Gluconeogenesis

Production of Glucose from Non-Hexose Precursors

  • Occurs primarily in the liver and proximal tubules of the kidneys.

  • Importance of gluconeogenesis in maintaining blood glucose levels, so as not to solely rely on liver glycogen stores.

  • Liver glycogen lasts 12-24 hrs when fasting and resting

    • After 22 hr fasting, ~64% of glc in circulation was from gluconeogenesis

    • After 46 hr fasting, ~90% of glc in circulation was from gluconeogenesis

Gluconeogenesis

  • Definition: Gluconeogenesis is essentially a reversal of glycolysis, with three key enzymatic steps differing from glycolysis.

    • **Key Enzymatic Steps:

    1. Hexokinase vs. Glucose-6-phosphatase:

      • In glycolysis, hexokinase converts glucose to glucose-6-phosphate. In gluconeogenesis, glucose-6-phosphatase converts glucose-6-phosphate to glucose.

    2. Phosphofructokinase-1 vs. Fructose-1,6-bisphosphatase:

      • Phosphofructokinase-1 in glycolysis is replaced by fructose-1,6-bisphosphatase in gluconeogenesis to convert fructose-1,6-bisphosphate to fructose-6-phosphate.

    3. Pyruvate Kinase vs. Pyruvate Carboxylase and PEP Carboxykinase:

      • The pyruvate kinase reaction in glycolysis is reversed by the actions of pyruvate carboxylase and phosphoenolpyruvate (PEP) carboxykinase.

      • May involve an intermediary conversion to malate by malate dehydrogenase.

Enzymatic Reactions in Glycolysis and Gluconeogenesis

  • A complex interrelationship exists between glycolysis and gluconeogenesis, where different enzymes catalyze opposing reactions:

    • Key Reactants & Products:

    • Glucose in glycolysis is converted to glucose-6-phosphate through hexokinase (ATP is consumed).

    • In gluconeogenesis, glucose-6-phosphate is converted back to glucose using glucose-6-phosphatase.

    • Fructose-6-phosphate converts to fructose-1,6-bisphosphate via phosphofructokinase in glycolysis, while the reverse happens using fructose-1,6-bisphosphatase in gluconeogenesis.

    • Convert Glyceraldehyde-3-phosphate (G3P) to 1,3-bisphosphoglycerate, then to 3-phosphoglycerate, and so forth, until reaching glucose again.

    • Acetyl-CoA: A crucial substrate in gluconeogenesis that provides the necessary acetyl groups for glucose synthesis, although it cannot be converted back to glucose directly.

Substrates for Gluconeogenesis

  • Lactate:

    • Produced during anaerobic glycolysis, converting pyruvate to lactate via lactate dehydrogenase (LDH), which requires NADH and yields NAD+.

    • Lactate is released by muscles or red blood cells into the bloodstream, and converted back to glucose in the liver, which is then transported back to muscles.

  • Alanine:

    • Produced from the breakdown of proteins or during the transamination of amino acids, alanine can also be converted to pyruvate through alanine aminotransferase (ALT) in the liver, providing a substrate for gluconeogenesis.

    • All amino acids except 2, leucine and lysine - can be used for gluconeogenesis, as they can be converted into intermediates that enter the gluconeogenic pathway.

      • They’re purely ketogenic, meaning they can be used to make ketones

  • Glycerol:

    • Derived from lipolysis in adipose tissue, serves as a gluconeogenic substrate.

Directional Flow in Metabolism

  • Flow of gluconeogenic substrates during prolonged fasting:

    • Sources: Glycerol and fatty acids from adipose, amino acids from protein degradation, and lactate released by red blood cells.

    • Conversion to glucose occurs mainly in the liver and proximal renal tubules.

Regulation of Pyruvate Kinase

  • Pyruvate kinase is regulated differently in the liver versus other glycolytic tissues:

    • Inactive under certain conditions, such as the presence of ATP, acetyl-CoA, and long-chain fatty acids.

    • Active when ADP is present, indicating a requirement for energy production.

Regulation of Gluconeogenesis

  • Influenced by various hormones:

    • Glucagon: Increases gluconeogenesis, particularly in the liver.

    • Insulin: Acts oppositely to glucagon, inhibiting gluconeogenesis and stimulating glycolysis.

      • Glc-6-P → Fruc-6-P with PFK-1 (activated with F2,6BP)→ Fruc 1,6 BP

        • Bypassed in gluconeogenesis

    • Cortisol (Glucocorticoid): Promotes gluconeogenesis.

      • In gluconeogenesis, Fruc 1,6 BP with FBPase-1 (inhibited by FBPase-1)→ Fruc-6-P → Glc-6-P

    • Thyroid hormones and several transcription factors like CREB, FoxO1, etc.

    • Enzyme that makes F2,6BP is phosphofructokinase-2 (PFK-2) (6PF-2K), which regulates the levels of fructose 2,6-bisphosphate

    • Enzyme that makes F2,6BP into Fruc-1-P is fructose bisphosphatase-2 (FBPase-2) [F2,6-Pase]

      • It removes a phosphate group, converting fructose 2,6-bisphosphate into fructose 6-phosphate

    • Increased F2,6BP = increased stimulation of glycolysis and inhibition of gluconeogenesis, facilitating the conversion of glucose to pyruvate.

    • Decreased F2,6BP = decreased stimulation of glycolysis and increased gluconeogenesis, promoting the generation of glucose from non-carbohydrate sources.

  • Protein Kinase A is activated by glucagon or epinephrine

    • Low glucose, glucagon epinephrine activates protein kinase a, which in turn catalyzes the phosphorylation of key enzymes involved in gluconeogenesis, enhancing the liver's ability to synthesize glucose.

      • Phosphate is added to biphasic regulation of fructose-1,6-bisphosphate, thereby promoting the conversion of pyruvate to phosphoenolpyruvate.

  • Thyroid-stimulating hormone

    • also plays a role in metabolic regulation, influencing gluconeogenesis through its effect on basal metabolic rate and aiding in the overall homeostasis of blood glucose levels.

  • Thyroid Hormone

    • increases the rate of gluconeogenesis by stimulating the expression of key enzymes, such as phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase, which are crucial for glucose production.

  • CREB, FoxO1, CREBH, C/EBPs, NRS

    • transcription factors also modulate gluconeogenesis by regulating the expression of these enzymes, thereby impacting glucose metabolism under various physiological conditions.

Glucose Production Data

  • Study data from Ahlborg et al. (1974) presents glucose production during rest and exercise:

    • Overall Glucose Production (mmol/min):

    • Rest: 0.82

    • During exercise (40 min): 1.86

    • Post-exercise (240 min): 1.46

    • Uptake of Glucogenic Nutrients:

    • Lactate: 0.13, 0.23, 0.29 mmol/min

    • Pyruvate: 0.01, 0.02, 0.03 mmol/min

    • Glycerol: 0.02, 0.10, 0.21 mmol/min

    • Amino Acids: 0.05, 0.09, 0.12 mmol/min

    • Percent contribution to overall glucose production varies, peaking at 45% in prolonged fasting conditions.

Energy cost of gluconeogenesis?