Which is correct for catabolism of 18 C fatty acid?

Options

A. Mitochondria, beta oxidation, 148 ATP molecules
B. Mitochondria, beta oxidation, 140 ATP molecules
C. Cytosol, beta oxidation 146 ATP molecules
D. Cytosol, beta oxidation, 140 ATP molecules

Answer

To determine the correct statement regarding the catabolism of an 18-carbon fatty acid, we need to analyze the process of beta-oxidation and the ATP yield from this catabolic pathway.

### Step-by-Step Solution:

1. **Understanding Catabolism of Fatty Acids**:
- The catabolism of fatty acids is primarily known as beta-oxidation. This process breaks down fatty acids into acetyl-CoA units.

2. **Location of Beta-Oxidation**:
- Beta-oxidation occurs in the mitochondria of cells. This is crucial because it distinguishes the correct options based on where the process takes place.

3. **Calculating Acetyl-CoA Production**:
- An 18-carbon fatty acid will yield 9 acetyl-CoA molecules. This is because each acetyl-CoA is derived from a 2-carbon unit, and 18 divided by 2 equals 9.

4. **ATP Yield from Acetyl-CoA**:
- Each acetyl-CoA that enters the TCA cycle can generate approximately 10 ATP (3 NADH, 1 FADH2, and 1 GTP, which is equivalent to ATP). Therefore, 9 acetyl-CoA would yield about 90 ATP.

5. **ATP Yield from Beta-Oxidation**:
- During the beta-oxidation of the 18-carbon fatty acid, 8 cycles of beta-oxidation occur (since each cycle shortens the fatty acid by 2 carbons). Each cycle produces 1 FADH2 and 1 NADH. Therefore, 8 FADH2 and 8 NADH are produced, which contribute additional ATP:
- 8 NADH = 8 x 2.5 ATP = 20 ATP
- 8 FADH2 = 8 x 1.5 ATP = 12 ATP

6. **Total ATP Calculation**:
- Adding the ATP from acetyl-CoA and beta-oxidation:
- From acetyl-CoA: 90 ATP
- From beta-oxidation: 20 ATP (NADH) + 12 ATP (FADH2) = 32 ATP
- Total ATP = 90 + 32 = 122 ATP

7. **Final ATP Yield**:
- However, when considering the total net gain, we need to subtract the 2 ATP that are used in the activation of the fatty acid before it enters beta-oxidation. Therefore, the final total is:
- Total = 122 - 2 = 120 ATP

8. **Conclusion**:
- The correct statement regarding the catabolism of an 18-carbon fatty acid is that it occurs in the mitochondria and involves beta-oxidation, yielding a total of 120 ATP. However, the question states 148 ATP, which is incorrect based on standard calculations.