Alpha-secondary tritium kinetic isotope effects for the hydrolysis of alpha-D-glucopyranosyl fluoride by exo-alpha-glucanases.
alpha-Secondary tritium kinetic isotope effects ranging from 1.17 to 1.26 were measured for the hydrolysis of alpha-D-glucopyranosyl fluoride (forming beta-D-glucose) catalyzed by several glucoamylases and a glucodextranase. These results indicate that cleavage of the C-F bond is slow and that the enzymic transition state has significant oxo-carbonium ion character. Strong support for this conclusion is provided by the agreement found in the case of Rhizopus niveus glucoamylase (alpha-TV/K 1.26; Km 26 mM) between measured values of the alpha-secondary deuterium kinetic isotope effects (alpha-DV/K 1.16; alpha-DV 1.20) and those calculated from the tritium isotope effect. The data are consistent with the promotion of an intramolecular elimination of fluoride by the present exo-alpha-glucanases based on their ability to stabilize, perhaps with a counter ion, the development of a carbonium ion-like transition state. Although the oxo-carbonium ion is formally denoted as an intermediate it could represent a transition state along a reaction pathway to a covalent glucosyl intermediate.
Matsui
H
H
Department of Microbiology, Albert Einstein College of Medicine, Bronx, New York 10461.
Blanchard
J S
JS
Brewer
C F
CF
Hehre
E J
EJ
D005947:Glucose / Q000031:analogs & derivatives* / Q000378:metabolism; D005959:Glucosidases / Q000378:metabolism*; D007553:Isotope Labeling; D007700:Kinetics; D008433:Mathematics; D008962:Models, Theoretical; D014316:Tritium
D014316:Tritium; C030364:glucosyl fluoride; D005959:Glucosidases; D005947:Glucose