Minimal enzyme cascades for the aromatic-to-aromatic upgrading of lignin monomers

Lignin valorization is critical to the economic viability of lignocellulosic biorefineries and could serve as an additional revenue stream for the pulp and paper industry. Emergent lignin-focused biomass pretreatment processes release a variety of aromatic monomers that could be directly upgraded into value-added aromatics for use as monomers, flavors, and fragrances. To date, most biological approaches for lignin valorization use a two-stage molecular disassembly→targeted reconstruction strategy that leverages microbial funnels to route lignin monomers (>C6) into C2-C3 metabolic intermediates before building them back up into C6-C12 value-added chemicals. Here, we review the use of minimal enzyme cascades (<4 enzymes) that preserve the aromatic core for the atom-efficient transformation of ferulic acid (FA) and p-coumaric acid (pCA) into value-added products. We focus on FA/pCA because they are the primary aromatic monomers released from grassy lignin deconstruction and have been extensively upgraded in the literature, yielding over thirty value-added chemicals via minimal enzyme cascades. Opportunities in this space include artificial intelligence/machine learning-driven retro-biosynthetic approaches to predict minimal enzyme cascades for novel chemicals, coupled with cell-free expression systems for rapid validation prior to implementation in microbial hosts. Key challenges include aromatic toxicity to the microbial host, enzyme inhibition, poor heterologous expression of plant enzymes in microbes, and separation of aromatic production from lignin-derived aromatic feed streams. The high price point of value-added aromatics (>$15/kg), coupled with the low cost of lignin monomers (∼$4/kg), creates an opportunity for atom-efficient aromatic-to-aromatic lignin upgrading.