Fructose 1,6-bisphosphatase substrate cycle

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

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 ... 

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. 

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. 

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

The carbons of PEP reverse the steps of glycolysis, forming fmctose 1,6-bisphos-phate. Fructose 1,6-bisphosphatase acts on this bisphosphate to release inorganic phosphate and prodnce fmctose 6-phosphate. A futile substrate cycle is prevented at... 

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. 

Since is positive, cycling will increase the sensitivity of the flux to regulator X provided that s j>r ,), that is, if the sensitivity of the subsequent reaction of the system to the concentration of the product, P, is greater than the sensitivity of the reverse reaction of the cycle to the concentration of P. This situation can be met in several ways (9). For example, in the fructose 6-phosphate/fructose bisphosphate cycle, fructose bisphosphatase (equivalent to C above) has aK for fructose 1,6-bis-phosphate of about 1 pM (or at least two orders of magnitude lower than many other glycolytic enzymes for their glycolytic substrates) so that it is probably saturated with fructose bisphosphate in vivo (equivalent to P in... 

Figure 20-h Flow chart of pentose phosphate pathway and its connections with the pathway of glycolysis. The full pathway, as indicated, consists of three interconnected cycles in which glucose 6-phosphate is both substrate and end product. The reactions above the broken line are nonreversible, whereas all reactions under that line are freely reversible apart from that catalyzed by fructose-1,6-bisphosphatase. 

Phosphatases specific for such substrates as glucose-6-phosphate, fructose-l,6-bisphosphate, and phospho-glycolate help to drive metabolic cycles (Chapter 17). The 335-residue fructose-1,6-bisphosphatase associates to form a tetramer with D2 symmetry. ° The allosteric enzyme exists in two conformational states (see Chapter 11). Activity is dependent upon Mg + or other suitable divalent cation, e.g., Mn or Zn, and is further enhanced by K+ or NH3. While the dimetal sites depicted in Figs. 12-23 and 12-24 are quite rigid and undergo little change upon formation of complexes with substrates or products, the active site of fructose-1,6-bisphosphatase is more flexible. There are three metal-binding sites but they contain no histidine side chains and have been seen clearly only in a product complex. Perhaps because of the need for... 

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