Fructose 2,6-bisphosphate

Fructose 2,6-bisphosphate (or fructose 2,6-diphosphate), abbreviated Fru-2,6-P2, is a metabolite which 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-P2 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-P2 is performed through the phosphorylation of fructose 6-phosphate using ATP by the PFK-2 portion of the enzyme. The breakdown of Fru-2,6-P2 is catalyzed by dephosphorylation by FBPase-2 to produce Fructose 6-phosphate and Pi.

Reaction scheme of breakdown of fructose 2,6-bisphosphate to fructose 6-phosphate.

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.

Fru-2,6-P2 stimulates glucose breakdown further through reduction of gluconeogensis through allosteric inhibition of fructose 1,6-bisphosphatase. Increased levels of Fru-2,6-P2 show dramatically reduced rates of FBPase-1 activity, with greater degrees of inhibition occurring at lower concentrations of Fru-2,6-P2. This inhibition also appears synergistic with AMP, another allosteric inhibitor of FBPase-1. This coordination between Fru-2,6-P2 and AMP allows similar inhibition of FBPase-1 with dramatically reduced levels of inhibitory AMP. Inhibition by Fru-2,6-P2, however, appears reduced at elevated pH and temperature levels.

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 and glucagon which affect the enzyme through phosphorlyation/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 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, as a phosphoprotein phosphatase dephosphorylates the enzyme to activate the PFK-2 and inhibit the FBPase-2 activities. With additional Fru-2,6-P2 present, activation of PFK-1 occurs to stimulate glycolysis while inhibiting gluconeogenesis.

Regulation of sucrose production
Fru-2,6-P2 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 1/6th of triose phosphate can be converted into sucrose or stored as starch. Fru-2,6-P2 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-P2 is inhibited, thus producing sucrose. Fru-2,6-P2 production is activated when plants are in the dark and photosynthesis and triose phosphates are not produced.