Fructose 2,6-bisphosphate: A mediator of hormone action at the fructose 6-phosphate/fructose 1,6-bisphosphate substrate cycle

Fructose 2,6-bisphosphate was discovered in the course of studies on the regulation of hepatic 6-phosphofructo-1-kinase activity by glucagon. The compound is acid-labile but stable to heating in alkali. These properties are due to the presence of a phosphate on the hemiketalic hydroxyl group at C2. Fructose 2,6-bisphosphate can be synthesized chemically by reacting fructose 1,6-bisphosphate with dicyclohexylcarbodiimide followed by base-catalyzed ring opening of the fructose 1,2-cyclic, 6-bisphosphate intermediate. Fructose 2,6-bisphosphate is a potent allosteric activator of 6-phosphofructo-1-kinase and an inhibitor of fructose 1,6-bisphosphatase. It potentiates the effect of AMP on both enzymes. Since 6-phosphofructo-1-kinase is one of the most important control elements in glycolysis, it is likely that fructose 2,6-bisphosphate is a significant physiological regulator of this process in liver. Likewise, fructose 2,6-bisphosphate is probably a significant physiological regulator of fructose 1,6-bisphosphatase, a key gluconeogenic enzyme. For example, the effect of glucagon to enhance carbon flux through fructose 1,6-bisphosphatase and to inhibit flux through 6-phosphofructo-l-kinase in intact hepatocytes is explained by the ability of the hormone to lower the level of fructose 2,6-bisphosphate. This decrease in fructose 2,6-bisphosphate is brought about, at least in part, by a cAMP-dependent phosphorylation and inactivation of the enzyme responsible for its synthesis, 6-phosphofructo-2-kinase. This novel enzyme catalyzes the transfer of the γ phosphate of ATP to the C2 position of fructose 6-phosphate. Phosphorylation of this enzyme is catalyzed by the cAMP-dependent protein kinase in vitro with concomitant inhibition of enzyme activity. Fructose 1,6-bisphosphatase and 6-phosphofructo-l-kinase are also substrates for the cAMP-dependent protein kinase both in vitro and in vivo. However, a role for phosphorylation in regulating their activity remains uncertain. The discovery of the unique sugar phosphate, fructose 2,6-bisphosphate, has been an important advance in our understanding of the regulation of carbohydrate metabolism in liver.