Cyclic Phosphofructokinase

Figure 11-2 Roles of phosphofructose kinase and fructose 1,6-bisphosphatase in the control of the breakdown and storage (—+) of glycogen in muscle. The uptake of glucose from blood and its release from tissues is also illustrated. The allosteric effector fructose 2,6-bisphosphate (Fru-2,6-P2) regulates both phosphofructokinase and fructose 2,6-bisphosphatase. These enzymes are also regulated by AMP if it accumulates. The activity of phosphofructokinase-2 (which synthesizes Fru-2,6-P2) is controlled by a cyclic AMP-dependent kinase and by dephosphorylation by a phosphatase.

The regulatory role of calcium ions in intermediary metabolism is well documented. Calcium has been shown to be involved in activation or inhibition of specific enzyme systems [105], For example, it activates cyclic nucleotide phosphodiesterase, phosphofructokinase, fructose 1 6 biphosphatase, glycerol phosphate dehydrogenase, pyruvate dehydrogenase phosphatase and pyruvate dehydrogenase kinase. Calcium ions inhibit pyruvate kinase, pyruvate carboxylase, Na+/K+-AT-Pase and adenylate cyclase. 

Brunswick (1971) had only achieved 20% incorporation of label previously in their respective studies on yeast alcohol dehydrogenase, Brunswick and Cooperman (1973) achieved more than 70 % incorporation of ligand into phosphofructokinase using either the dialysis or resin technique with the photoaffinity label of cyclic AMP. 

F-2,6-BE Thus, F-2,6-BP levels are decreased and phosphofructokinase activity is decreased. In liver and muscle, F-2,6-BP is the major allosteric activator of phosphofructokinase. In skeletal muscle, however, the kinase responsible for the synthesis of F-2,6-BP is activated, not inhibited, by cyclic AME Thus, muscle sees an increase in glycolysis following epinephrine stimulation, while the liver experiences a decrease in glycolytic activity. In both tissues, glycogen phosphorylase is activated and glycogenolysis occurs. Under these conditions, glucose is utilized in muscle for ATP production relative to contractile activity, while the liver produces glucose for export to the blood. 

The photoaffinity label O -(ethyl-2-diazomalonyl)adenosine-3, 5 -cyclic phosphate (17) has been used to label rabbit muscle phosphofructokinase. If... 

Srinivasan, N. G., Wariso, B. A., Kulkarni, G., Rao, G. S. J. and Harris, B. G. (1988) Phosphofructokinase from Dirofilaria immitis. Stimulation of activity by phosphorylation with cyclic AMP-dependent protein kinase. J. Biol. Chem. 263 3482-3485. 

Phosphofructokinase 2 (PFK-2) is negatively regulated by phosphorylation in the liver (the enzyme that catalyzes the phosphorylation is the cyclic adenosine monophosphate [cAMP]-dependent protein kinase). However, in skeletal muscle, PFK-2 is not regulated by phosphorylation. This is because the skeletal muscle isozyme of PFK-2 lacks the regulatory serine residue, which is phosphorylated in the liver. However, the cardiac isozyme of PFK-2 is phosphorylated and activated by a kinase cascade initiated by insulin. This allows the heart to activate glycolysis and to use blood glucose when blood glucose levels are elevated. 

Such an effect appears to be mediated by regulating the activity of the rate-limiting enzyme of glycolysis, phosphofructokinase. Cyclic AMP affects such activity by relieving the inhibition by ATP of effective levels of the substrate fructose-1,6-phosphate and by converting an inactive form of phosphofructokinase to an active form through subunit aggregation. 

HT activates the adenyl cyclase of the liver fluke Fasciola hepatica) and increases the activity of phosphofructokinase. which is the rate-limiting step of glycolysis in this organism, via the production of 3. 5 -cyclic AMP. 

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