Regulation of fructose-6-phosphate 2-kinase by phosphorylation and dephosphorylation: possible mechanism for coordinated control of glycolysis and glycogenolysis.

The kinetic properties and the control mechanism of fructose-6-phosphate 2-kinase (ATP: D-fructose-6-phosphate 2-phosphotransferase) were investigated. The molecular weight of the enzyme is approximately 100,000 as determined by gel filtration. The plot of initial velocity versus ATP concentration is hyperbolic with a Km of 1.2 mM. However, the plot of enzyme activity as a function of fructose-6-phosphate is sigmoidal. The apparent K0.5 for fructose-6-phosphate is 20 microM. Fructose-6-phosphate 2-kinase is inactivated by the catalytic subunit of cyclic AMP-dependent protein kinase, and the inactivation is closely correlated with phosphorylation. The enzyme is also inactivated by phosphorylase kinase in the presence of Ca2+ and calmodulin. The phosphorylated fructose-6-phosphate 2-kinase, which is inactive, is activated by phosphorylase phosphatase and alkaline phosphatase. The possible physiological significance of these observations in the coordinated control of glycogen metabolism and glycolysis is discussed.