Biochemical Characterization of an Extracellular -Glucosidase from the Fungus, Penicillium italicum, Isolated from Rotten Citrus Peel

A ${\beta}$-glucosidase from Penicillium italicum was purified with a specific activity of 61.8 U/mg, using a chromatography system. The native form of the enzyme was an 88.5-kDa tetramer with a molecular mass of 354 kDa. Optimum activity was observed at pH 4.5 and $60^{\circ}C$, and the half-lives were 1,737, 330, 34, and 1 hr at 50, 55, 60, and $65^{\circ}C$, respectively. Its activity was inhibited by 47% by 5 mM $Ni^{2+}$. The enzyme exhibited hydrolytic activity for p-nitrophenyl-${\beta}$-D-glucopyranoside (pNP-Glu), p-nitrophenyl-${\beta}$-D-cellobioside, p-nitrophenyl-${\beta}$-D-xyloside, and cellobiose, however, no activity was observed for p-nitrophenyl-${\beta}$-D-lactopyranoside, p-nitrophenyl-${\beta}$-D-galactopyranoside, carboxymetyl cellulose, xylan, and cellulose, indicating that the enzyme was a ${\beta}$-glucosidase. The $k_{cat}/K_m\;(s^{-1}mM^{-1})$ values for pNP-Glu and cellobiose were 15,770.4 mM and 6,361.4 mM, respectively. These values were the highest reported for ${\beta}$-glucosidases. Non-competitive inhibition of the enzyme by both glucose ($K_i=8.9mM$) and glucono-${\delta}$-lactone ($K_i=11.3mM$) was observed when pNP-Glu was used as the substrate. This is the first report of non-competitive inhibition of ${\beta}$-glucosidase by glucose and glucono-${\delta}$-lactone.