β-Glucosidase components of the cellulolytic system of the edible straw mushroom, Volvariella volvacea

Abstract

The edible straw mushroom, Volvariella volvacea (V-14), produced β-glucosidase when grown in liquid culture on a variety of carbon sources including cellulose, cellobiose, salicin, sorbose, lactose, esculin, cotton wool, and filter paper. Two cell-associated β-glucosidases, BGL-I and BGL-II, were purified 32-fold and 23-fold, respectively, from extracts of cellulose-grown mycelium. The purification procedure included DEAE-Sepharose chromatography, Sephacryl S-200 chromatography, and fast protein liquid chromatography (FPLC) using Mono- Q and Mono-P anion-exchange columns. The enzymes were found to be homogeneous and to have native molecular weights of 158 kDa (BGL-I) and 256 kDa (BGL-II) by gel filtration. The isoelectric points for BGL-I and BGL-II were 5.6 and 5–5.2, respectively. Both isozymes displayed relatively broad pH optima with maximum reaction velocities recorded at pH 7.0 for BGL-I and pH 6.2 for BGL-II, and were rapidly denatured at temperatures of 60°C and above. Purified BGL-I and BGL-II were both active against p-nitrophenyl-β- d-glucopyranoside, p-nitrophenyl-α- l-arabinofuranoside, p-nitrophenyl-α- d-glucopyranoside, cellobiose, salicin, and esculin, but only BGL-I was active against p-nitrophenyl-β- d-xylopyranoside. BGL-I and BGL-II exhibited K m values for p-nitrophenyl-β- d-glucopyranoside of 90 and 500 μ m, respectively. Isozyme activities were adversely affected by several reported β-glucosidase inhibitors, various metal ions, and lignin-derived aromatic acids and aldehydes. Glucose production from microcrystalline cellulose by a commercial cellulase preparation was enhanced by 9.7% in reaction mixtures supplemented with BGL-II.