Gluconeogenesis Study Notes

University of Waterloo - Faculty of Applied Health Sciences: Chapter 17 - Gluconeogenesis

Overview of Gluconeogenesis

• Definition: Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precursors.

  • Significance: Essential for maintaining blood glucose levels, especially during fasting or starvation.

  • Primary Site: Liver is the main gluconeogenic tissue, with some gluconeogenesis occurring in the kidneys.

  • Metabolic Importance: Major source of energy for the brain and the sole energy source for red blood cells.

Sources of Blood Glucose During a Normal Day

• Percentage Distribution:

  • Diet: Varies, but contributes significantly per meal (breakfast, lunch, dinner).

  • Gluconeogenesis: Plays a role when dietary intake is insufficient.

  • Glycogenolysis: Breakdown of glycogen to release glucose, but is limited in duration.

  

Non-Carbohydrate Precursors for Gluconeogenesis

1. Pyruvate:

   • Primarily formed from muscle-derived lactate via lactate dehydrogenase.

2. Amino Acids:

   • Carbon skeletons of some amino acids can be transformed into gluconeogenic intermediates.

3. Glycerol:

   • Derived from the hydrolysis of triacylglycerols and converted to dihydroxyacetone phosphate (DHAP) for gluconeogenesis or glycolysis.

   • Process includes:

     • Hydrolysis producing glycerol.

     • Conversion to glycerol phosphate and subsequently to DHAP.

Key Processes in Gluconeogenesis

• Gluconeogenic Pathway:

  • Describes the conversion of pyruvate back to glucose through a series of enzymatic steps that reverse glycolysis but with distinct enzymes for irreversible steps.

Enzymatic Steps Involved:

1. Pyruvate Carboxylase:

   • Converts pyruvate to oxaloacetate in the mitochondria.

   • Requires ATP and bicarbonate.

   • Reaction: $CO_2 + Pyruvate <br>ightarrow Oxaloacetate$ (involves biotin).

2. Phosphoenolpyruvate Carboxykinase (PEPCK):

   • Converts oxaloacetate to phosphoenolpyruvate (PEP) in the cytoplasm.

   • Uses GTP as phosphate donor, coupling with the decarboxylation reaction.

   • Reaction: $Oxaloacetate + GTP <br>ightarrow PEP + GDP + CO_2$.

3. Fructose 1,6-bisphosphatase:

   • Key regulatory enzyme, converting fructose 1,6-bisphosphate to fructose 6-phosphate.

   • Highly regulated and irreversible step.

   • Reaction: $Fructose 1,6-bisphosphate + H_2O <br>ightarrow Fructose 6-phosphate + Pi$.

4. Glucose 6-phosphatase:

   • Final step producing free glucose from glucose 6-phosphate.

   • Primarily found in the liver to allow for glucose export into the bloodstream.

   • Reaction: $Glucose 6-phosphate + H_2O <br>ightarrow Glucose + Pi$.

Energetics of Gluconeogenesis vs. Glycolysis

• Glycolysis requires energy:

  • Reaction: $Glucose + 2 ADP + 2 NAD^+ <br>ightarrow 2 Pyruvate + 2 ATP + 2 NADH + 2 H_2O$; $ΔG° = -90 kJ/mol$.

• Gluconeogenesis produces free glucose using energy:

  • Reaction: $2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 6 H<em>2O ightarrow Glucose + 4 ADP + 2 GDP + 6 P</em>i + 2 NAD^+$; $ΔG°' = -48 kJ/mol$.

Regulation of Glycolysis and Gluconeogenesis

• Reciprocal Regulation: One pathway is activated while the other is inhibited.

  • Principle: Prevents futile cycling and energy wastage.

  • Example: In conditions of high glucose, glycolysis is activated; in low glucose, gluconeogenesis is favored.

The Role of the Liver in Glucose Homeostasis

• Function: Liver converts lactate from tissues to glucose, supporting the body's glucose needs.

  • Cori Cycle: Describes the metabolic exchange where muscle generates lactate, which is then converted back to glucose by the liver.

  • Historical Note: Gerty and Carl Cori were awarded the Nobel Prize in 1947 for their research in this area.

  

Conclusion

• Gluconeogenesis is crucial for glucose homeostasis, especially during fasting. The liver plays a central role in metabolic regulation, and the enzymatic processes involved require careful control to ensure efficient energy use and avoidance of waste.