Gluconeogenesis

• Gluconeogenesis is the synthesis of glucose from pyruvate (non-carbohydrate)
• The major site of gluconeogenesis is the liver, with a small amount taking place in the kidney.
• Noncarbohydrate precursors of glucose are first converted into pyruvate or enter the pathway at later intermediates. The major noncarbohydrate precursors are lactate, amino acids, and glycerol.

  • Lactate is converted into pyruvate by lactate dehydrogenase in the liver
  • The hydrolysis of triacylglycerls yields glycerol and fatty acids. Glycerol is a precursor of glucose but can also be metabolized by glycolysis
• What is the first step in gluconeogenesis?

  The carboxylation of pyruvate to form oxaloacetate using ATP by pyruvate carboxylase in the mitochondria.
• What cofactor is required by pyruvate carboxylase?

  Biotin is a covalently attached prosthetic group that serves as the carrier of activated CO2.
• What are the three stages of carboxylation of pyruvate?

  
• Oxaloacetate is shuttled into the cytoplasm and converted into phosphoenolpyruvate.
• What does phosphoenolpyruvate carboxykinase do?

  Decarboxylates and phosphorylates oxaloacetate to generate phosphoenolpyruvate by hydrolyzing GTP.
• Decarboxylations often drive reactions that are otherwise highly endergonic.
• Adding a phosphoryl group to oxaloacetate is highly unfavourable so decarboxylation powers the addition of the phosphoryl group
• Why is a carboxylation and a decarboxylation required to

  form phosphoenolpyruvate from pyruvate?


  The presence of a phosphoryl group traps the unstable enol form of pyruvate as phosphoenolpyruvate.
• What is the the first irreversible step and its significance?

  The reactions that catalyze the formation of glucose are near equilibrium at intracellular conditions, so when conditions favour gluconeogenesis, the reverse reactions will take place until the next irreversibly step is reached.

  The first irreversible step is the hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate and Pi catalyzed by fructose 1,6-bisphosphatase
• What does fructose 1,6-bisphosphatase do?


  The first irreversible step is the hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate and Pi which is catalyzed by fructose 1,6-bisphosphatase
• Free glucose is not generated in the cytoplasm, where and how is it formed?

  Glucose 6- phosphate is transported into the lumen of the endoplasmic reticulum of liver cells, where it is hydrolyzed to glucose by glucose 6-phosphatase bound to the lumen side of the ER membrane.
• pyruvate is converted to oxaloacetate in the mitochondria
• how are gluconeogenesis and glycolysis reciprocally regulated when energy is needed?

  Energy charge determines which process is more active.

  If energy is needed, the concentration of AMP is high which stimulates phosphofructokinase but inhibits fructose 1,6-bisphosphatase (converts fructose 1,6-bisphosphate to fructose 6-phosphate in glucogenesis). Subsequently, glycolysis is turned on while glucogenesis is turned off.
• What happens when energy is not needed?

  Energy charge determines which process is more active.

  If energy is not needed, the concentration of citrate and ATP is high which inhibits phosphofructokinase while citrate activates fructose 1,6-bisphosphatase (to make fructose 6-phosphate). Glycolysis is inhibited and glucogenesis promoted
• How are glycolysis and gluconeogenesis regulated at the liver?

  When energy charge is high, ATP and alanine are also high both of which inhibit pyruvate kinase to slow glycolysis. Acetyl CoA indicates that the citric acid cycle is producing energy and activates pyruvate carboxylase to promote gluconeogenesis.

  When energy charge is low, ADP is high which inhibits pyruvate carboxylase and phosphoenolpyruvate carboxykinase.
• What does pyruvate carboxylase do?

  Adds a carboxyl group to pyruvate to form oxaloacetate.
• Fructose 2,6-bisphosphate is also an inhibitor of fructose 1,6-bisphosphatase. So when blood glucose is low, fructose 2,6-bisphosphate is dephosphorylated to form fructose 6-phosphate which no longer activates phosphofructokinase