描述
Fatty acid degradation in most organisms occurs primarily via the beta-oxidation cycle. In mammals, beta-oxidation occurs in both mitochondria and peroxisomes, whereas plants and most fungi harbor the beta-oxidation cycle only in the peroxisomes. Although the fatty acid oxidation scheme works neatly for even- numbered chain lengths, it can’t work completely for fatty acids that contain an odd number of carbons or branched chain fatty acids such as phytanic acid. Beta-oxidation of these compounds leads to propionyl-CoA and acetyl-CoA, rather than to two acetyl-CoA at the final step. The propionyl-CoA is not a substrate for the TCA cycle or other simple pathways. For example, phytanic acid, found in animal milk, can’t be oxidized directly by beta-oxidation because the addition of water is a problem at the branched beta-carbon. The first step in the digestion of this compound is the oxidation of the carbon by molecular oxygen. Then the original carboxyl group is removed as CO2, leaving a shorter chain. This chain can now be accommodated by the beta-oxidation reactions, because the new beta-carbon now lacks a methyl group. In mitochondria, the beta-oxidation pathway includes four reactions that occur in repeating cycles with each fatty acid molecule. In each cycle, a fatty acid is progressively shortened by two carbons as it is oxidized and its energy captured by the reduced energy carriers NADH and FADH2. At the end of each cycle of four reactions, one acetyl-CoA two-carbon unit is released from the end of the fatty acid, which then goes through another round of beta-oxidation, continuing to oxidize and shorten even-chain fatty acids until they are entirely converted to acetyl-CoA. The acetyl-CoA generated in beta-oxidation enters the TCA cycle, where it is further oxidized to CO2, producing more reduced energy carriers, NADH and FADH2. These carriers produced in the TCA cycle, along with those produced directly in beta-oxidation, transfer their energy to the electron transport chain where they drive the creation of the proton gradient that supports mitochondrial ATP production. Another destination of acetyl-CoA is the production of ketone bodies by the liver that are transported to tissues like the heart and brain for energy.