Gluconeogenesis: Synthesis and Regulation

• Gluconeogenesis is a metabolic process synthesizing glucose from non-carbohydrate precursors.
• It mainly occurs in the liver and, to a lesser extent, in the kidneys.
• This process is vital for maintaining blood glucose levels during fasting, starvation, and intense exercise.
• It ensures a continuous energy supply to the brain and other glucose-dependent tissues.

Precursors for Gluconeogenesis

• Main precursors include:
  • Lactate: Produced by anaerobic glycolysis in muscles and erythrocytes.
  • Glycerol: Released from the breakdown of triglycerides in adipose tissue.
  • Amino acids: Derived from protein catabolism, especially alanine.
  • Propionate: Produced during odd-chain fatty acid metabolism.

Key Enzymes and Reactions

• Gluconeogenesis shares enzymes with glycolysis but bypasses three irreversible steps using different enzymes.
• Irreversible steps in glycolysis and their corresponding enzymes in gluconeogenesis are:
  • Hexokinase → Glucose-6-Phosphatase
    • Hexokinase phosphorylates glucose to glucose-6-phosphate.
    • Glucose-6-phosphatase, in the endoplasmic reticulum of liver and kidney cells, hydrolyzes glucose-6-phosphate to glucose.
    • Muscles and the brain lack this enzyme, preventing them from releasing glucose into the bloodstream.
  • Phosphofructokinase-1 (PFK-1) → Fructose-1,6-Bisphosphatase (FBPase-1)
    • PFK-1 phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate.
    • FBPase-1 hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate.
    • AMP and fructose-2,6-bisphosphate inhibit this step, while ATP activates it.
  • Pyruvate Kinase → Pyruvate Carboxylase and Phosphoenolpyruvate Carboxykinase (PEPCK)
    • Pyruvate kinase converts phosphoenolpyruvate (PEP) to pyruvate.
    • The bypass involves two enzymes.
    • Pyruvate carboxylase converts pyruvate to oxaloacetate in the mitochondria, using biotin as a cofactor.
    • PEPCK converts oxaloacetate to PEP, using GTP, and occurs in the mitochondria or cytosol, depending on the species.

Steps of Gluconeogenesis

• Conversion of Pyruvate to Phosphoenolpyruvate (PEP)

  • Pyruvate is carboxylated to oxaloacetate by pyruvate carboxylase in the mitochondrial matrix.
  • Oxaloacetate is converted to PEP by PEPCK.
  • The conversion of oxaloacetate to PEP occurs in the mitochondria or cytosol, depending on the PEPCK isoform.
  • Transport of oxaloacetate from the mitochondria to the cytosol involves converting it to malate or aspartate.

• Formation of Fructose-6-Phosphate

  • Fructose-1,6-bisphosphate is dephosphorylated to fructose-6-phosphate by fructose-1,6-bisphosphatase.
  • This is an irreversible, major regulatory step.

• Formation of Glucose

  • Glucose-6-phosphate is dephosphorylated to glucose by glucose-6-phosphatase in the endoplasmic reticulum.
  • The liver and kidney contain this enzyme, enabling these organs to release glucose into the bloodstream.

Regulation of Gluconeogenesis

• Gluconeogenesis is tightly regulated to maintain blood glucose homeostasis.

• Regulation occurs through:

  • Allosteric regulation of key enzymes.
  • Hormonal control via insulin and glucagon.
  • Substrate availability.

• Allosteric Regulation

  • Fructose-1,6-bisphosphatase is inhibited by AMP and fructose-2,6-bisphosphate, indicating high energy charge and abundant glucose.
  • Pyruvate carboxylase is activated by acetyl-CoA, signaling the need to process pyruvate.
  • PFK-1 and FBPase-1 are reciprocally regulated by fructose-2,6-bisphosphate.
    • High fructose-2,6-bisphosphate levels stimulate glycolysis and inhibit gluconeogenesis.
    • Low levels stimulate gluconeogenesis and inhibit glycolysis.

• Hormonal Control

  • Insulin inhibits gluconeogenesis by:
    • Reducing the expression of PEPCK and glucose-6-phosphatase genes.
    • Stimulating glycolysis, which decreases gluconeogenic precursor levels.
  • Glucagon stimulates gluconeogenesis by:
    • Increasing the expression of PEPCK and glucose-6-phosphatase genes.
    • Inhibiting glycolysis by decreasing fructose-2,6-bisphosphate levels.

• Substrate Availability

  • Lactate, glycerol, and amino acid availability affects gluconeogenesis rate.
  • High concentrations of these substrates promote gluconeogenesis.

Energetics of Gluconeogenesis

• More ATP and GTP is requires than is produced by glycolysis making gluconeogenesis is energy-consuming
• Net reaction from pyruvate:
  • 2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 6 H₂O → Glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD⁺

Significance of Gluconeogenesis

• Maintenance of Blood Glucose Levels:
  • Prevents hypoglycemia during fasting, starvation, and prolonged exercise.
  • Ensures a continuous glucose supply to the brain.
• Regulation of Acid-Base Balance:
  • Consumes protons, helping maintain blood pH.
• Detoxification of Lactate:
  • Converts lactate produced in muscles back to glucose in the liver (Cori cycle).
• Metabolic Interconnections:
  • Interconnected with the urea cycle, fatty acid metabolism, and amino acid metabolism.

Explore gluconeogenesis, the metabolic process of synthesizing glucose from non-carbohydrate precursors. Learn about its occurrence in the liver and kidneys. Understand the vital role it plays in maintaining blood glucose levels during fasting, starvation, and exercise.