Oxidation of Fatty Acids:- Acetyl-CoA Can Be Further Oxidized in the Citric Acid Cycle
The acetyl-CoA produced from the oxidation of fatty acids can be oxidized to CO2 and H2O by the citric acid cycle. The following equation represents the balance sheet for the second stage in the oxidation of palmitoyl CoA, together with the coupled phosphorylations of the third stage:
8 Acetyl-CoA+16O2+80Pi+80ADP→8CoA+80ATP +16CO2+16H2O (17–5)
Combining Equations 17–4 and 17–5, we obtain the overall equation for the complete oxidation of palmitoyl CoA to carbon dioxide and water:
Palmitoyl-CoA+23O2+108Pi+108ADP→CoA+108ATP +16CO2+23H2O (17–6)
Table 17–1 summarizes the yields of NADH, FADH2, and ATP in the successive steps of palmitoyl-CoA oxidation. Note that because the activation of palmitate to palmitoyl-CoA breaks both phosphoanhydride bonds in ATP (Fig. 17–5), the energetic cost of activating a fatty acid is equivalent to two ATP, and the net gain per molecule of palmitate is 106 ATP. The standard free-energy change for the oxidation of palmitate to CO2 and H2O is about 9,800 kJ/mol. Under standard conditions, the energy recovered as the phosphate bond energy of ATP is 106 x 30.5 kJ/mol=3,230 kJ/mol, about 33% of the theoretical maximum. However, when the free-energy changes are calculated from actual concentrations of re actants and products under intracellular conditions, the free-energy recovery is more than 60%; the energy conservation is remarkably efficient.
