Abstract:

Yeast glycogen synthetase (uridine diphosphate glucose:glycogen α-4-glucosyltransferase, EC 2.4.1.11) is moderately stimulated by glucose 6-phosphate at neutral pH. Addition of certain anions at relatively high concentrations (0.1-0.2 M) inhibits the enzyme, but glucose 6-phosphate reverses the inhibition. Consequently, the relative stimulation by the phosphoric ester is much larger in the presence of inhibitors. Chloride was used as model inhibitor in the following study. Substrate kinetics are Michaelian both without and with added inhibitor. The inhibition is of the mixed or of the competitive type. On the other hand, sigmoid curves are obtained when the reaction rate is represented as a function of glucose 6-phosphate concentration in the presence of chloride, or as a function of chloride concentration in the presence of glucose 6-phosphate. Treatment of the enzyme with fluorodinitrobenzene with uridine diphosphate glucose as protector leads to a total loss of sensitivity against chloride, while the inhibition by uridine diphosphate, a product of the glycogen synthetase reaction, is maintained. It is concluded that chloride and other inhibitors are allosteric, i.e., bind to a site different from that of uridine diphosphate glucose. The direct activating effect of glucose 6-phosphate is also obtained with a number of other substances, but the reversal of inhibition by anions is only shared by glucosamine 6-phosphate. A tentative model of the enzyme, compatible with the results, includes a single site for each substrate, an unspecific site for an activating ion, and several sites for both glucose 6-phosphate and the allosteric inhibitor.