Purification and characterization of extracellular β-xylosidase and α-arabinosidase from the plant pathogenic fungus Cochliobolus carbonum

The filamentous fungus Cochliobolus carbonum produces extracellular β-xylosidase (Xyp) and α-arabinofuranosidase (Arf) when grown on maize cell walls. Low concentrations of sucrose in a medium containing other carbon sources stimulate production of both enzymes, while high sucrose concentrations partially repress enzyme production. Xyp and Arf were purified 49-fold and 162-fold with 24% and 15% yields, respectively, by cation-exchange, hydrophobic-interaction, and gel-filtration HPLC. Xyp has a molecular mass of 42 kDa, a low level of α-arabinosidase, but no arabinanase activity, a pH optimum of 5.5–6.5, and a temperature optimum of 37 °C. Two peptides derived from Xyp show strong similarity to a bacterial bifunctional β-d-xylopyranosidase/α-l-arabinofuranosidase. Arf has a molecular mass of 63 kDa, a low level of β-xylosidase and arabinanase activity, a pH optimum of 3.5–4.0, and a temperature optimum of 50 °C. Xyp but not Arf is glycosylated. Xyp liberates xylose from oat spelt xylan, while Arf releases both arabinose and larger oligosaccharides, suggesting that while Xyp behaves as a β-xylosidase on natural substrates, Arf acts as both an α-arabinosidase and an arabinanase.