Overview:

Fatty acid breakdown brings about the oxidation of long-chain fatty acids. The fatty acids are first transformed to their acyl CoA (coenzyme A) derivatives and then degraded through the successive removal of two-carbon units from the end of the fatty acid as acetyl CoA. The pathway generates FADH₂   and NADH directly. The acetyl CoA generates can also enter the citric acid cycle and produce further NADH and FADH₂. The NADH and FADH₂ are then oxidized through the respiratory electron transport chain to yield energy in the form of ATP.

Activation:      

Fatty acid breakdown occurs in the cytosol of prokaryotes in plants in peroxisomes and in the mitochondrial matrix of all other eukaryotes.  By entering the mitochondrial matrix, the fatty acid is activated through creating a thioester link with CoA. This reaction is catalyzed through acyl CoA synthase (also known as fatty acid thiokinase) which is present on the outer mitochondrial membrane, and uses a molecule of ATP. The whole reaction is irreversible due to the subsequent hydrolysis of PP_i to two molecules of P_i.