gluconeogenesis

1. Introduction to Gluconeogenesis

• Definition: Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactate, glycerol, and glucogenic amino acids.

• Purpose: It is a critical pathway for maintaining blood glucose levels, especially during fasting, starvation, or prolonged exercise, when carbohydrate intake is insufficient.

• Primary Sites: The primary site of gluconeogenesis is the liver ($90\%$). The kidneys also contribute significantly ($10\%$), especially during prolonged fasting.

2. Substrates for Gluconeogenesis

• Lactate: Produced during anaerobic glycolysis in muscle and red blood cells. It's converted to pyruvate by lactate dehydrogenase.

• Glycerol: Derived from the hydrolysis of triglycerides in adipose tissue. It enters gluconeogenesis at the level of dihydroxyacetone phosphate (DHAP).

• Glucogenic Amino Acids: All amino acids except leucine and lysine can be converted to intermediates of the citric acid cycle or pyruvate.

• Propionyl-CoA: Derived from odd-chain fatty acids and some amino acids, it enters gluconeogenesis via succinyl-CoA.

3. Key Enzymes Bypassing Irreversible Glycolysis Steps

Gluconeogenesis is essentially the reverse of glycolysis, but it must bypass three irreversible steps of glycolysis using unique enzymes:

1. Pyruvate to Phosphoenolpyruvate (PEP):

   • Pyruvate Carboxylase: Converts pyruvate to oxaloacetate (OAA) in the mitochondrial matrix. This step requires ATP and $CO_2$ (biotin cofactor).

   • PEP Carboxykinase (PEPCK): Converts OAA to PEP. This enzyme exists in both mitochondrial and cytosolic forms, varying by species.

2. Fructose-1,6-bisphosphate to Fructose-6-phosphate:

   • Fructose-1,6-bisphosphatase: Hydrolyzes fructose-1,6-bisphosphate to fructose-6-phosphate, releasing inorganic phosphate ($P_i$).

3. Glucose-6-phosphate to Glucose:

   • Glucose-6-phosphatase: Hydrolyzes glucose-6-phosphate to free glucose. This enzyme is primarily found in the liver and kidney, explaining why these are the main gluconeogenic organs.

4. Overview of the Gluconeogenesis Pathway

1. Mitochondrial Reactions:

   • Pyruvate, from lactate or amino acids, enters the mitochondria.

   • Pyruvate Carboxylase converts pyruvate to oxaloacetate (OAA).

   • OAA is converted to malate (by malate dehydrogenase) to be transported out of the mitochondria into the cytosol, or in some cases, OAA is directly converted to PEP if mitochondrial PEPCK is present.

2. Cytosolic Reactions:

   • Malate is converted back to OAA in the cytosol.

   • PEP Carboxykinase converts OAA to PEP.

   • PEP then proceeds through a series of reversible reactions, essentially the reverse of glycolysis, to form fructose-1,6-bisphosphate.

   • Fructose-1,6-bisphosphatase converts fructose-1,6-bisphosphate to fructose-6-phosphate.

   • Fructose-6-phosphate is isomerized to glucose-6-phosphate.

3. Endoplasmic Reticulum (ER) Reaction (for final glucose release):

   • Glucose-6-phosphate is transported into the ER lumen.

   • Glucose-6-phosphatase hydrolyzes glucose-6-phosphate to glucose.

   • Free glucose is then transported out of the ER into the cytosol and subsequently released into the bloodstream.

5. Energy Requirements

Gluconeogenesis is an energy-intensive process. The synthesis of one molecule of glucose from two molecules of pyruvate requires:

• $4\ ATP$

• $2\ GTP$

• $2\ NADH$

6. Regulation

• Reciprocal Regulation with Glycolysis: When glycolysis is active, gluconeogenesis is inhibited, and vice versa. Key regulatory points include the irreversible steps.

• Hormonal Control:

  • Glucagon (fasting state): Promotes gluconeogenesis by increasing the activity of gluconeogenic enzymes (e.g., PEPCK) and inhibiting glycolytic enzymes (e.g., PFK-2).

  • Insulin (fed state): Inhibits gluconeogenesis by decreasing the expression and activity of gluconeogenic enzymes and promoting glycolysis.

  • Cortisol: Stimulates gluconeogenesis, particularly by increasing the breakdown of muscle proteins to provide amino acids as substrates.

• Allosteric Regulation:

  • Fructose-2,6-bisphosphate: This molecule strongly activates phosphofructokinase-1 (PFK-1) in glycolysis and inhibits fructose-1,6-bisphosphatase in gluconeogenesis.

  • Acetyl-CoA: Activates pyruvate carboxylase (promoting gluconeogenesis) and inhibits pyruvate dehydrogenase (inhibiting glycolysis).

  • AMP: Activates PFK-1 (glycolysis) and inhibits fructose-1,6-bisphosphatase (gluconeogenesis).