Inhibition of α-Glucan Phosphorylase by α-D-Glucopyranosyl Fluoride

Abstract

α-D-Glucopyranosyl fluoride was found to inhibit strongly the action of α-glucan phosphorylase b [EC 2. 4. 1. 1] from rabbit muscle, and that of the enzyme from potato tubers rather weakly. The inhibition is highly specific, being competitive with respect to glucose 1-phosphate and noncompetitive with respect to polysaccharide, during polysaccharide synthesis. In the reverse process, it is competitive with respect to P_i. These results have been explained by assuming that the inhibitor binds to the glucose 1-phosphate site of the enzyme, occupying both subsites which normally bind the glucosyl and phosphate moities of the substrate, but does not directly interact with the polysaccharide site. Based on this assumption, the dissociation constants of the enzyme-inhibitor and enzyme-polysaccharide-inhibitor complexes have been evaluated (0.43 and 0.20m_M for the muscle enzyme, respectively; 24 and 23m_M for the potato enzyme, respectively). Glucosyl fluoride also acts as a noncompetitive inhibitor with respect to AMP. A high concentration of AMP causes an inhibitory effect on the action of the muscle enzyme, the effect being manifested in the presence of glucosyl fluoride.