Fatty acids are central molecules in lipid metabolism. They carry most of the energy in a triacylglycerol and oxidation of them releases energy. The oxidation of a fatty acid is a five-part process that starts in the cytoplasm and ends in the mitochondria: (1) acyl-CoA formation, (2) acyl-CoA transport, (3) beta-oxidation, (4) citric acid cycle, and (5) oxidative phosphorylation. While the last citric acid cycle and oxidative phosphorylation steps are the same as in slow respiration, the other three steps are unique to fatty acid oxidation. Acyl-CoA is formed in the cytoplasm by acyl-CoA synthesis, which requires two ATP. The acyl-CoA is then transported into the mitochondrial matrix for oxidation. This transportation is carried out by the carnitine acyltransferase, and once inside, the acyl-CoA cannot be transported out. Beta-oxidation involved sequential cycles of four reactions where electrons are transferred to FAD and NAD+ and carbons are removed as acetyl-CoA, a two-carbon molecule. These four reactions continue to cycle until the whole fatty acid chain is oxidized, two carbons at a time. One thing to note is that the oxidation of unsaturated fatty acids requires additional enzymes and varies based on the position of the double bonds. Fatty acids with double bonds are less reduced than saturated fatty acids and therefore yield fewer ATP per carbon.
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