Integration with carbohydrate metabolism

Fed State & Lipogenesis - Glucose-to-Fat Pipeline

• Driving Force: High insulin:glucagon ratio after a meal.
• Glycolysis: Glucose → Pyruvate. Also produces Dihydroxyacetone phosphate (DHAP) → Glycerol-3-Phosphate (the triglyceride backbone).
• Citrate Shuttle: Primary mechanism to move Acetyl-CoA from mitochondria to the cytosol for synthesis.
  • High ATP/NADH levels inhibit isocitrate dehydrogenase, causing citrate to accumulate and exit the mitochondria.

• Key Regulatory Steps:
  • Acetyl-CoA Carboxylase (ACC): Irreversible, rate-limiting enzyme. Converts Acetyl-CoA → Malonyl-CoA.
    • Activation: Insulin, Citrate.
    • Inhibition: Glucagon, Palmitoyl-CoA.
  • Fatty Acid Synthase: Synthesizes palmitate from Malonyl-CoA, requires NADPH from HMP Shunt.

⭐ Malonyl-CoA not only serves as a building block for fatty acids but also inhibits carnitine acyltransferase I (CAT-I), preventing newly synthesized fatty acids from entering the mitochondria for β-oxidation.

Fasting State & Ketogenesis - Fat-to-Fuel Switch

• Hormonal Trigger: ↓ Insulin and ↑ Glucagon activate Hormone-Sensitive Lipase (HSL) in adipose tissue, initiating lipolysis.
• Fatty Acid Influx: Free Fatty Acids (FFAs) are released from adipose tissue, travel bound to albumin, and are taken up by the liver.
• Hepatic β-Oxidation: In liver mitochondria, FFAs are oxidized into a large volume of Acetyl-CoA, which inhibits the Pyruvate Dehydrogenase Complex.
• Ketogenesis: Excess Acetyl-CoA is shunted into the ketogenesis pathway.
  • Rate-Limiting Enzyme: Mitochondrial HMG-CoA synthase.
  • Products: Acetoacetate and β-hydroxybutyrate (transportable energy) and acetone (exhaled).

⭐ The liver produces ketone bodies but cannot use them as fuel because it lacks the enzyme thiophorase (succinyl-CoA acetoacetate-CoA transferase).

Regulatory Crossroads - The On/Off Switches

• Acetyl-CoA Carboxylase (ACC): Rate-limiting enzyme for fatty acid synthesis.
  • Activators: Insulin, Citrate. 📌 Insulin & Citrate Activate Carboxylase.
  • Inhibitors: Glucagon, Palmitoyl-CoA (long-chain fatty acyl-CoA).
• Carnitine Palmitoyltransferase I (CPT-1): Rate-limiting for β-oxidation.
  • Inhibitor: Malonyl-CoA (the product of ACC).
• Reciprocal Regulation: High insulin (well-fed state) → ↑ACC activity → ↑Malonyl-CoA → ↓CPT-1 activity, preventing futile cycling.

⭐ Malonyl-CoA is the pivotal molecule ensuring that fatty acid synthesis and oxidation do not occur simultaneously.

High‑Yield Points - ⚡ Biggest Takeaways

• Citrate shuttles acetyl-CoA from mitochondria for FA synthesis; it also inhibits glycolysis via PFK-1 inhibition.
• A high insulin/glucagon ratio (fed state) activates acetyl-CoA carboxylase (ACC), promoting lipid storage.
• Malonyl-CoA (from ACC) inhibits CAT-I, blocking newly synthesized FAs from mitochondrial entry and oxidation.
• The glycerol-3-P backbone for TG synthesis is derived from glycolytic intermediate DHAP.
• In fasting, hepatic β-oxidation drives ketone body formation from excess acetyl-CoA.