Reductive amination cascades in cell?free and resting whole cell formats for valorization of lignin deconstruction products
Amine containing side chains can confer useful properties to polymers but are not naturally found in lignin monomers. Furthermore, their selective installation can be challenging using traditional synthetic strategies. Here, we propose a sustainable process for synthesizing value?added aryl amine monomers from lignin?derived carboxylates, using carboxylic acid reductase/??transaminase enzymatic functionalization for eventual synthesis of materials.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 ??transaminase (TA). Unlike previous studies that have paired CAR and TA enzymes, here we examine multiple homologs of each of these enzymes and a broader set of candidate substrates. In addition, we compare the performance of these systems in cell?free and resting whole?cell biocatalysis formats using the conversion of vanillate to vanillyl amine as model chemistry. We also demonstrate that resting whole cells can be recycled for multiple batch reactions. We used the knowledge gained from this study to produce several amines from carboxylic acid precursors using one?pot biocatalytic reactions, several of which we report for the first time. 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.