Lignocellulolytic enzymes in circular biorefineries: Challenges, barriers, and integrative solutions for next...

The sustainable bioeconomy fundamentally relies on the effective valorization of lignocellulosic biomass into renewable fuels and high-value biochemicals. Although extensive research has been conducted over several decades, commercialization remains hindered by enzyme instability, low catalytic efficiency, high production costs, and the intrinsic recalcitrance of biomass. While previous reviews have separately discussed lignocellulolytic enzymes, biomass conversion, or circular bioeconomy strategies, comprehensive integration of recent molecular, microbial, and process-engineering advancements remains limited. Therefore, this review provides a multidisciplinary perspective integrating enzyme engineering, microbial co-culturing, heterologous gene expression, advanced biomass pretreatment, and bioprocess optimization within a unified biorefinery framework. The review critically links molecular-level improvements, including protein engineering, codon optimization, and thermostability enhancement, with industrial process strategies such as consolidated bioprocessing, consolidated bio-saccharification, and techno-economic optimization. Emerging approaches including CRISPR/Cas9-mediated strain improvement, artificial cellulosomes, nanobiotechnology, and AI-assisted modeling are discussed as supportive tools for improving biomass conversion efficiency and industrial applicability. In addition, major bottlenecks such as enzyme instability, biomass recalcitrance, and process incompatibilities are critically evaluated along with recent strategies to overcome these limitations. By integrating molecular biology, microbial engineering, and process-level innovations, this review provides a comprehensive framework for the development of sustainable and scalable lignocellulosic biorefineries supporting the circular bioeconomy.