Phosphofructokinase substrate cycle

Substrate cycles generate heat, a property that is apparently put to good use by cold bumblebees whose thoracic temperature must reach at least 30°C before they can fly. The insects apparently use the fructose bisphosphatase-phosphofructokinase substrate cycle (Fig. 11-2, steps b and c) to warm their flight muscles.268 It probably helps to keep us warm, too. 

Clark, M. G., Bloxham, D.P., Holland, P. C., and Lardy, H. A., 1973, Estimation of the fructose diphosphatase-phosphofructokinase substrate cycle in the flight muscle of Bombus affinis, Biochem. J. 134 589. 

If fructose-1,6-bisphosphatase and phosphofructokinase acted simultaneously, they would constitute a substrate cycle in which fructose-1,6-bisphosphate and fructose-6-phosphate became interconverted with net consumption of ATP ... 

FIGURE 15-21 Regulation of fructose 1,6-bisphosphatase-1 (FBPase-1) and phosphofructokinase-1 (PFK-1). The important role of fructose 2,6-bisphosphate in the regulation of this substrate cycle is detailed in subsequent figures. 

It has been proposed that the substrate cycle involving phosphofructokinase and fructose bisphosphatase is used by bumblebees to warm their flight muscles to 30°C before flight begins. 

The effects of ATP, AMP, and fructose 2,6-bisphos-phate on phosphofructokinase have been discussed in Chapter 11, Section C. Fructose 2,6-P2 is a potent allosteric activator of phosphofructokinase and a strong competitive inhibitor of fructose 1,6-bisphosphatase (Fig. 11-2). It is formed from fructose 6-P and ATP by the 90-kDa bifunctional phosphofructo-2-kinase/ fructose 2,6-bisphosphatase. Thus, the same protein forms and destroys this allosteric effector. Since the bifunctional enzyme is present in very small amounts, the rate of ATP destruction from the substrate cycling is small. 

In glycolysis, the enzymes phosphofructokinase (PFK) and fructose 1,6-bisphosphatase (FBP) form a substrate cycle ... 

Does the existence of the phosphofructokinase-fructose-1,6-bisphosphatase substrate cycle have any energetic disadvantages ... 

Substrate cycling also provides a means of increasing the sensitivity and speed of metabolic regulation. The increased rate of glycolysis in response to a need for ATP for muscle contraction would imply a more or less instantaneous 1000-fold increase in phosphofructokinase activity if phosphofructokinase were inactive and fructose 1,6-bisphosphatase active. If there is moderate activity of phosphofructokinase, but greater activity of fructose 1,6-bisphosphatase, so that the metabolic flux is in the direction of gluconeogenesis, then a more modest increase in phosphofructokinase activity and decrease in fructose 1,6-bisphosphatase activity will achieve the same reversal of the direction of flux. 

A substrate cycle is produced by the simultaneous operation of opposing but chemically distinct reactions (32). For example, the enzymes 6-phosphofructokinase (PFK) and fructose bisphosphatase (FBPase) catalyze nonequilibrium and opposing reactions so that they can produce the fructose 6-phosphate/fructose bisphosphate substrate cycle, as follows ... 

The enzyme phosphofructokinase is allosteric, that is, it is made up of equivalent units that possess specific reaction sites for the fixation of the substrate and product. Each unit exists in two conformational states one active with more affinity for the substrate, and one inactive. The reaction products of phosphofructokinase (FDP and ADP) displace the conformational equilibrium in favor of the active form of the enzyme. This may create a destabilizing effect on the excess entropy production. In the glycolytic cycle, the allosteric properties of the phosphofructokinase may lead to oscillations. Consider the following simple model... 

Phosphofructokinase, the enzyme that catalyzes the third reaction in glycolysis, is a key regulatory enzyme in the pathway. ATP is an allosteric inhibitor of phosphofructokinase, whereas AMP and ADP are allosteric activators. Another allosteric inhibitor of phosphofructokinase is citrate. As we will see in the next chapter, citrate is the first intermediate in the citric acid cycle. The citric acid cycle is a pathway that results in the complete oxidation of the pyruvate produced by glycolysis. A high concentration of citrate signals that sufficient substrate is entering the citric acid cycle. The inhibition of phosphofructokinase by citrate is an example of feedback inhibition the product, citrate, allosterically inhibits the activity of an enzyme early in the pathway. 

---