Fructose 2,6-bisphosphate
Fructose 2,6-bisphosphate, abbreviated Fru-2,6-P2, is a metabolite that allosterically affects the activity of the enzymes phosphofructokinase 1 and fructose 1,6-bisphosphatase to regulate glycolysis and gluconeogenesis. Fru-2,6-P2 itself is synthesized and broken down in either direction by the integrated bifunctional enzyme phosphofructokinase 2, which also contains a phosphatase domain and is also known as fructose-2,6-bisphosphatase. Whether the kinase and phosphatase domains of PFK-2/FBPase-2 are active or inactive depends on the phosphorylation state of the enzyme.
Fructose-6-p-phosphate is phosphorylated by the kinase domain of PFK-2/FBPase-2 to Fru-2,6-P2 when PFK-2/FBPase-2 is active in a dephosphorylated state. This dephosphorylated state is favored by high levels of insulin, which activates the phosphatase domain.
The synthesis of Fru-2,6-P2 is performed through a bifunctional enzyme containing both PFK-2 and FBPase-2, which is dephosphorylated, allowing the PFK-2 portion to phosphorylate fructose 6-phosphate using ATP. The breakdown of Fru-2,6-P2 is catalyzed by the phosphorylation of the bifunctional enzyme, which allows FBPase-2 to dephosphorylate fructose 2,6-bisphosphate to produce fructose 6-phosphate and Pi.
Effects on glucose metabolism
Fru-2,6-P2 strongly activates glucose breakdown in glycolysis through allosteric modulation of phosphofructokinase 1. Elevated expression of Fru-2,6-P2 levels in the liver allosterically activates phosphofructokinase 1 by increasing the enzyme’s affinity for fructose 6-phosphate, while decreasing its affinity for inhibitory ATP and citrate. At physiological concentration, PFK-1 is almost completely inactive, but interaction with Fru-2,6-P2 activates the enzyme to stimulate glycolysis and enhance breakdown of glucose.Cellular stress as a result of oncogenesis or DNA damage among others, activates certain genes by the tumor suppressor p53. One such gene encodes TP53-inducible glycolysis and apoptosis regulator ; an enzyme that inhibits glycolysis, monitors the cellular levels of reactive oxygen species, and protects cells from apoptosis. The structure of TIGAR is shown to be nearly identical to FBPase-2 on the bifunctional enzyme. TIGAR removes the allosteric effector, Fru-2,6-P2., therefore the activator does not enhance the affinity of the enzyme for its substrate. Furthermore, TIGAR also removes the glycolytic intermediate fructose 1,6-bisphosphate, the product of the PFK catalyzed third reaction of glycolysis and the substrate for the following aldolase fourth reaction of glycolysis.
Production regulation
The concentration of Fru-2,6-P2 in cells is controlled through regulation of the synthesis and breakdown by PFK-2/FBPase-2. The primary regulators of this are the hormones insulin, glucagon, and epinephrine which affect the enzyme through phosphorylation/dephosphorylation reactions.Activation of the glucagon receptor triggers production of cyclic adenosine monophosphate, which activates protein kinase A. PKA phosphorylates the PFK-2/FBPase-2 enzyme at an NH2-terminal Ser residue with ATP to activate the FBPase-2 activity and inhibit the PFK-2 activity of the enzyme, thus reducing levels of Fru-2,6-P2 in the cell. With decreasing amounts of Fru-2,6-P2, glycolysis becomes inhibited while gluconeogenesis is activated.
Insulin triggers the opposite response by activating protein phosphatases that dephosphorylate PFK-2, thereby inhibiting the FBPase-2 domain. With additional Fru-2,6-P2 present, activation of PFK-1 occurs to stimulate glycolysis while inhibiting gluconeogenesis. As of 2023, which specific phosphatases are involved in mediating insulin's downstream effect specifically on PFK-2 are currently unclear; protein phosphatase 1 is known to be involved in mediating insulin's downstream effect of dephosphorylating glycogen synthase, thereby activating it.