Hydrolysis of para-Substituted Phenyl-β-D-xylosides by β-Xylosidase from Malbranchea pulchella var. sulfurea No. 48

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The rates of hydrolysis of aryl-β-D-xylosides (e.g. p-NO2-phenyl-, p-Cl-phenyl-, phenyl-, p-CH3-phenyl- and p-CH3O-phenyl-β-D-xylosides) by Malbranchea β-xylosidase were investigated. The Michaelis constants, Km, were almost independent of aglycone, whereas maximum velocity, Vmax, showed a marked dependence. In all hydrolysis reactions studied, the molecular activities, k0 (Vmax/e0, where e0 is the molar concentration of enzyme), were markedly increased by introducing both electron-withdrawing and electron-donating substituents in the para position of the phenyl ring. Between the values of log k0 and inductive sigma constants, σ, V-shaped Hammett plots were obtained. When electron-withdrawing substituents were introduced, the Hammett reaction constant, ρ, was + 1.27, when electron-donating substituents were introduced, its value was -2.07. On enzymic hydrolysis of p-nitrophenyl- and phenyl-β-D-xyloside, the reaction products, in both cases, were found to be α-D-xylose with inversion of configuration.
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