Question 18.ex.1: How much ATP is formed by the complete catabolism of stearic...

Step [1] Determine the amount of ATP required to synthesize the acyl CoA from the fatty acid.
• Since the conversion of stearic acid (C_{17}H_{35}COOH) to an acyl CoA (C_{17}H_{35}COSCoA) requires the hydrolysis of two phosphate bonds, this is equivalent to the energy released when 2 ATPs are converted to 2 ADPs.
• Thus, the first step in catabolism costs the equivalent of 2 ATPs—that is, –2 ATPs.

Step [2] Add up the ATP generated from the coenzymes produced during 𝛃-oxidation.
• As we learned in Section 18.9A, each cycle of β-oxidation produces one molecule each of NADH and FADH2. To cleave eight carbon–carbon bonds in stearic acid requires eight cycles of β-oxidation, so 8 NADH and 8 FADH_{2}are produced.

8 NADH × 2.5 ATP/NADH = 20 ATP
8 FADH_{2} × 1.5 ATP/FFADH_{2} = 12 ATP
From reduced coenzymes: 32 ATP

• Thus, 32 ATPs would be produced from oxidative phosphorylation after the reduced coenzymes enter the electron transport chain.

Step [3] Determine the amount of ATP that results from each acetyl CoA, and add the results for steps [1]–[3].
• From Section 18.9A, stearic acid generates nine molecules of acetyl CoA, which then enter the citric acid cycle and go on to produce ATP by the electron transport chain and oxidative phosphorylation. As we learned in Section 18.7C, each acetyl CoA results in 10 ATPs.

9 acetyl CoA × 10 ATP/acetyl CoA = 90 ATP

• Totaling the values obtained in steps [1]–[3]:

–2) + 32 + 90 = 120 ATP molecules from stearic acid