Essential for the oxidation of fatty acids in the mitochondria. The fatty acids are activated by conversion to their CoA thioesters. This activation requires ATP and the enzyme, acyl CoA synthase or thiokinase. There are several thiokinases, which differ with respect to their specificity for the different fatty acids. The activation step is dependent on the release of energy from ATP. Once the fatty acid is activated, it is bound to carnitine with the release of CoA. The acylcarnitine is then translocated through the mitochondrial membranes into the mitochondrial matrix via the carnitine- acylcarnitine translocase. As one molecule of acylcarnitine is passed into the matrix, one molecule of carnitine is translocated back to the cytosol and the acylcarnitine is converted back to acyl CoA. The acyl CoA can then enter the P-oxidation pathway. Without carnitine, the oxidation of fatty acids, especially the long-chain fatty acids, cannot proceed. Acyl CoA cannot traverse the membrane into the mitochondria and thus requires a translocase for its entry. The translocase requires carnitine.