Structural and Functional Characterization of an Amidase Targeting a Polyurethane for Sustainable Recycling

Abstract

Global plastic production exceeded 400 million tons in 2022, urgently demanding improved waste management and recycling strategies for a circular plastic economy. While the enzymatic hydrolysis of polyethylene terephthalate (PET) has become feasible on industrial scales, efficient enzymes targeting other hydrolyzable plastic types, such as polyurethanes (PURs), are lacking. Recently, enzymes of the amidase signature (AS) family, capable of cleaving urethane bonds in a polyether-PUR analog and a linear polyester-PUR, have been identified. Herein, we present high-resolution crystal structures of the AS enzyme UMG-SP3 in three states: ligand-free, bound with a suicidal inhibitor mimicking the transition state, and bound with a monomeric PUR degradation product. Besides revealing the conserved core and catalytic triad akin to other AS family members, the UMG-SP3 structures show remarkable flexibility of loop regions. Particularly, Arg209 in loop 3 adopts two induced-fit conformations upon ligand binding. Through structure-guided kinetic studies and enzyme engineering, we mapped structural key elements that determine the enhanced hydrolysis of urethane and amide bonds in various small molecules, including a linear PUR fragment analog. Our findings contribute critical insights into urethanase activity, aiding PUR degradation campaigns and sustainable plastic recycling efforts in the future.

Keywords: amidase; crystal structure; enzyme catalysis; protein engineering; thermoset plastic.