Affinity of Glucose Analogs for α-Glucan Phosphorylases from Rabbit Muscle and Potato Tubers

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Abstract
The action of phosphorylase b from rabbit muscle and potato phosphorylase was inhibited to various extents by several glucose analogs. Like glucose itself, all of the glucosidic oxygen-substituted analogs tested in kinetic experiments showed a nonlinear competitive inhibition for muscle phosphorylase b and a linear competitive one for potato phosphorylase. 5-Thio-n-D-glucose, one of the ring oxygen-substituted analogs, also inhibited the action of muscle phos-phorylase b in the same manner, while the inhibition pattern of 5-amino-D-glucose (nojirimycin) was of a linear noncompetitive type. Since the conformation of 5-amino-n-glucose in aqueous solution is half-chair (Reese et al. (1971) Carbohyd. Res. 18, 381–388), the unusual kinetic behavior of the compound toward muscle phosphorylase b was supposed to be due to its halfchair conformation. In the glucosidic oxygen-substituted analogs, the affinity for both muscle phosphorylase b and potato phosphorylase decreased with decreasing order of magnitude of electronegativity of the glucosidic atom. The strong positive correlation between the affinity and the electronegativity suggests that D-glucose-1-P, the substrate, may bind to phosphorylase with the formation of a hydrogen bond between its glucosidic oxygen and a hydrogen donor of the enzyme.