Glycolysis Glucose phosphatase

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... 

Glucose 6-phosphate can then be metabolized by glycolysis in the liver or muscle, or it can be dephosphorylated by the action of glucose 6-phosphatase mainly in the liver and released into the bloodstream for use by other tissues of the body. 

H. The initial irreversible step of glycolysis is bypassed by glucose 6-phosphatase, which catalyzes the dephosphorylation of glucose 6-phosphate to form glucose (Figure 6-8). 

Gluconeogenesis uses seven of the reactions in glycolysis, but three are replaced by the sum of the pyruvate carboxylase and phosphoeno/pyruvate carboxykinase reactions, the fructose 1,6-biphosphatase reaction, and the glucose 6-phosphatase reaction. Tables 4.7, 4.8, and 4.9 give the thermodynamic properties of these reactions and the net reaction for gluconeogenesis. 

Pyruvate is converted to oxaloacetate (by pyruvate carboxylase). The oxaloacetate is decarboxylated and phosphorylated to phosphoenolpyruvate (PEP) by phosphoenolpyruvate carboxykinase (PEP carboxykinase). PEP is converted to fructose 1,6-bisphosphate by a direct reversal of several reactions in glycolysis. Next, fructose 1,6-bisphosphate is dephosphorylated to fructose 6-phosphate (by fructose 1,6-bisphosphatase) and this is then converted to glucose 6-phosphate (by phosphoglucoisomerase). Finally, glucose 6-phosphate is dephosphorylated (by glucose 6-phosphatase) to yield glucose. 

Fig. 1. Comparison of gluconeogenesis and glycolysis. The three steps of glycolysis that are irreversible are numbered. (1) Flexokinase in glycolysis is reversed by glucose 6-phosphatase in gluconeogenesis (2) PFK in glycolysis is reversed by fructose 1,6-bisphosphatase in gluconeogenesis (3) pyruvate kinase in glycolysis is reversed by two sequential reactions in gluconeogenesis catalyzed by pyruvate carboxylase and PEP carboxykinase.

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