Fatty acids are oxidized within mitochondria by most body cells except those of the brain and intestine to produce energy. The usual source for the substrate is release from adipocytes. This can occur at times of starvation and exercise. The rate of oxidation is determined by the rate of triglyceride degradation within adipocytes as most enzymes within the pathway are not physiologically saturated.
The fatty acid passes into the cytoplasm of the cell where it forms a thioester derivative by combination with coenzyme A. The fatty acid chain is then carried into mitochondria after transfer to a carnitine molecule. The carnitine compound is conveyed inwards by a specific membrane transporter. On the other side of the membrane, coenzyme A is swapped for carnitine. The enzyme catalyzing this reaction is carnitine palmitoyltransferse.
Then, saturated fatty acids are oxidized by the action of 4 enzymes. Oxidation precedes cleavage of 2 carbon atoms in each cycle. Sequentially, the enzymes are:
- acyl-CoA dehydrogenase: reduces FAD
- enoyl-CoA hydratase
- 3-hydroxyacyl-CoA dehydrogenase: reduces NAD
- acetyl-CoA acetyl transferase
Energy is released in 3 ways:
- FADH2 passing to the electron carrier chain
- NADH via same route
- acetyl-CoA, one of end products, passes into tricarboxylic acid cycle
Exceptions to this sequence include:
- unsaturated fatty acids: converted by 2 enzymes into saturated forms before oxidation
- fatty acid chains with odd numbers of carbon atoms: oxidized via the normal scheme until reach 3 carbon residue then converted to 4 carbon molecule