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FIGURE 24.6 • Fatty acids are degraded by repeated cycles of oxidation at the ft-carbon and cleavage of the —Cft bond to yield acetate units.

is acetyl-CoA, not free acetate. Because the entire process begins with oxidation of the carbon that is "ft" to the carboxyl carbon, the process has come to be known as ^-oxidation.

Coenzyme A Activates Fatty Acids for Degradation

The process of ft-oxidation begins with the formation of a thiol ester bond between the fatty acid and the thiol group of coenzyme A. This reaction, shown in Figure 24.7, is catalyzed by acyl-CoA synthetase, which is also called acyl-CoA ligase or fatty acid thiokinase. This condensation with CoA activates the fatty acid for reaction in the ft-oxidation pathway. For long-chain fatty acids, this reaction normally occurs at the outer mitochondrial membrane, prior to entry of the fatty acid into the mitochondrion, but it may also occur at the surface of the endoplasmic reticulum. Short- and medium-length fatty acids undergo this activating reaction in the mitochondria. In all cases, the reaction is accompanied by the hydrolysis of ATP to form AMP and pyrophosphate. As shown in Figure 24.7, the two combined reactions have a net aG°' of about —0.8 kJ/mol, so that the reaction is favorable but easily reversible. However, there is more to the story. As we have seen in several similar cases, the pyrophosphate produced in this reaction is rapidly hydrolyzed by inorganic pyrophos-phatase to two molecules of phosphate, with a net AG°' of about —33.6 kJ/mol.

AG0' for acyl-CoA synthesis

Net AG0'

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