FIGURE 12-18 Oxidation of fatty acids in mitochondria and in peroxisomes. In both mitochondrial oxidation (a) and peroxisomal oxidation (b), fatty acids are converted to acetyl CoA by a series of four enzyme-catalyzed reactions (shown down the center of the figure). A fatty acyl CoA molecule is converted to acetyl CoA and a fatty acyl CoA shortened by two carbon atoms. Concomitantly, one FAD molecule is reduced to FADH₂ and one NAD⁺ molecule is reduced to NADH. The cycle is repeated on the shortened acyl CoA until fatty acids with an even number of carbon atoms are completely converted to acetyl CoA. In mitochondria, electrons from FADH₂ and NADH enter the electron-transport chain and are ultimately used to generate ATP; the acetyl CoA generated is oxidized in the citric acid cycle, resulting in the release of CO₂ and ultimately the synthesis of additional ATP. Because peroxisomes lack the protein complexes composing the electron-transport chain and the enzymes of the citric acid cycle, oxidation of fatty acids in these organelles yields no ATP.