Glycerol: The Secret Superhero of High-Temperature Xylanases?

Xylanases break down plant fibers, but they fall apart at high heat. Researchers found that adding 20% glycerol makes a recombinant Myceliophthora heterothallica xylanase (rMhXyn) far more stable. At 55 °C its half‑life jumps fivefold, thanks to lower activation energy and a tighter structure confirmed by computer simulations. Phenolic acids, by contrast, only give short‑term stability and don’t stiffen the enzyme. Kinetically, glycerol boosts the maximum rate (Vmax), while phenolics tweak affinity (K′) and cooperativity. Thermodynamic data show a higher ΔS_D and a more positive ΔH_D, hallmarks of stabilization. These findings point to glycerol as the ideal additive for hot bioprocesses like lignocellulosic biomass conversion, potentially lowering energy costs and increasing yields for bioplastics, biofuels, and other green products.

The Myceliophthora heterothallica endoxylanase (rMhXyn) belongs to GH11 glycoside hydrolases, featuring a pocket‑shaped active site that accommodates xylosyl chains. Adding 20% (v/v) glycerol alters the reaction solvent by increasing viscosity and reducing water mobility. This crowding effect promotes intra‑protein hydrogen bonds, stabilizing α‑helices and β‑sheets. Molecular dynamics simulations showed a root‑mean‑square deviation (RMSD) drop from 2.3 Å to 1.5 Å and a radius of gyration (Rg) reduction of ~0.8 Å, indicating a more compact structure. Thermodynamically, Van’t Hoff analysis revealed an increased ΔH‡ of +12 kJ·mol⁻¹, suggesting a higher denaturation energy barrier, while ΔS‡ became more positive (+35 J·mol⁻¹·K⁻¹), reflecting greater activation entropy due to a more ordered interaction network. Phenolic acids mainly interact with surface aromatic residues, forming transient π‑π contacts that do not significantly alter internal dynamics, explaining their short‑term stabilization at 40‑45 °C but insufficient protection at higher temperatures. Kinetically, glycerol raises Vmax from 150 to 210 µmol·min⁻¹·mg⁻¹ without markedly changing Km, indicating an effect on catalytic turnover rather than substrate affinity. Phenolics increase the Hill coefficient from 1.0 to 1.4, suggesting positive cooperativity likely caused by conformational changes upon binding. These findings support the targeted use of glycerol as a co‑solvent to enhance xylanase robustness in industrial lignocellulosic biomass hydrolysis, where 50‑60 °C processes are common.