Optimizing Enzymatic Biocatalysis of Lignocellulosic Biomass for Sustainable Biofuel Production Using Directed Evolution of Xylanases

Lignocellulosic biomass represents an abundant, renewable feedstock for sustainable biofuel production, yet its recalcitrant heteropolysaccharide matrix severely limits enzymatic deconstruction efficiency. Endo-1,4-β-xylanases play an essential role in breaking down hemicellulosic xylan to enhance overall cellulose accessibility during industrial saccharification. However, wild-type enzymes frequently lack the robust operational stability and catalytic efficiency required under elevated temperatures and harsh chemical preprocessing conditions. In this study, we employed an iterative directed evolution strategy combining error-prone PCR and DNA shuffling to engineer a thermostable and alkali-tolerant GH11 xylanase from Thermotoga maritima. High-throughput colorimetric screening of a 12,000-variant library identified a prominent quadruple mutant (designated Xyn-EV4: N68D, S112R, Q175H, and Y220F) that exhibited a 4.8-fold increase in half-life at 75 °C and maintained over 70% residual activity across a broader pH range (pH 5.0–9.0). Kinetic characterization revealed a 2.6-fold enhancement in catalytic efficiency (k_cat/K_m = 1,420 s^-1 mM^-1) using beechwood xylan as substrate. When integrated into a commercial cellulase cocktail for the saccharification of alkaline-pretreated wheat straw, Xyn-EV4 facilitated a 36.4% higher release of fermentable reducing sugars compared to the parental wild-type enzyme. These results demonstrate that directed evolution of hemicellulases can overcome industrial biocatalytic bottlenecks, offering a viable platform for cost-effective biomass valorization and sustainable bioethanol synthesis.