Biosynthesis of Bacterial Glycogen

Abstract

A number of differences in the kinetic properties of the Escherichia coli B ADP-glucose pyrophosphorylase have been found depending on whether Mg++ or Mn++ served to fulfill the divalent cation requirement. With Mg++ as cation, the curve for rate plotted against ATP concentration was sigmoidal, and the concentration of ATP necessary for half-maximal activity was not related to the Mg++ concentration. If Mn++ served as metal, however, an anomalous curve for rate plotted against ATP concentration was obtained, and maximal activity was always reached at a 1:1 ratio between ATP concentration and Mn++ concentration. In the presence of Mg++, the curves of rate as a function of concentration of the activator, fructose 1,6-diphosphate, were sigmoidal. With Mn++ as metal ion, the curve for rate plotted against activator concentration was hyperbolic (in the presence of AMP inhibitor, however, this changed to a sigmoid shape). Regardless of whether Mg++ or Mn++ served as cation, an interaction between the activator, fructose 1,6-diphosphate, and the inhibitors AMP, Pi, and ADP, was found such that the sensitivity of the enzyme to inhibition (as measured by concentration of inhibitor required to produce 50% inhibition of the rate) was modulated by the activator concentration. With Mg++ as metal, the n values of the Hill plots for the inhibitors progressively decreased with decreasing fructose 1,6-diphosphate concentration, indicating a lessening degree of cooperative interaction between sites binding inhibitor; furthermore, the enzyme was relatively insensitive to these inhibitors in the absence of activator. When Mn++ served to fulfill the metal requirement, the n values of the Hill plot (or interaction coefficients) for the inhibitors appeared to remain constant with decreasing fructose 1,6-diphosphate concentration; also, the rate was quite sensitive to these inhibitors in the absence of activator. These results are discussed in light of the enzyme's function in control of glycogen biosynthesis.