When fasting (meaning, in this case, not eating any carbohydrates), the pancreas releases a polypeptide hormone, glucagon, that stimulates gluconeogenesis in the liver (1). Glucagon functions via a regulating bifunctional enzyme (1). The enzyme dephosphorylated acts as phophofructokinase-2 (PFK-2) and glucagon induces phosphorylation to produce fructose bisphosphatase-2 (FBPase-2) (1). The suppression of PFK-2 and increased activity of FBPase-2 reduces concentration of fructose 2,6-biphosphate (1).
Fructose 2,6-biphosphate’s presence regulates glycolysis and gluconeogenesis (1). It is the key positive modulator for glycolytic enzyme phophofructokinase (PFK) (not same as PFK-2), and PFK’s increased activity stimulates glycolysis by suppressing activity of fructose biphophatase (not same as FBase-2) (1). The reduced presence of fructose 2,6-biphosphate and, thus, increased activity of gluconeogenesis restores blood glucose levels successfully (1).
Phew! Now on to dessert.
In response to restored blood glucose levels from gluconeogenesis or when breaking a fast, the picture is reversed. The presence of glucose stimulates the pancreas to release insulin (1). Insulin reduces fructose 2,6-biphosphate via dephosphorylation of phosphofructokinase-2, and positively affects activity of glycogen synthase (also through dephosphorylation) stimulating glycogenesis (1).
So just remember, dessert equals dephosphorylation and phasting equals phosphorylation!
1. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009.