Kinetic analysis of the activation of Zymomonas mobilis glucokinase by phosphate.

A detailed kinetic analysis of glucokinase EC 2.7.1.2 from Zymomonas mobilis has been carried out. This enzyme has an absolute requirement for inorganic phosphate as activator, and the kinetic behaviour can be interpreted as a steady-state ordered mechanism in which glucose is the first substrate. Values for each of the kinetic constants have been obtained for the conditions I = 0.12, 30 degrees C, and pH 7.0. Direct binding studies have confirmed that ATP does not bind to the enzyme without glucose present. Phosphate does not affect ATP binding to the enzyme-glucose complex; when saturated with both ATP and glucose, the dissociation constant for phosphate (determined kinetically) is 0.045 mM. When saturated with the other substrate and phosphate, the Km values for glucose and MgATP are 0.095 mM and 0.19 mM, respectively. The ionic form of phosphate is not important, as the apparent Km for phosphate did not change significantly over the pH range 6.4 to 7.5. Raising the temperature increased Vmax at the high rate of 10% per degree, which correlates well with the fermentation rates between 20 and 30 degrees C, giving further support to the concept that glucokinase is the rate-controlling enzyme in Z. mobilis glucose fermentation.