamates with varied alcohol moieties, representative of intermediates from chemical PUR recycling. Notably, TflABH and MthABH are the first urethanases from an esterase superfamily shown to efficiently hydrolyze at least four of the five tested PU-related substrates. Among these, TflABH displayed exceptional thermostability, with a melting temperature (Tm) at least 12 °C higher than those of the other urethanases evaluated. Optimal reaction conditions are established for all three enzymes, revealing pH optima of 7.0 for MthABH, 8.0 for TflABH, and 9.5 for OspAmd, while temperature optima clustered closely around 56–60 °C. Importantly, OspAmd demonstrates greater catalytic efficiency in the hydrolysis of methylenedianiline-MeOH, achieving conversions up to 50% after 48 h, approximately threefold higher than benchmark enzymes. These findings highlight the potential of OspAmd, in particular, as a promising biocatalyst for the enzymatic recycling of polyurethanes.