Purification and properties of an extracellular xylanase from the thermophilic fungus Humicola grisea var. thermoidea

Abstract

Humicola grisea var. thermoidea mycelium grown on xylan as the sole source of carbon produced at least two extracellular xylanases. The main xylanolytic component (form 2; 90% of recovered activity) was purified to homogeneity. The apparent molecular mass of the purified enzyme was estimated to be 23 000 and 25 550 Da by Bio-Gel P-60 filtration and urea–SDS–PAGE, respectively. The purified enzyme was a glycoprotein with 45% carbohydrate content and pH and temperature optima of 5.5 and 70 °C, respectively. The apparent K_m and V_max values determined with larch-wood xylan were 3.3 mg/mL and 229 μmol∙min⁻¹∙mg protein⁻¹, respectively. The enzyme was highly specific for xylan and degraded this substrate to produce xylo-oligosaccharides, suggesting that it is a β-1,4-endoxylanase (EC 3.2.1.8). The minor enzymatic component of H. grisea extracellular xylanase activity (form 1) was partially purified and some of its properties were studied for comparative purposes. The results obtained suggested that the mode of action of xylanases form 1 and 2 on xylan differs. Key words: xylanase, hemicellulase, enzyme purification, endoxylanase, Humicola grisea.