Fructose 1,6-bisphosphate dephosphorylation in gluconeogenesis

Hydrolysis of fructose 1,6-bisphosphate by fructose 1,6-bisphosphatase,found in the liver and kidneys, bypasses the irreversible phosphofructokinase-1 (PFK-1) reaction of glycolysis and provides an energetically favorable pathway for the formation of fructose 6-phosphate (Fig. 1). This reaction is an important regulatory site of gluconeogenesis.

Figure 1: Dephosphorylation of fructose 1,6-bisphosphate. AMP = adenosine monophosphate; = phosphate.

1. Regulation by intracellular energy levels: Fructose 1,6-bisphosphatase is inhibited by a rise in the adenosine monophosphate (AMP)/ATP ratio, which signals a low-energy state in the cell. Conversely, low AMP and high ATP levels stimulate gluconeogenesis, an energy-requiring pathway.
2. Regulation by fructose 2,6-bisphosphate: Fructose 1,6- bisphosphatase is inhibited by fructose 2,6-bisphosphate, an allosteric effector whose concentration is influenced by the insulin/glucagon ratio. When glucagon is high, the effector is not made by hepatic PFK-2 , and thus, the phosphatase is active (Fig. 2 ). [Note: The signals that inhibit (low energy, high fructose 2,6-bisphosphate) or activate (high energy, low fructose 2,6-bisphosphate) gluconeogenesis have the opposite effect on glycolysis, providing reciprocal control of the pathways that synthesize and oxidize glucose .]

Figure 2: Effect of elevated glucagon on the intracellular concentration of fructose 2,6-bisphosphate in the liver. AMP and ADP = adenosine mono- and diphosphates; cAMP = cyclic AMP; PFK-2 = phosphofructokinase-2; FBP-2 =fructose 2,6-bisphosphatase; FBP-1 = fructose 1,6-bisphosphatase; and =phosphate.