Natural diversity screening, assay development, and characterization of nylon-6 enzymatic depolymerization

Abstract

Successes in biocatalytic polyester recycling have raised the possibility of deconstructing alternative polymers enzymatically, with polyamide (PA) being a logical target due to the array of amide-cleaving enzymes present in nature. Here, we screen 40 potential natural and engineered nylon-hydrolyzing enzymes (nylonases), using mass spectrometry to quantify eight compounds resulting from enzymatic nylon-6 (PA6) hydrolysis. Comparative time-course reactions incubated at 40-70 °C showcase enzyme-dependent variations in product distributions and extent of PA6 film depolymerization, with significant nylon deconstruction activity appearing rare. The most active nylonase, a NylC_K variant we rationally thermostabilized (an N-terminal nucleophile (Ntn) hydrolase, NylC_K-TS, T_m = 87.4 °C, 16.4 °C higher than the wild-type), hydrolyzes 0.67 wt% of a PA6 film. Reactions fail to restart after fresh enzyme addition, indicating that substrate-based limitations, such as restricted enzyme access to hydrolysable bonds, prohibit more extensive deconstruction. Overall, this study expands our understanding of nylonase activity distribution, indicates that Ntn hydrolases may have the greatest potential for further development, and identifies key targets for progressing PA6 enzymatic depolymerization, including improving enzyme activity, product selectivity, and enhancing polymer accessibility.

Polyamides (PAs) or nylons are types of plastics with wide applications, but due to their accumulation in the environment, strategies for their deconstruction are of interest. Here, the authors screen 40 potential nylon-hydrolyzing enzymes (nylonases) using a mass spectrometry-based approach and identify a thermostabilized N-terminal nucleophile hydrolase as the most promising for further development, as well as crucial targets for progressing PA6 enzymatic depolymerization.