Lignin, as one of the most abundant natural aromatic polymers, holds significant potential for valorization into high-value products. However, its complex and heterogeneous structure presents a major challenge for bioconversion. This review focuses on the bioconversion of lignin-derived syringyl monomers, which are characterized by 3,5-dimethoxy-substituted aromatic rings that create metabolic bottlenecks during microbial O-demethylation and ring cleavage. Recent advances are summarized in lignin depolymerization, biological funneling, and synthetic biology-driven pathway design for converting syringyl monomers into value-added products. Special attention is given to critical enzymes, including Rieske non-heme iron monooxygenases, tetrahydrofolate-dependent O-demethylase systems, and cytochrome P450 peroxygenases, which are pivotal for demethylation. Furthermore, metabolic engineering strategies are discussed to enhance the efficiency of lignin conversion and address challenges such as cofactor imbalance and intermediate toxicity. Finally, this review discusses future research priorities, including broad-substrate-range O-demethylase engineering, host tolerance improvement, validation using realistic lignin-derived streams, and industrial translation. Advancing the bioconversion of lignin syringyl monomers could enable sustainable production of renewable products and supports a circular bioeconomy.
Yu-Lu Wang, Hong-Wei Zhu, Xin-Yu Song et al.· International Journal of Bio...· 0 citations
The enzymatic breakdown of lignin generates a spectrum of aromatic monomers — including vanillin, guaiacol, syringaldehyde, vanillic acid, ferulic acid, and p-coumaric acid — that serve as platform chemicals for pharmaceuticals, fragrances, resins, and bio-based polymers. However, its complex and recalcitrant structure necessitates highly efficient enzymatic systems for depolymerization. This review systematically classifies ligninolytic enzymes into five functional categories: laccases, peroxidases (including manganese peroxidases, lignin peroxidases, and versatile peroxidases), cytochrome P450s, dye-decolorizing peroxidases (DyPs), and auxiliary enzymes, evaluating their distinct roles and synergies in lignin breakdown. Laccases emerge as particularly versatile biocatalysts due to their widespread occurrence, broad substrate specificity, and operational flexibility under diverse conditions. Peroxidases drive critical oxidative reactions, while DyPs represent a functionally robust peroxidase class with superior stability under extreme pH, temperature, and pressure. Complementary enzymes such as etherases and lignin-mimetic systems further expand the toolbox for lignin valorization. To overcome inherent limitations of native enzymes, protein engineering strategies were highlighted to enhance catalytic efficiency, stability, and substrate affinity. Additionally, enzyme immobilization on advanced matrices (e.g., metal-organic frameworks) is discussed as a breakthrough approach to improve reusability and reaction scalability. These integrated advancements pave the way for sustainable lignin valorization.
Bo-Chao Gao, E. Fordjour, Jian-Ning Hu et al.· BioResources· 0 citations
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