Beta-Oxidation

4 min read Updated Apr 18, 2026

Beta-oxidation breaks fatty acids down two carbons at a time, producing acetyl-CoA, NADH, and FADH2. It happens in the mitochondrial matrix. Each round chops off a 2-carbon acetyl-CoA from the carboxyl end of the fatty acid.

β-oxidation: burning a fatty acid

Pathway map

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FADH₂ per turn NADH per turn Acetyl-CoA out CPT-I: the regulated step

CPT-I is the switchThe carnitine shuttle, not any matrix enzyme, is the rate-limiting step. Malonyl-CoA, the first committed intermediate of fatty acid synthesis, inhibits CPT-I. So the moment the cell starts building fat it stops burning it: one molecule enforces the whole reciprocal relationship.

Where the products goAcetyl-CoA goes to the TCA cycle if oxaloacetate is available, and to ketone bodies if it is not. FADH₂ and NADH go straight to the electron transport chain, which is why β-oxidation is useless without oxygen.

Odd and unsaturated chainsAn odd-numbered chain ends in propionyl-CoA, which becomes succinyl-CoA and enters the TCA cycle. This is the one part of a fatty acid that can contribute to glucose. Double bonds need an extra isomerase and reductase, costing a little yield.

Four steps, over and over, two carbons at a time. Oxidize, hydrate, oxidize, cut. Palmitate (16 carbons) goes round seven times to make 8 acetyl-CoA, 7 NADH, and 7 FADH₂, which is roughly 106 ATP after subtracting the 2 ATP equivalents spent activating it.

The key insight is that beta-oxidation feeds into the same machinery as glucose oxidation. Acetyl-CoA from beta-oxidation enters the TCA cycle (citric acid cycle / Krebs cycle) exactly like acetyl-CoA from PDH. The NADH and FADH2 from both the beta-oxidation cycle and the subsequent TCA cycle donate electrons to the electron transport chain, driving ATP synthesis. So fat and glucose merge at acetyl-CoA and share the same downstream pipeline to ATP.

Getting Fatty Acids Into the Matrix

Short and medium fatty acids diffuse freely into mitochondria. Long-chain fatty acids (>12 carbons) need the carnitine shuttle:

Fatty acid is activated in the cytoplasm to fatty acyl-CoA (uses 2 ATP equivalents).
Carnitine palmitoyltransferase I (CPT-I) on the outer membrane swaps CoA for carnitine.
Fatty acyl-carnitine crosses the inner membrane via a translocase.
CPT-II inside the matrix swaps carnitine back for a matrix CoA.

The Four-Step Cycle

Each round of beta-oxidation chops off one acetyl-CoA.

1
Acyl-CoA → trans-enoyl-CoA
Acyl-CoA dehydrogenase · oxidation (double bond formation)
+1 FADH₂
2
Enoyl-CoA → 3-hydroxyacyl-CoA
Enoyl-CoA hydratase · hydration (water added across double bond)
–
3
3-Hydroxyacyl-CoA → 3-ketoacyl-CoA
3-Hydroxyacyl-CoA dehydrogenase · oxidation (hydroxyl to ketone)
+1 NADH
4
3-Ketoacyl-CoA + CoA → acetyl-CoA + (n−2)-acyl-CoA
Thiolase · cleavage (releases acetyl-CoA, shortens chain by 2C)
+acetyl-CoA

Per round: 1 FADH₂ + 1 NADH + 1 acetyl-CoA, plus a fatty acyl-CoA shortened by 2 carbons. Palmitate (C16) = 7 rounds = ~106 ATP total.

ATP Yield from Palmitate (C16)

Palmitate → 8 acetyl-CoA, 7 FADH2, 7 NADH, minus 2 ATP for initial activation.

8 acetyl-CoA × 10 ATP per TCA turn = 80 ATP
7 FADH2 × 1.5 = 10.5 ATP
7 NADH × 2.5 = 17.5 ATP
Minus 2 ATP (initial activation)
Total: ~106 ATP per palmitate

Compare with glucose (~32 ATP). Palmitate has 2.5x the carbons, but 3.3x the ATP - more efficient per carbon because fatty acid carbons are more reduced.

Odd-Chain Fatty Acids

Most dietary fatty acids are even-chain, so beta-oxidation yields only acetyl-CoA (and therefore cannot contribute to gluconeogenesis). But about 1-5 percent of dietary fatty acids have an odd number of carbons. Beta-oxidation proceeds normally for the first several rounds, but the final round produces one acetyl-CoA plus a 3-carbon propionyl-CoA.

Propionyl-CoA cannot enter the TCA cycle directly. It is converted to succinyl-CoA in three steps:

Propionyl-CoA carboxylase (needs biotin, B7) adds CO2 to make D-methylmalonyl-CoA.
Methylmalonyl-CoA epimerase converts it to L-methylmalonyl-CoA.
Methylmalonyl-CoA mutase (needs cobalamin, B12) rearranges it to succinyl-CoA.

Succinyl-CoA is a TCA intermediate, so its carbons CAN feed gluconeogenesis. That means odd-chain fatty acids, unlike even-chain ones, can produce net glucose. The same propionyl-CoA → succinyl-CoA route is also used by the carbon skeletons of Val, Met, Ile, and Thr.

What are the four steps of a single round of beta-oxidation?

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Oxidation (FAD → FADH2), Hydration, Oxidation (NAD+ → NADH), Thiolysis (releases acetyl-CoA and leaves a fatty acyl-CoA shortened by 2 carbons). Mnemonic: O-H-O-T. Each round produces 1 FADH2 + 1 NADH + 1 acetyl-CoA.

Why does fat yield more ATP per gram than carbohydrate?

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Fatty acid chains are highly reduced (long stretches of C-H bonds). Beta-oxidation generates many NADH and FADH2 per molecule because every carbon must be oxidized. Carbohydrates are partially oxidized already (C-O bonds), so they carry fewer electrons per gram. Per gram: fat ~9 kcal vs. carb ~4 kcal; per C16 palmitate ~106 ATP vs. per glucose ~32 ATP.

How does malonyl-CoA prevent futile fatty acid cycling?

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Malonyl-CoA is the first committed intermediate of fatty acid synthesis. It allosterically inhibits carnitine palmitoyltransferase I (CPT-I), the rate-limiting step of fatty acid import into the mitochondrion for beta-oxidation. When synthesis is active (malonyl-CoA high), CPT-I is inhibited, preventing simultaneous synthesis and breakdown. A classic example of reciprocal regulation at a branch point.