To make Acetyl CoA, you have to have two carbons. When fatty acids are oxidized, they are usually even-numbered carbon chains (8, 10, 12, 14, 16...). However, very seldomly, there are odd-numbered fatty acid chains. By definition, the last three carbons cannot be converted to acetyl CoA.
What happens to these?
When you get to a five-carbon fatty acid
- undergoes beta oxidation to AcetylCoA and Propionyl CoA
- Three more steps will convert Propionyl CoA to Succinyl CoA
- Succinyl CoA can contribute carbons to gluconeogenesis! (exceptions to the rules)