Metabolism

Beta-oxidation: how fat is burned for energy

Fat is the body's largest energy store, and beta-oxidation is the pathway that converts it into something the Krebs cycle can use.

Getting the fatty acid into the mitochondrion

  1. Activation — in the cytosol, the fatty acid is joined to coenzyme A to form fatty acyl-CoA. This costs the equivalent of 2 ATP.
  2. The carnitine shuttle — long-chain fatty acyl-CoA cannot cross the inner mitochondrial membrane directly. Carnitine palmitoyltransferase I (CPT-1) transfers it to carnitine for transport, and it is reassembled inside.

Why CPT-1 matters

CPT-1 is the rate-limiting step of fat oxidation, and it is inhibited by malonyl-CoA — a molecule produced when fatty acid synthesis is active. That reciprocal control prevents the cell from building and breaking down fat simultaneously.

The four repeating steps

Each cycle removes two carbons from the fatty acid chain, in the same order every time:

  1. Oxidation — produces FADH2
  2. Hydration — water is added across the double bond
  3. Oxidation — produces NADH
  4. Thiolysis — the chain is cleaved, releasing acetyl-CoA

A mnemonic that holds: oxidise, hydrate, oxidise, cleave. The cycle repeats until the chain is fully converted to acetyl-CoA, which feeds the Krebs cycle.

Why fat yields more energy

Palmitate (16 carbons) undergoes 7 cycles, producing 8 acetyl-CoA, 7 FADH2, and 7 NADH — roughly 106 net ATP, versus about 30–32 for glucose. Fatty acids are far more reduced (more C-H bonds, less oxygen) than carbohydrate, so there are simply more electrons to harvest. That's the same reason fat supplies 9 kcal/g against carbohydrate's 4.

Where ketones come in

When carbohydrate is scarce, oxaloacetate is diverted toward gluconeogenesis, so acetyl-CoA cannot all enter the Krebs cycle. The liver converts the excess into ketone bodies, which other tissues — including the brain — can use. This is the biochemistry underlying the ketogenic diet.

Frequently asked questions

What are the four steps of beta-oxidation?

Oxidation producing FADH2, hydration, a second oxidation producing NADH, and thiolysis which cleaves off acetyl-CoA. The cycle repeats until the fatty acid chain is fully converted.

What is the rate-limiting step of fat oxidation?

Carnitine palmitoyltransferase I (CPT-1), which transfers long-chain fatty acyl-CoA onto carnitine for transport into the mitochondrion. It is inhibited by malonyl-CoA.

Why does fat provide more energy than carbohydrate?

Fatty acids are more reduced, with more carbon-hydrogen bonds and less oxygen, so more electrons can be harvested per gram. This is why fat supplies about 9 kcal/g compared with 4 kcal/g for carbohydrate.

Keep reading: The Krebs cycle explained · The ketogenic diet · What fat actually does

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