Gluconeogenesis Overview

Gluconeogenesis Overview

• Conversion of lactate to glucose via gluconeogenesis utilizes glycolytic reactions in reverse.
• Occurs primarily in the liver; also in kidneys and small intestine under specific conditions.
• This pathway maintains blood glucose levels during fasting or low carbohydrate intake.

Energy Requirements and Bypasses

• Gluconeogenesis requires energy input, specifically ATP.
• Three irreversible glycolysis reactions must be bypassed:
  • Hexokinase (or Glucokinase)
  • Phosphofructokinase
  • Pyruvate Kinase
• Two bypasses involve hydrolysis reactions, utilizing enzymes like glucose-6-phosphatase and fructose-1,6-bisphosphatase.

Key Enzymatic Reactions

• Hexokinase/Glucokinase: Converts glucose + ATP to glucose-6-phosphate + ADP.
• Glucose-6-Phosphatase: Converts glucose-6-phosphate + H2O to glucose + Pi.
• Phosphofructokinase: Converts fructose-6-phosphate + ATP to fructose-1,6-bisphosphate + ADP.
• Fructose-1,6-bisphosphatase: Converts fructose-1,6-bisphosphate + H2O to fructose-6-phosphate + Pi.
• Pyruvate Carboxylase: Converts pyruvate + HCO3- + ATP to oxaloacetate + ADP + Pi.
• PEP Carboxykinase: Converts oxaloacetate + GTP to PEP + GDP + CO2.

Substrates and Inputs

• Major sources for gluconeogenesis during fasting include amino acid catabolism and glycerol from fat breakdown.
• Amino acids converted to pyruvate, oxaloacetate, or precursors for gluconeogenesis, especially from muscle.

Regulation Mechanisms

• Glycolysis and gluconeogenesis are regulated to prevent futile cycles, saving energy.
• Local Regulation: Reciprocal allosteric regulation by AMP and ATP.
• Phosphofructokinase is inhibited by ATP, activated by AMP; Fructose-1,6-bisphosphatase is inhibited by AMP.
• Global Regulation: The hormone glucagon triggers a cAMP cascade in liver, inhibiting glycolysis while stimulating gluconeogenesis.
• Protein Kinase A activates gluconeogenesis enzymes and inhibits glycolysis enzymes such as pyruvate kinase.

Cori Cycle

• Explains glucose consumption by muscles and the lactate production process.
• Lactate is transported to the liver for glucose synthesis through gluconeogenesis.

Summary of Energy Transactions

• Glycolysis yields 2 ATP, while gluconeogenesis expends 6 ATP (net expended ~P of 4 in the process).
• Glycolysis: glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 pyruvate + 2 NADH + 2 ATP.
• Gluconeogenesis: 2 pyruvate + 2 NADH + 4 ATP + 2 GTP → glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi.

Interplay of Metabolic Pathways

• Glycolysis and gluconeogenesis are spontaneity driven; conditions ensure that they don't operate simultaneously.
• Fructose-2,6-bisphosphate functions as a key regulator, favoring glycolysis when glucose is abundant and inhibiting gluconeogenesis.

Gluconeogenesis Overview

• Converts lactate to glucose, primarily in the liver.
• Also occurs in kidneys and small intestine under specific conditions.
• Utilizes some glycolytic reactions in reverse, alongside unique reactions for synthesizing glucose.
• Requires energy input in the form of ATP.
• Maintains glucose levels in blood even without dietary intake, supplying fast-twitch muscle fibers.

Key Enzymes and Bypass Reactions

• Key Glycolysis Enzymes: Hexokinase, Phosphofructokinase, Pyruvate Kinase – these have large negative Gibbs free energy values, making them essentially irreversible.
• Gluconeogenesis must bypass these steps using different enzymes:
  • Hexokinase/Glucokinase vs. Glucose-6-phosphatase: Converts glucose-6-phosphate to glucose.
  • Phosphofructokinase vs. Fructose-1,6-bisphosphatase: Converts fructose-1,6-bisphosphate to fructose-6-phosphate.
  • Pyruvate Kinase vs. Pyruvate Carboxylase and PEP Carboxykinase: Converts pyruvate to phosphoenolpyruvate (PEP).

Enzyme Details

• Glucose-6-Phosphatase: Found in the endoplasmic reticulum of liver cells, hydrolyzes glucose-6-phosphate to glucose.
• Fructose-1,6-bisphosphatase: Catalyzes conversion of fructose-1,6-bisphosphate to fructose-6-phosphate.
• Pyruvate Carboxylase and PEP Carboxykinase: Work together to convert pyruvate to oxaloacetate, then to PEP in a two-step process requiring ATP and GTP.

Energy Dynamics

• Gluconeogenesis costs 6 high-energy phosphate bonds (4 ATP, 2 GTP) to produce glucose from 2 pyruvate.
• Glycolysis yields 2 high-energy phosphates.
• If both pathways were active simultaneously, they would waste 4 high-energy phosphates per cycle, termed a "futile cycle".

Regulation Mechanisms

• Local Control: Reciprocal allosteric regulation by adenine nucleotides; ATP inhibits phosphofructokinase but stimulates fructose-1,6-bisphosphatase, ensuring energy conservation.
• Global Control:
  • Hormonal regulation by glucagon during low blood glucose levels via a cAMP cascade.
  • Protein Kinase A phosphorylates enzymes, inhibiting glycolysis and stimulating gluconeogenesis.
  • Enhances transcription of PEP Carboxykinase to increase gluconeogenesis.

Substrates for Gluconeogenesis

• Major substrates include:
  • Amino acids, mainly derived from muscle protein catabolism.
  • Glycerol from triglyceride breakdown in adipose tissue.
• Oxaloacetate depletion prevents acetyl CoA from entering the Krebs Cycle, crucial for gluconeogenesis during fasting.

Summary of Reaction Pathways

• Glycolysis: Converts glucose to pyruvate, yielding 2 ATP and 2 NADH.
• Gluconeogenesis: Converts 2 pyruvate back to glucose, consuming 6 high-energy phosphates.

Enzyme Names

• Gluconeogenesis enzymes listed in red, glycolysis enzymes in blue, illustrating their coordinated regulation and cross-utilization in carbohydrate metabolism.

Carbohydrate Metabolism

• Primary source of dietary carbohydrates for humans is starch from plants, supplemented by glycogen from animals, sucrose, and lactose.
• Digestion converts carbohydrates into monosaccharides, primarily glucose, which is transported to the liver.
• Liver plays a crucial role in storage and distribution of glucose and energy sources in the body.

Fates of Dietary Glucose

• Glucose is catabolized for ATP production, stored as glycogen in liver and muscle, or converted to fatty acids for triglyceride storage in adipose tissue.

Glycolysis

• Glycolysis cleaves glucose (C6H12O6) into two pyruvate molecules (C3H3O3-) while producing a net of 2 ATP per glucose.
• NAD functions as an electron acceptor; its limited supply necessitates its re-oxidation for glycolysis to continue.

Anaerobic Glycolysis

• In absence of oxygen, pyruvate is reduced to lactate, allowing glycolysis to continue in heavily exercising muscles.
• This conversion helps regenerate NAD from NADH, essential for continued ATP production.

Key Enzymatic Reactions in Glycolysis

• Triose Phosphate Isomerase (TIM): Converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate to facilitate glycolysis progression.
• Glyceraldehyde-3-phosphate Dehydrogenase: Catalyzes the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, using NAD+.
• Phosphoglycerate Mutase: Shifts phosphate from C3 to C2 to form 2-phosphoglycerate.
• Enolase: Dehydrates 2-phosphoglycerate to phosphoenolpyruvate, a reaction requiring Mg++ ions.
• Pyruvate Kinase: Catalyzes the conversion of phosphoenolpyruvate to pyruvate, producing ATP in the process.

Lactate and Its Role

• Lactate generated can be re-converted to pyruvate by lactate dehydrogenase in other tissues, facilitating energy production.
• Plays a crucial role as a fuel source for cardiac muscle and brain neurons, where astrocytes convert glucose to lactate for neuronal uptake.

Alcohol Fermentation

• Some anaerobic organisms convert pyruvate into ethanol, excreting it as a waste product.
• NADH is oxidized back to NAD+ in this process via alcohol dehydrogenase.

Summary of Glycolysis

• Overall equation for glycolysis:
  • Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP
• Fermentation equation from glucose to lactate:
  • Glucose + 2 ADP + 2 Pi → 2 Lactate + 2 ATP
• Anaerobic metabolism yields only 2 ATP.

Glucokinase vs. Hexokinase

• Glucokinase, found in the liver, has a high KM for glucose and is active at high glucose concentrations.
• It is not inhibited by glucose-6-phosphate and allows glucose uptake in high concentrations.
• Subject to regulation via glucokinase regulatory protein (GKRP), which alters based on metabolic state, allowing modulation of glucose phosphorylation.

Glucose Metabolism in Liver

• Glucose-6-phosphatase catalyzes the release of inorganic phosphate from glucose-6-phosphate, regulating glucose homeostasis.