Pyruvate to Phosphoenolpyruvate Requires Two High Energy Phosphates

Conversion of Pyruvate to Phosphoenolpyruvate Requires Two High Energy Phosphates 

The first step in the gluconeogenic direction involves the formation of phosphoenolpyruvate from pyruvate. Reversal of the pyruvate kinase reaction requires at least two ATP-to-ADP conversions. One means by which this is done is shown in figure 12.26. 

The first step in the conversion of pyruvate to PEP entails the carboxylation of pyruvate to form oxaloacetate. This reaction is catalyzed by pyruvate carboxylase. As in 

The carboxylation of pyruvate supplies a significant portion of the thermodynamic push for the next step in the sequence. This is because the free energy change for decarboxylation of /3-keto carboxylic acids such as oxaloacetate is large and negative. The oxaloacetate formed from pyruvate by carboxylation is converted to phosphoenolpyruvate in a reaction catalyzed by phosphoenolpyruvate carboxyki-nase. In many species, including mammals, this reaction involves a GTP-to-GDP conversion. 

The use of GTP, UTP or CTP in a metabolic reaction or sequence is energetically equivalent to the use of ATP, because the nucleoside diphosphate that is produced is rephos-phorylated at the expense of ATP in the reaction catalyzed by nucleoside diphosphate (NDP) kinase. 

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Gluconeogenesis Consumes ATP Conversion of Pyruvate to Phosphoenolpyruvate Requires Two High Energy Phosphates Conversion of Phosphoenolpyruvate to Fructose-1,6-bisphosphate Uses the Same Enzymes as Glycolysis... 

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