Fructose 2,6-bisphosphate
Fructose 2,6-bisphosphate, abbreviated Fru-2,6-"P", is a metabolite that allosterically affects the activity of the enzymes phosphofructokinase 1 (PFK-1) and fructose 1,6-bisphosphatase (FBPase-1) to regulate glycolysis and gluconeogenesis. Fru-2,6-"P" itself is synthesized and broken down by the bifunctional enzyme phosphofructokinase 2/fructose-2,6-bisphosphatase (PFK-2/FBPase-2). The synthesis of Fru-2,6-"P" 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-"P" 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 P. Effects on glucose metabolism. Fru-2,6-"P" strongly activates glucose breakdown in glycolysis through allosteric modulation (activation) of phosphofructokinase 1 (PFK-1). Elevated expression of Fru-2,6-"P" 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-"P" 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 is for the expression of TP53-induced glycolysis and apoptosis regulator (TIGAR); 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-"P", therefore the activator does not enhance the affinity of the enzyme (PFK1) for its substrate (fructose 6-phosphate). 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-"P" 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. Release of the hormone glucagon triggers production of cyclic adenosine monophosphate (cAMP), which activates a cAMP-dependent protein kinase. This kinase phosphorylates the PFK-2/FBPase-2 enzyme at an NH-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-"P" in the cell. With decreasing  amounts of Fru-2,6-"P", glycolysis becomes inhibited while gluconeogenesis is activated. Insulin triggers the opposite response. As a phosphoprotein phosphatase, insulin dephosphorylates the enzyme, thus activating the PFK-2 and inhibiting the FBPase-2 activities. With additional Fru-2,6-"P" present, activation of PFK-1 occurs to stimulate glycolysis while inhibiting gluconeogenesis. Regulation of sucrose production. Fru-2,6-"P" plays an important role in the regulation of triose phosphates, the end products of the Calvin Cycle. In the Calvin Cycle, 5/6th of triose phosphates are recycled to make ribulose 1,5-bisphosphate. The remaining 1/6 of triose phosphate can be converted into sucrose or stored as starch. Fru-2,6-"P" inhibits  production of fructose 6-phosphate, a necessary element for sucrose synthesis. When the rate of photosynthesis in the light reactions is high, triose phosphates are constantly produced and the production of Fru-2,6-"P" is inhibited, thus producing sucrose. Fru-2,6-"P" production is activated when plants are in the dark and photosynthesis and triose phosphates are not produced.