Reductive amination cascades in cell-free and resting whole cell formats for valorization of lignin deconstruction products

Abstract

The selective introduction of amine groups within deconstruction products of lignin could provide an avenue for valorizing waste biomass while achieving a green synthesis of industrially relevant building blocks from sustainable sources. Here, we built and characterized enzyme cascades that create aldehydes and subsequently primary amines from diverse lignin-derived carboxylic acids using a carboxylic acid reductase (CAR) and an ω-transaminase (TA). We report a detailed characterization of two reaction formats - cell-free biocatalysis and resting whole-cell biocatalysis - using the conversion of vanillate to vanillyl amine as model chemistry. We showed that resting whole-cell reactions could be competitive with cell-free reactions (97% and 70% yields, respectively) while avoiding steps of cell lysis, purification, and cofactor supplementation. We also demonstrated the recyclability of resting whole cells for multiple batches of catalysis without significant loss in productivity. Finally, we used the knowledge gained from this study to produce several amines from carboxylic acid precursors using one-pot biocatalytic reactions. These results expand our knowledge of these industrially relevant enzyme families to new substrates and contexts for environmentally friendly and potentially low-cost synthesis of diverse aryl aldehydes and amines.