Phosphorylation of fructose-6-phosphate

Fructose 2,6-bisphosphate is formed by phosphorylation of fructose 6-phosphate by phosphofructoki-nase-2. The same enzyme protein is also responsible for its breakdown, since it has fructose-2,6-hisphos-phatase activity. This hifrmctional enzyme is under the allosteric control of fructose 6-phosphate, which stimulates the kinase and inhibits the phosphatase. Hence, when glucose is abundant, the concentration of fructose 2,6-bisphosphate increases, stimulating glycolysis by activating phosphofructokinase-1 and inhibiting... 

The enzyme catalyses the phosphorylation of fructose 6-phosphate to form fructose 2,6-bisphosphate. 

A typical chemical system is the oxidative decarboxylation of malonic acid catalyzed by cerium ions and bromine, the so-called Zhabotinsky reaction this reaction in a given domain leads to the evolution of sustained oscillations and chemical waves. Furthermore, these states have been observed in a number of enzyme systems. The simplest case is the reaction catalyzed by the enzyme peroxidase. The reaction kinetics display either steady states, bistability, or oscillations. A more complex system is the ubiquitous process of glycolysis catalyzed by a sequence of coordinated enzyme reactions. In a given domain the process readily exhibits continuous oscillations of chemical concentrations and fluxes, which can be recorded by spectroscopic and electrometric techniques. The source of the periodicity is the enzyme phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate by ATP, resulting in the formation of fructose-1,6 biphosphate and ADP. The overall activity of the octameric enzyme is described by an allosteric model with fructose-6-phosphate, ATP, and AMP as controlling ligands. 

Phosphorylation of Fructose 6-Phosphate to Fructose 1,6-Bisphosphate In the second of the two priming reactions of glycolysis, phosphofructokinase-1 (PFK-1) catalyzes the transfer of a phosphoryl group from ATP to fructose 6-phosphate to yield fructose 1,6-bisphos-phate ... 

The most important control element is the allosteric enzyme phosphofructoki-nase. It catalyzes the phosphorylation of fructose 6-phosphate to fructose 1,6-diphosphate,... 

Fructose-1,6-bisphosphate is converted to fructose-6-phos-phate by hydrolysis of the phosphoryl ester bond at C-l in a reaction catalyzed by fructose bisphosphate phosphatase. The standard free energy change for this reaction is about —4 keal/mol, corresponding to an equilibrium constant of about 103. Thus, the two conversions (the phosphorylation of fructose-6-phosphate to form fructose 1,6-bisphosphate with ATP as the phosphate donor, and the hydrolysis of fructose-bisphosphate to form fructose-6-phosphate) are both thermodynamically favored under any conditions that are likely to exist in a living cell. These two reactions constitute a pseudocycle and, consistent with the principles enunciated in the previous chapter, the pathways have evolved so the number of ATP-to-ADP conversions is greater in one direction than in the other. 

In vitro,d an uncoupled phosphorylation of fructose-6-phosphate to form the fructose-1,6-bisphosphate has a positive ArG° but in vivo, it is coupled with the ATP-ADP hydrolysis through the enzyme, phosphofructokinase, to yield an overall negative reaction free energy. 

Phosphorylation of glucose Phosphorylation of fructose 6-phosphate Dephosphorylation of 2 molecules of 1,3-BPG Dephosphorylation of 2 molecules of phosphoenolpyruvate... 

A pair of reactions such as the phosphorylation of fructose 6-phosphate to fructose 1,6-bisphosphate and its hydrolysis back to fructose 6-phosphate is called a substrate cycle. As already mentioned, both reactions are not simultaneously fully active in most cells, because of reciprocal allosteric controls. However, isotope-labeling studies have shown that some fructose 6-phosphate is phosphorylated to fructose 1,6-bisphosphate even during gluconeogenesis. There also is a limited degree of cycling in other pairs of opposed irreversible reactions. This cycling was regarded as an imperfection in metabolic control, and so substrate cycles have sometimes been called... 

Pyrnvate kinase catalyzes one of the three irreversible steps in the glycolytic pathway, the others being the phosphorylation of glucose to glucose 6-phosphate and the phosphorylation of fructose 6-phosphate to fructose 2,6-bisphosphate. 

Phosphofructokinase. This enzyme is traditionally thought of as the first enzyme that commits glucose metabolism to the glycolytic pathway. This enzyme requires ATP and Mg2+, as was the case with hexokinase, and catalyzes the phosphorylation of fructose-6-phosphate in the number one position, forming fructose-1,6-bisphosphate. 

The second glycolytic reaction that cannot participate in gluconeogenesis is the phosphorylation of fructose 6-phosphate by PFK-1 (Table 14-2, step (3)). Because this reaction is highly exergonic and therefore irreversible in intact cells, the generation of fructose 6-phosphate from fructose 1,6-bisphosphate (Fig. 14-16) is catalyzed by a different enzyme, Mg -dependent fructose 1,6-bisphosphatase (FBPase-1), which promotes the essentially irreversible hydrolysis of the C-1 phosphate (not phosphoryl group transfer to ADP) ... 

As in the reaction in Step 1, the endei onic reaction of phosphorylation of fructose-6-phosphate is coupled to the exergonic reaction of hydrolysis of ATP, and the overall reaction is exergonic. See Table 17.1. 

The hydrolysis of fructose-1,6-Msphosphate is a strongly exergonic reaction. The reverse reaction in glycolysis, phosphorylation of fructose-6-phosphate, is irreversible because of the energy supplied by ATP hydrolysis. 

The glycolytic pathway down to pyruvate involves 10 enzyme-catalyzed steps with three kinases that use ATP (Fig. 11-32). The reaction catalyzed by phosphofructokinase (PFK) plays the major part in controlling the overall flux of the whole pathway. It catalyzes the phosphorylation of fructose 6-phosphate (Fru6P) to fructose 1,6-bisphosphate (Frul6P2). Hexokinase and pyruvate kinase (PK) have relatively higher maximal velocities. PFK is the target for more effector molecules than the other two kinases (see Fig. 11-14). 

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