HYDROLYSIS OF PARA-SUBSTITUTED PHENYL-BETA-D-XYLOSIDES BY BETA-XYLOSIDASE FROM MALBRANCHEA-PULCHELLA VAR SULFUREA NO-48

The rates of hydrolysis of aryl-.beta.-D-xylosides (e.g., p-NO2-phenyl-, p-Cl-phenyl-, phenyl-, p-CH3-phenyl- and p-CH3O-phenyl-.beta.-D-xylosides) by Malbranchea .beta.-xylosidase were investigated. The Km were almost independent of aglycone, 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, .sigma., V-shaped Hammett plots were obtained. When electron-withdrawing substituents were introduced, the Hammett reaction constant, .rho., was +1.27, when electron-donating substituents were introduced, its value was -2.07. On enzymic hydrolysis of p-nitrophenyl- and phenyl-.beta.-D-xyloside, the reaction products, in both cases, were found to be .alpha.-D-xylose with inversion of configuration.