Hydrolysis ofpara-Substituted Phenyl-β-D-xylosides by β-Xylosidase fromMalbranchea pulchellavar.sulfureaNo. 48

Abstract

The rates of hydrolysis of arylβ-D-xylosides (e.g. p-NO₂-phenyl-, p-Cl-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, V_max, showed a marked dependence. In all hydrolysis reactions studied, the molecular activities, k₀ (V_max/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 k₀and inductive sigma constants, σ, V-shaped Hammett plots were obtained. When electron-withdrawing substituents were introduced, the Hammett reaction constant, p, 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.