Gluconeogenesis Study Notes

Gluconeogenesis Overview

  • Definition: Gluconeogenesis is the process of generating glucose from non-carbohydrate precursors when blood glucose levels are low (hypoglycemia).

  • Precursors:

    • Lactate
    • Glucogenic amino acids (especially alanine)
    • Glycerol (from triglycerides)

  • Primary Site:

    • Liver (main site of gluconeogenesis)
    • Kidneys and small intestines are also capable but function locally.

Pathway Characteristics

  • Reverse Glycolysis:
    • Gluconeogenesis is essentially the reverse of the glycolytic pathway but includes three bypass steps for irreversible glycolytic reactions.

Bypass Enzymes

  • Bypass 1: Pyruvate to Phosphoenolpyruvate (PEP)

    • Pyruvate carboxylase converts pyruvate to oxaloacetate (OAA) in the mitochondria.
    • OAA is then converted to malate (MDH), transported out to the cytosol, and converted back to OAA.
    • Finally, OAA is converted to PEP by Phosphoenolpyruvate carboxykinase (PEPCK).
    • Energy Cost: 2 ATP per molecule of pyruvate.

  • Bypass 2: Fructose-1,6-bisphosphate (F-1,6-BP) to Fructose-6-phosphate (F6P)

    • F-1,6-BP is converted to F6P by Fructose-1,6-bisphosphatase (F-1,6-BPase).

  • Bypass 3: Glucose-6-phosphate (G6P) to Glucose

    • G6P is converted to glucose by Glucose-6-phosphatase in the smooth endoplasmic reticulum. Only the liver and kidney can transport glucose back to blood.

The Glucose-Alanine Cycle

  • Muscle tissue can convert amino acids like alanine to glucose via the Glucose-Alanine cycle, contributing to gluconeogenesis.

Regulation of Gluconeogenesis

  • Allosteric Regulation:

    • Negative effectors of glycolysis can enhance gluconeogenesis.
    • E.g., ATP acts as an inhibitor for glycolysis pathway enzymes while enhancing gluconeogenesis.
    • Acetyl CoA activates pyruvate carboxylase (Bypass 1).
    • Fructose-2,6-bisphosphate inhibits F-1,6-BPase (Bypass 2).

  • Hormonal Regulation:

    • Activating Hormones: Glucagon and Epinephrine (increase cAMP, enhancing gluconeogenesis)
    • Inhibiting Hormones: Insulin (lowers cAMP, inhibits gluconeogenesis).

Other Sources of Glucose in Gluconeogenesis

  • Triglycerides and Glycerol:
    • Triglycerides can be converted to glycerol and subsequently phosphorylated to form glycerol-3-phosphate, which can enter the gluconeogenic pathway as DHAP, requiring 1 ATP.

Summary of Key Terms

  • G6P: Glucose-6-Phosphate
  • F-1,6-BP: Fructose-1,6-Bisphosphate
  • PEP: Phosphoenolpyruvate
  • Alanine: An important amino acid in gluconeogenesis, specifically involved in the glucose-alanine cycle.
  • cAMP: Cyclic Adenosine Monophosphate, important in hormonal regulation of gluconeogenesis.