While short‐chain fatty acids can move across the mitochondrial membrane directly and are then activated in the mitochondrial matrix, the inner mitochondrial membrane is impermeable to longer fatty acids, such as palmitate. A small molecule, carnitine, serves as a carrier across the mitochondrial membrane. This pathway requires no chemical energy supply; rather, the fact that the fatty acid in the mitochondrial matrix is being broken down by oxidation drives the process.
Carnitine acyltransferase I, which is located on the outer mitochondrial membrane, transfers the fatty acyl group from fatty acyl‐CoA to the hydroxyl (OH) group of carnitine. The acyl‐ carnitine then moves across the intermembrane space to a translocase enzyme, which, in turn, moves the acyl‐carnitine to carnitine acyltransferase II, which exchanges the carnitine for Coenzyme A.
Carnitine is used as a dietary supplement by endurance athletes and in the treatment of certain metabolic diseases. Apparently the extra carnitine allows more rapid transport of fatty acids and a more efficient energy metabolism.