High-level production of a thermoacidophilic β-glucosidase from Penicillium citrinum YS40-5 by solid-state fermentation with rice bran

A high yield of β-glucosidase (EC 3.2.1.21) of 159.1 U/g-solid activity on 4-nitrophenyl β-d-glucopyranoside (pNPG) was achieved by rice bran-based solid-state fermentation (SSF) of the recently characterized fungus Penicillium citrinum YS40-5. The enzyme was both thermophilic and acidophilic at the optimized temperature and pH of 70 °C and 5.0, respectively. Over 95% of the original β-glucosidase activity was maintained after a prolonged storage at ambient temperature for 4 weeks. The kinetic parameters Vmax, Km and KI were 85.93 U/mg, 1.2 mM and 17.59 mM with pNPG, and 72.49 U/mg, 32.17 mM and 8.29 mM with cellobiose, respectively. The protein band with β-glucosidase activity was characterized by native PAGE followed by MUG-zymogram analysis, and its identity confirmed by nanoLC-MS/MS. A 3.43-fold synergistic effect by combining this β-glucosidase with Trichoderma reesei cellulases was observed, indicating this enzyme could potentially be used for improving the efficiency of cellulosic bioconversion.