Integration of metabolic pathways

Overview - The Metabolic Metro Map

• Metabolism is a network of pathways linked by shared intermediates.
• Key Junctions: Glucose-6-Phosphate (G6P), Pyruvate, and Acetyl-CoA act as metabolic traffic circles.
• These hubs direct substrates towards energy production (catabolism) or storage/synthesis (anabolism) based on the cell's energy state.

⭐ The Pyruvate Dehydrogenase Complex reaction (Pyruvate → Acetyl-CoA) is irreversible. This is why fatty acids (which yield Acetyl-CoA) cannot be used for net glucose synthesis.

Fed State - Post-Meal Power-Up

• Primary Driver: High insulin-to-glucagon ratio promotes anabolism.
• Main Fuel: Glucose.
• Metabolic Priorities: Use glucose, store the excess.
  • Liver & Muscle: ↑ Glycolysis & ↑ Glycogenesis (storage as glycogen).
  • Adipose Tissue: ↑ Glucose uptake for glycerol synthesis; ↑ Lipoprotein Lipase (LPL) activity to store dietary fats as triacylglycerols (TAGs).
  • Protein: ↑ Amino acid uptake and protein synthesis.

⭐ GLUT4 is the key! This insulin-responsive transporter moves to the membrane in muscle and adipose tissue to facilitate glucose uptake.

Fasting State - Fasting Fuel Frenzy

• Hormonal Profile: ↓ Insulin, ↑ Glucagon, ↑ Epinephrine.
• Primary Goal: Maintain euglycemia for brain and RBCs.
• Liver Metabolism:
  • Glycogenolysis: Initial glucose source (first 12-18 hours).
  • Gluconeogenesis: Becomes primary source later. Key substrates: lactate, alanine, and glycerol.
• Adipose Tissue:
  • Lipolysis: Hormone-sensitive lipase breaks down triglycerides into fatty acids and glycerol.
• Fuel Utilization:
  • Muscle/Liver: Switch to fatty acid oxidation.
  • Brain: Utilizes glucose, then ketone bodies after prolonged fasting.

⭐ Alanine is the principal amino acid shuttle from muscle to liver for gluconeogenesis (Cahill Cycle).

Starvation State - Ketone Survival Mode

• Timeline: Prolonged fasting (> 3 days).
• Primary Goal: Muscle protein-sparing by ↓ gluconeogenesis.
• Liver Metabolism:
  • ↑ Fatty acid oxidation produces excess Acetyl-CoA.
  • Acetyl-CoA is converted to ketone bodies (β-hydroxybutyrate & acetoacetate).
• Brain Fuel Shift: Adapts to use ketone bodies as its main fuel. This spares glucose for obligate tissues like RBCs.

⭐ The brain derives up to 75% of its energy from ketone bodies during prolonged starvation, a key adaptation to spare essential proteins.

Organ-Specific Metabolism - The Body's Orchestra

• Liver: The central metabolic processor.
  • Fed: Glycolysis, glycogenesis, lipogenesis.
  • Fasting: Gluconeogenesis (primary source), glycogenolysis, ketogenesis, urea cycle.
• Skeletal Muscle: Major glucose user & storage site.
  • Fed: Glucose uptake (GLUT4) & glycogen storage.
  • Fasting: Switches to fatty acid oxidation. Exports lactate (Cori cycle) & alanine (Cahill cycle).
• Brain: High, constant energy demand.
  • Uses glucose almost exclusively (~120g/day).
  • Adapts to use ketone bodies in prolonged starvation.
• Adipose Tissue: Body's main energy reservoir.
  • Fed: Stores triglycerides.
  • Fasting: Releases free fatty acids & glycerol via lipolysis.

⭐ The brain's absolute requirement for glucose drives the body's gluconeogenic and ketogenic responses during fasting.

• Insulin (anabolic, fed state) and glucagon (catabolic, fasting state) are the primary hormonal regulators of metabolism.
• Acetyl-CoA is the central metabolic hub, linking carbohydrate, fat, and protein breakdown.
• The liver is the key organ for maintaining blood glucose homeostasis through gluconeogenesis and glycogenolysis.
• The brain depends on glucose but adapts to use ketone bodies during prolonged starvation.
• Muscle utilizes glucose, fatty acids, and ketones, releasing alanine and lactate during exertion.