Fatty Acid Metabolism

Overview – Fatty Acid Metabolism

Fatty acid metabolism is the essential process by which the body mobilises, transports, and oxidises fats to produce energy, particularly during fasting or low-carbohydrate states. Fatty acids are a dense energy source, yielding more ATP per gram than carbohydrates or proteins. This metabolic pathway includes lipolysis, mitochondrial beta-oxidation, and integration with the TCA cycle and electron transport chain, making it a cornerstone of systemic energy homeostasis.

Definition

  • Fatty acids = simplest lipid form
    • Consist of a carboxylic acid group + long hydrocarbon chain
  • Classified by chain length:
    • Short–medium chain: mostly from dietary fats
    • Long chain: synthesised in liver and adipose tissue
  • Key functions:
    • Fuel – major ATP source via beta-oxidation
    • Energy storage – highest energy-density of any nutrient
    • Biosynthetic precursor – for triglycerides, phospholipids, hormones, ketones

Fatty Acid Oxidation

1. Lipolysis

  • Occurs in adipose tissue
  • Lipase enzyme breaks down triglycerides → releases fatty acids
  • Stimulated by:
    • Glucagon
    • Epinephrine
    • Triggered in response to falling blood glucose

2. Transport in Blood

  • Fatty acids are hydrophobic, require carriers
  • Bound to plasma albumin for transport

3. Cellular Uptake

  • Enter cells via specific transport proteins (e.g. SLC27 family)

4. Activation

  • In cytoplasm:
    • Fatty acid + Coenzyme A → Fatty acyl-CoA

5. Mitochondrial Transport

  • Fatty acyl-CoA transported across mitochondrial membranes via the carnitine shuttle
    • Essential step for long-chain fatty acids

6. Beta-Oxidation

  • Occurs in the mitochondrial matrix
  • Fatty acyl-CoA undergoes repetitive cleavage to generate:
    • Multiple acetyl-CoA units (2-carbon)
    • NADH and FADH₂ as electron carriers
  • Each acetyl-CoA enters the TCA cycle:
    • Combines with oxaloacetate to form citrate
FATTY ACID OXIDATION
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Ketogenesis & Ketone Oxidation

Ketogenesis

  • Excess acetyl-CoA (esp. during fasting or diabetes) → converted to ketone bodies in the liver mitochondria
    • Main product: β-hydroxybutyrate
  • Ketones are water-soluble, energy-rich molecules transported to peripheral tissues
Ketogenesis 
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Ketolysis (Ketone Oxidation)

  • In tissues like brain, heart, skeletal muscle:
    • β-hydroxybutyrate → converted back to acetyl-CoA
    • Acetyl-CoA enters TCA cycle for energy
Ketone Oxidation 
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

ATP Yield and Electron Transport Chain

Oxidative Phosphorylation

  • Final step in fatty acid metabolism
  • NADH and FADH₂ generated in beta-oxidation and TCA → enter electron transport chain
  • Located in inner mitochondrial membrane
  • Proton gradient generated → drives ATP synthase
  • Yields:
    • 1 NADH → ~3 ATP
    • 1 FADH₂ → ~2 ATP
  • Final by-products: CO₂ and H₂O
  • Fatty acid metabolism yields significantly more ATP than glucose metabolism
ELECTRON TRANSPORT CHAINS
CNX OpenStax, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons

Summary – Fatty Acid Metabolism

Fatty acid metabolism allows the body to produce energy from lipids during fasting, exercise, or carbohydrate scarcity. Through lipolysis, mitochondrial beta-oxidation, the TCA cycle, and the electron transport chain, fatty acids yield high ATP output and act as key metabolic fuels. For a broader context, see our Cell Biology & Biochemistry Overview page.

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