Shuttle systems carnitine

Answer The transport of fatty acid molecules into mitochondria requires a shuttle system involving a fatty acyl-carnitine intermediate. Fatty acids are first converted to fatty acyl-CoA molecules in the cytosol (by the action of acyl-CoA synthetases) then, at the outer mitochondrial membrane, the fatty acyl group is transferred to carnitine (by the action of carnitine acyl-transferase I). After transport of fatty acyl-carnitine through the inner membrane, the fatty acyl group is transferred to mitochondrial CoA. The cytosolic and mitochondrial pools of CoA are thus kept separate, and no labeled CoA from the cytosolic pool enters the mitochondrion. 

They are converted back to acyl CoAs by carnitine acyltransferases appropriate for their chain length and enter the normal pathways for p-oxidation and acetyl CoA metabolism. The electrons from NADH and acetyl CoA can also pass from the peroxisome to the cytosol. The export of NADH-containing electrons occurs through use of a shuttle system similar to those described for NADH electron transfer into the mitochondria. 

The most important process in the degradation of fatty acids is p-oxidation—a metabolic pathway in the mitochondrial matrix (see p. 164). initially, the fatty acids in the cytoplasm are activated by binding to coenzyme A into acyl CoA [3]. Then, with the help of a transport system (the carnitine shuttle [4] see p. 164), the activated fatty acids enter the mitochondrial matrix, where they are broken down into acetyl CoA. The resulting acetyl residues can be oxidized to CO2 in the tricarboxylic acid cycle, producing reduced... 

This, in a sense, creates a problem because CoA carries several negative charges and therefore we have now made a molecule (stearoyl CoA) that is much too hydrophilic to be able to cross the double membrane of the mitochondrion without help. The solution lies in a specific carrier system for shuttling the fatty acid chains across the membrane barriers. It uses a molecule called carnitine, which, unlike CoA, carries no net charge. There is a system of three membrane-boimd enzymes that (1)... 

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