Production of thermostable β-mannosidase by a strain of Thermoascus aurantiacus: Isolation, partial purification and characterization of the enzyme

The production of an extracellular thermostable β-mannosidase by the thermophilic fungus Thermoascus aurantiacus Miehe was investigated in shake flask cultures. Among five different carbon sources and inducers tested, locust bean gum at 15 g l−1 induced the highest (4 nkat ml−1) β-mannosidase activity. Although the fungus grew very well on mannose at an initial concentration of 20 g l−1 there was practically no enzyme synthesis. Low level of enzyme synthesis occurred only when mannose had dropped to about 1 g l−1. The β-mannosidase was purified to apparent homogeneity by ethanol precipitation, differential solubility at pH 4.0 and cation ion exchange chromatography. The enzyme had a molecular mass of 99.9 kDa and a pI value of 4.8. The β-mannosidase showed maximum activity at pH 2.5–3.0 and 76 °C. It exhibited highest pH stability at 5.9 after 5 h incubation at 50 °C. It had a half-life of 10 min at 76 °C. The Km and Vmax values for p-nitrophenyl-β-d-mannopyranoside (p-NP-β-MP) were 1.1 mM and 61 nkat mg−1, respectively. Low transglycosylation activity was observed when the enzyme was incubated with p-NP-β-MP as glycosyl donor and methyl-α-d-mannopyranoside as acceptor at 50 °C and pH 5.0 and small amounts of β-configured methylmannobioside was formed.