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  • Title: Structure-reactivity relationships for beta-galactosidase (Escherichia coli, lac Z). 2. Reactions of the galactosyl-enzyme intermediate with alcohols and azide ion.
    Author: Richard JP, Westerfeld JG, Lin S, Beard J.
    Journal: Biochemistry; 1995 Sep 19; 34(37):11713-24. PubMed ID: 7547903.
    Abstract:
    Velocities for the synthesis of beta-D-galactopyranosyl derivatives by transfer of the galactosyl group from beta-galactosidase to seven alkyl alcohols, glucose, and azide ion have been determined as the difference in the velocities for beta-galactosidase-catalyzed cleavage of 4-nitrophenyl beta-D-galactopyranoside to give 4-nitrophenoxide anion (v PNP) and hydrolysis of this compound to give D-galactose (v Gal). Rate constant ratios kROH/ks (M-1) for partitioning of the galactosylated enzyme between reaction with alkyl alcohols and solvent determined by this method are in good agreement with values of kROH/ks (M-1) determined by analysis of alcohol inhibition of enzyme-catalyzed hydrolysis of the corresponding alkyl beta-D-galactopyranosides. Absolute rate constants kROH (M-1 s-1) for reaction of alkyl alcohols with the galactosylated enzyme intermediate were calculated from the corresponding rate constant ratio kROH/ks (M-1) and ks = 710 s-1. A Brønsted parameter of (beta nuc) ROH = -0.19 +/- 0.10 was determined from the second-order rate constants for the reactions of alcohols with the galactosylated enzyme. The large difference between (beta 1g)kcat/Km = -0.75 +/- 0.14 for cleavage of alkyl beta-D-galactopyranosides to form the galactosylated enzyme and (beta nuc)ROH = -0.19 for the reverse synthesis reaction requires that the equilibrium constants for galactosyl group transfer from alkyl beta-D-galactopyranosides to the enzyme increase sharply with decreasing pKa of the alkyl alcohol leaving group. These data give beta eq = -0.56 +/- 0.05 for the reaction of alkyl beta-D-galactopyranosides with ethanol to form ethyl beta-D-galactopyranoside and alkyl alcohol. Several effects that lead to this increased ease of cleavage of alkyl beta-D-galactopyranosides with decreasing basicity of the alkoxy group are discussed. A second-order rate constant of kGlc = 1.2 x 10(4) M-1 s-1 was determined for reaction of glucose with the galactosylated enzyme. The relatively low reactivity of glucose is surprising, because an earlier observation that the galactosylated enzyme complex generated by the cleavage of lactose undergoes release of glucose and synthesis of allolactose at nearly equal rates suggests that the binding of glucose to the galactosylated enzyme should be partly irreversible and that it takes place near the encounter-controlled limit. The data suggest a significant stabilization of nonproductive complexes formed by binding of glucose to the galactosylated enzyme. beta-Galactosidase catalyzes the hydrolysis of beta-D-galactopyranosyl azide, but not the synthesis of this compound by reaction of azide ion with the galactosylated enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)
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