Beta oxidation

Beta oxidation is the main route by which cells break down fatty acids for energy. It runs mostly in the mitochondrial matrix, with very long chains first trimmed in peroxisomes. The name comes from the beta carbon, the second carbon along from the acid group, which is oxidized in each round.

In the cytoplasm, the fatty acid is activated by attachment to coenzyme A, forming a fatty acyl CoA. Long chains cannot cross the inner mitochondrial membrane on their own, so they are ferried across bound to carnitine by the carnitine shuttle. Inside the matrix, each turn of the cycle has four reactions: an oxidation that makes FADH2, the addition of water, a second oxidation that makes NADH, and a split that releases one acetyl CoA and leaves a chain two carbons shorter. The cycle repeats until the whole chain has been cut into acetyl CoA units; palmitic acid, with 16 carbons, yields eight. The acetyl CoA enters the citric acid cycle, and the NADH and FADH2 feed the electron transport chain.

During fasting, the liver turns much of its acetyl CoA into ketone bodies for other tissues. Inherited faults in beta oxidation enzymes, such as MCAD deficiency, which affects medium-length chains, can cause dangerously low blood sugar during fasting or illness, and many are now detected by newborn screening.