Microbial Lignin Valorization to Protocatechuic Acid and Catechol: Biofunneling Pathways and Metabolic Engineering...

Lignin, an abundant and renewable aromatic biopolymer, represents a largely underutilized resource for the sustainable production of high-value chemicals. Among lignin-derived intermediates, protocatechuic acid (PCA) and catechol have emerged as key platform molecules due to their versatile applications in pharmaceuticals, polymers, and fine chemicals. This review provides a critical overview of microbial lignin valorization focusing on the microbial conversion of lignin-derived aromatics into PCA and catechol. It highlights recent advances in lignin depolymerization techniques, including thermochemical and biological approaches, and examines their influence on the generation of bioavailable aromatic feedstocks. We systematically discuss microbial biofunneling pathways that converge diverse lignin-derived compounds into PCA and catechol, emphasizing the role of central metabolic nodes and enzymatic transformations such as O-demethylation, hydroxylation, and decarboxylation. We treat protocatechuate decarboxylase (PCADC) as the central enzymatic bridge linking PCA and catechol. However, it should be noted that many reported microbial production strategies have been demonstrated using purified lignin-derived aromatic model compounds (e.g., ferulate, vanillate, p-coumarate, and PCA) rather than authentic lignin streams, highlighting the need for improved integration of lignin depolymerization and downstream bioconversion processes. Furthermore, the review explores state-of-the-art metabolic engineering strategies, including gene deletions, pathway rewiring, transporter engineering, and CRISPR-based regulation, to enhance product yields and selectivity. Despite significant progress, several challenges persist, including lignin recalcitrance, heterogeneity of depolymerization products, toxicity of intermediates, and limited enzyme efficiency. This review identifies key knowledge gaps and proposes future directions for integrating synthetic biology, adaptive evolution, and systems-level optimization to develop robust microbial cell factories. Overall, this work provides a strategic framework for advancing lignin bioconversion into PCA and catechol, contributing to the development of sustainable biorefineries and a circular bioeconomy.