A heterodimeric xylosidase (E C. 3 2 1 37) with robust activity is secreted among the plant cell wall degrading enzymes produced by the saprophytic fungus Humicola insolens. The xylosidase has been purified to homogeneity by gel filtration and cation exchange chromatography, and demonstrated to be composed of two protein subunits of 68 and 17kDa with a molecular mass in solution of approximately 85kDa based on a combination of SDS-PAGE. size exclusion chromatography and analytical ultracentrifugation. Peptidesequence identities from the subunits indicate the 68 kDa subunit contains a catalytic protein domain and the 17 kDa subunit a carbohydrate binding module. The xylosidase has wide biotechnological potential with maximum activity exhibited at 70:C and kinetic constants with p-nitrophenol xylopyranoside substrate that suggest it has the highest catalytic efficiency recorded to date (Vmax 22.17 mu moles/min/mg. Km 1.74 mM and Kcat 6787/s). (C) 2009 Elsevier Inc. All rights reserved.