fatty acids; the tissues of the central nervous system cannot use fatty acids, despite containing mitochondria, because long-chain fatty acids (as opposed to medium-chain fatty acids) cannot cross the blood-brain barrier into the interstitial fluids that bathe these cells. Once inside the cell, long-chain-fatty-acid—CoA ligase catalyzes the reaction between a fatty acid molecule with ATP (which is broken down to AMP and inorganic pyrophosphate) to give a fatty acyl-adenylate, which then reacts with free coenzyme A to give a fatty acyl-CoA molecule. In order for the acyl-CoA to enter the mitochondrion the carnitine shuttle is used: Acyl-CoA is transferred to the hydroxyl group of carnitine by carnitine palmitoyltransferase I, located on the cytosolic faces of the outer and inner mitochondrial membranes. Acyl-carnitine is shuttled inside by a carnitine-acylcarnitine translocase, as a carnitine is shuttled outside. Acyl-carnitine is converted back to acyl-CoA by carnitine palmitoyltransferase II, located on the interior face of the inner mitochondrial membrane. The liberated carnitine is shuttled back to the cytosol, as an acyl-CoA is shuttled into the mitochondrial matrix. Beta oxidation, in the mitochondrial matrix, then cuts the long carbon chains of the fatty acids (in the form of acyl-CoA molecules) into a series of two-carbon (acetate) units, which, combined with co-enzyme A, form molecules of acetyl CoA, which condense with oxaloacetate to form citrate at the "beginning" of the citric acid cycle. It is convenient to think of this reaction as marking the "starting point" of the cycle, as this is when fuel - acetyl-CoA - is added to the cycle, which will be dissipated as CO and HO with the release of a substantial quantity of energy captured in the form of ATP, during the course of each turn of the cycle and subsequent oxidative phosphorylation. Briefly, the steps in beta oxidation are as follows: Dehydrogenation by acyl-CoA dehydrogenase, yielding 1 FADH Hydration by enoyl-CoA hydratase Dehydrogenation by 3-hydroxyacyl-CoA dehydrogenase, yielding 1 NADH + H+ Cleavage by thiolase, yielding 1 acetyl-CoA and a fatty acid that has now been shortened by 2 carbons (forming a new, shortened acyl-CoA) This beta oxidation reaction is repeated until the fatty acid has been completely reduced to acetyl-CoA or, in, the case of fatty acids with odd numbers of carbon atoms, acetyl-CoA and 1 molecule of propionyl-CoA per molecule of fatty acid. Each beta oxidative cut of the acyl-CoA molecule eventually yields 5 ATP molecules in oxidative phosphorylation.