One turn of the cycle in the mitochondrion, shown on a four-carbon chain: butyryl-CoA to two acetyl-CoA
Route: Butyryl-CoA → Crotonyl-CoA → (3S)-3-Hydroxybutyryl-CoA → Acetoacetyl-CoA → Acetyl-CoA
Fatty acids are broken down two carbons at a time. Each turn has four steps, shown here on butyryl-CoA because a short chain is easy to draw. Acyl-CoA dehydrogenase uses FAD to put a double bond between carbons 2 and 3 (the alpha and beta carbons), which gives crotonyl-CoA. The FADH₂ passes its electrons on to the respiratory chain. Enoyl-CoA hydratase adds water across the double bond and makes the (S) form of 3-hydroxybutyryl-CoA. 3-Hydroxyacyl-CoA dehydrogenase uses NAD⁺ to turn the hydroxyl group into a ketone, which gives acetoacetyl-CoA. Thiolase attacks the ketone with a new CoA and cuts the chain between carbons 2 and 3. A four-carbon chain gives two acetyl-CoA. A longer chain gives one acetyl-CoA and an acyl-CoA two carbons shorter, which goes around again. Palmitoyl-CoA needs seven turns and gives eight acetyl-CoA.
Practice this pathway All pathways
| Step | Enzyme | EC | In | Out |
|---|---|---|---|---|
| 1 | Short-chain acyl-CoA dehydrogenase | 1.3.8.1 | FAD | FADH₂ |
| 2 | Enoyl-CoA hydratase | 4.2.1.17 | H₂O | |
| 3 | 3-Hydroxyacyl-CoA dehydrogenase | 1.1.1.35 | NAD⁺ | NADH |
| 4 | 3-Ketoacyl-CoA thiolase | 2.3.1.16 | CoA-SH |
Enzyme names and EC numbers are those used on the interactive board. Checked against sources: see the claim table in the repository (tools/pathway_data/checks).