The interaction of 1-fluoro- d -glucopyranosyl fluoride with glucosidases

1-Fluoro-D-glucopyranosyl fluoride undergoes pH-independent loss of F- ion at a rate of 1 x 10(-8) s-1 at 50.0 degrees C, some 10(3)-fold slower than alpha-D-glucopyranosyl fluoride and 4 x 10(4)-fold slower than beta-D-glucopyranosyl fluoride. 2. The (inverting) amyloglucosidase II of Aspergillus niger hydrolyses the difluoride according to Michaelis-Menten kinetics (Km 34 mM and kcat. 0.27 s-1), by apparently the same (simple) mechanism by which it hydrolyses alpha-D-glucopyranosyl fluoride (Km 38 mM and kcat. 730 s-1), rather than by the Hehre resynthesis-hydrolysis mechanism used to transform beta-D-glucopyranosyl fluoride. 3. The difluoride is also a substrate for the (inverting) trehalase of pig kidney [Km 17.3 mM and Vmax. 6.2 x 10(-4) relative to alpha-D-glucopyranosyl fluoride (Km 38 mM]). 4. The quantitatively similar effect of fluorine substitution on the one-step enzymic reactions and on the non-enzymic reactions suggests that they go through similar (oxocarbonium-ion-like) transition states. 5. The difluoride is a substrate for the (retaining) beta-glucosidases from Aspergillus wentii (A3 enzyme) and sweet-almond meal (B isoenzyme) and for the retaining alpha-glucosidase from rice: comparison with the appropriate monofluoride reveals a variable rate-retarding effect of the second fluorine atom on kcat./Km that correlates with other measures of oxocarbonium ion character in the transition state. 6. The difluoride is a substrate for the (retaining) alpha-glucosidase from yeast, but also gives an insidious mimicry of active-site-directed irreversible inhibition, which we tentatively attribute either to formation of the non-covalent complex or to the fluoroglucosyl-enzyme increasing the well-known tendency of this enzyme to come out of solution by adsorption on the walls of the vessel.