Beta-Oxidation of Fatty Acids

1. 1

   Fatty acid + CoA-SH + ATP → fatty acyl-CoA + AMP + PPi

   Long-chain acyl-CoA synthetase (ACSL/thiokinase) ST-0073 Irreversible ATP CoA-SH PPi
   In Fatty acid CoA-SH ATP

   Out Fatty acyl-CoA AMP PPi
   › Notes

   A long-chain acyl-CoA synthetase (thiokinase) activates the fatty acid at the outer mitochondrial membrane/ER. ATP is converted to AMP and pyrophosphate; pyrophosphatase hydrolysis makes activation effectively irreversible and costs two ATP equivalents. This is necessary before transport or oxidation but is not the rate-limiting control point for long-chain oxidation.

2. 2

   Fatty acyl-CoA + carnitine → acylcarnitine + CoA-SH

   Carnitine palmitoyltransferase I (CPT-1) 2.3.1.21 ST-0074 Irreversible/directional rate-limiting CoA-SH Carnitine
   In Fatty acyl-CoA Carnitine

   Out Acylcarnitine CoA-SH
   › Notes

   Carnitine palmitoyltransferase I (CPT-1), located on the outer mitochondrial membrane, transfers the acyl group to carnitine. This is the principal rate-limiting/regulatory step for mitochondrial oxidation of long-chain fatty acids and is inhibited by malonyl-CoA.

3. 3

   Acylcarnitine (cytosolic side) → acylcarnitine (matrix side)

   Carnitine-acylcarnitine translocase (CACT) ST-0075 Irreversible/directional Carnitine
   In Acylcarnitine

   Out Acylcarnitine
   › Notes

   The inner-membrane carnitine–acylcarnitine translocase (CACT) exchanges incoming acylcarnitine for free carnitine moving outward. No ATP is directly consumed.

4. 4

   Acylcarnitine + CoA-SH → fatty acyl-CoA + carnitine

   Carnitine palmitoyltransferase II (CPT-2) 2.3.1.21 ST-0076 Irreversible/directional CoA-SH Carnitine
   In Acylcarnitine CoA-SH

   Out Fatty acyl-CoA Carnitine
   › Notes

   Matrix-facing CPT-2 regenerates fatty acyl-CoA and free carnitine. Carnitine returns through CACT; CPT-1, CACT, and CPT-2 together constitute the carnitine shuttle.

5. 5

   Fatty acyl-CoA + FAD → trans-Δ2-enoyl-CoA + FADH2

   Acyl-CoA dehydrogenase (VLCAD/LCAD/MCAD/SCAD) 1.3.8.- ST-0077 Irreversible/directional FAD FADH2
   In Fatty acyl-CoA FAD

   Out trans-Delta-2-Enoyl-CoA FADH2
   › Notes

   An acyl-CoA dehydrogenase catalyzes the first recurring oxidation. Very-long-, long-, medium-, or short-chain acyl-CoA dehydrogenase is selected by chain length; electrons pass from FADH2 through electron-transfer flavoprotein (ETF) and ETF:ubiquinone oxidoreductase to the ubiquinone pool. This is not directly ATP-producing but yields respiratory-chain reducing equivalents.

6. 6

   trans-Δ2-Enoyl-CoA + H2O → L-3-hydroxyacyl-CoA

   Enoyl-CoA hydratase (crotonase) 4.2.1.17 ST-0078 Irreversible/directional H2O
   In trans-Delta-2-Enoyl-CoA H2O

   Out L-3-Hydroxyacyl-CoA
   › Notes

   Enoyl-CoA hydratase (crotonase) hydrates the double bond stereospecifically. No redox cofactor is required.

7. 7

   L-3-Hydroxyacyl-CoA + NAD+ → 3-ketoacyl-CoA + NADH + H+

   L-3-hydroxyacyl-CoA dehydrogenase 1.1.1.35 ST-0079 Irreversible/directional NAD+ NADH
   In L-3-Hydroxyacyl-CoA NAD+

   Out 3-Ketoacyl-CoA NADH H+
   › Notes

   L-3-hydroxyacyl-CoA dehydrogenase oxidizes the β-hydroxyl group using NAD+. NADH donates electrons to complex I of the respiratory chain.

8. 8

   3-Ketoacyl-CoA + CoA-SH → acetyl-CoA + fatty acyl-CoA shortened by two carbons

   beta-Ketothiolase (acyl-CoA acetyltransferase) 2.3.1.9 ST-0080 Irreversible/directional CoA-SH
   In 3-Ketoacyl-CoA CoA-SH

   Out Acetyl-CoA
   › Notes

   β-Ketothiolase (acyl-CoA acetyltransferase) performs thiolysis. The shortened acyl-CoA re-enters steps 5–8 until the chain is fully converted to acetyl-CoA; for an even-chain saturated fatty acid, the final thiolysis yields two acetyl-CoA molecules.

9. 9

   Acetyl-CoA → CO2 in the tricarboxylic-acid cycle, or into ketogenesis in hepatic mitochondria

   (downstream fate - TCA cycle / ketogenesis; not a beta-oxidation enzyme) ST-0081 Irreversible/directional ATP
   In Acetyl-CoA
   › Notes

   This is not a β-oxidation reaction itself, but it determines the ultimate ATP yield. Odd-chain fatty acids end with propionyl-CoA, which is carboxylated by propionyl-CoA carboxylase (biotin, ATP), rearranged via methylmalonyl-CoA, and converted by methylmalonyl-CoA mutase (adenosylcobalamin) to succinyl-CoA.

Showing all 9 steps.

Overview

Mitochondrial β-oxidation degrades fatty acyl-CoA molecules by sequential removal of two-carbon acetyl-CoA units, producing NADH and FADH2 for oxidative phosphorylation. It predominates during fasting, prolonged exercise, and carbohydrate restriction, especially in liver, heart, and oxidative skeletal muscle. The liver uses much of the resulting acetyl-CoA to support ketogenesis when oxaloacetate is diverted toward gluconeogenesis.

Cellular location

Activation occurs on the cytosolic face of the outer mitochondrial membrane and endoplasmic reticulum; long-chain acyl groups cross the inner mitochondrial membrane through the carnitine shuttle. The core spiral occurs in the mitochondrial matrix. Very-long-chain fatty acids are initially shortened by peroxisomal β-oxidation, whereas branched-chain fatty acids may require peroxisomal α-oxidation before further degradation.

Net energetics

For palmitate (16:0), seven cycles yield 8 acetyl-CoA, 7 NADH, and 7 FADH2. Complete oxidation yields approximately 108 ATP (8 acetyl-CoA × 10 ATP, 7 NADH × 2.5 ATP, and 7 FADH2 × 1.5 ATP) minus 2 ATP equivalents for activation, for a net 106 ATP using modern P/O ratios. Older conventions report 129 ATP by assigning 3 ATP per NADH and 2 ATP per FADH2.

Clinical significance

β-Oxidation is essential for maintaining ATP production during fasting and provides the acetyl-CoA that activates pyruvate carboxylase and supports gluconeogenesis. Failure of this pathway causes hypoketotic hypoglycemia because hepatic energy production and ketone-body formation both fail. Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency classically presents in infancy or childhood with fasting intolerance, hypoketotic hypoglycemia, dicarboxylic aciduria, and risk of sudden death; management includes avoidance of prolonged fasting. CPT-1/CPT-2 and carnitine-transport defects similarly impair long-chain fatty-acid use.