Question

How many ATPs will be produced by oxidation of a fatty acid with 18 carbons, if we consider that one NADH produces 3 ATPs and one FADH2 yields 2 ATPs in respiratory chain?

• A. 146
• B. 112
• C. 100
• D. 172

Hint:

Always remember to subtract 2 ATP equivalents for the activation step ($\text{ATP} \rightarrow \text{AMP} + 2\text{P}_i$) to obtain the correct net ATP yield.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
To calculate the net ATP yield from the complete oxidation of an even-chain saturated fatty acid, we must determine the ATP generated from:
The $\text{NADH}$ and $\text{FADH}_2$ produced during beta-oxidation.
The Acetyl-CoA entering the citric acid cycle.
Finally, we must subtract the energy cost required for the initial activation of the fatty acid.
Key Formula or Approach:
For a saturated fatty acid with $N$ carbons:
- Number of Acetyl-CoA produced:
\[ N_{\text{acetyl}} = \frac{N}{2} \]
- Number of beta-oxidation cycles:
\[ N_{\text{cycles}} = \frac{N}{2} - 1 \]
- Each beta-oxidation cycle produces $1\text{ NADH}$ and $1\text{ FADH}_2$.
- Each Acetyl-CoA oxidized in the citric acid cycle yields $3\text{ NADH}$, $1\text{ FADH}_2$, and $1\text{ GTP (ATP equivalent)}$.
- The initial activation of the fatty acid to fatty acyl-CoA consumes 2 high-energy phosphate bonds ($\text{ATP} \rightarrow \text{AMP} + 2\text{P}_i$).

Step 2: Detailed Explanation:
Let us apply these values to an 18-carbon saturated fatty acid (stearic acid):
- Number of Acetyl-CoA:
\[ N_{\text{acetyl}} = \frac{18}{2} = 9 \]
- Number of beta-oxidation cycles:
\[ N_{\text{cycles}} = 9 - 1 = 8 \]
Next, we calculate the total cofactors produced:
- From 8 cycles of beta-oxidation:
\[ 8\text{ NADH and } 8\text{ FADH}_2 \]
- From 9 cycles of the citric acid cycle (from 9 Acetyl-CoA):
\[ 9 \times 3 = 27\text{ NADH} \]
\[ 9 \times 1 = 9\text{ FADH}_2 \]
\[ 9 \times 1 = 9\text{ GTP (ATP equivalent)} \]
Summing the cofactors:
- Total NADH = $8 + 27 = 35\text{ NADH}$
- Total $\text{FADH}_2$ = $8 + 9 = 17\text{ FADH}_2$
Calculating ATP yield using the given conversion values ($1\text{ NADH} = 3\text{ ATP}$ and $1\text{ FADH}_2 = 2\text{ ATP}$):
- ATP from NADH = $35 \times 3 = 105\text{ ATP}$
- ATP from $\text{FADH}_2$ = $17 \times 2 = 34\text{ ATP}$
- ATP from GTP = $9\text{ ATP}$
- Gross ATP = $105 + 34 + 9 = 148\text{ ATP}$
- Net ATP = Gross ATP - Activation cost = $148 - 2 = 146\text{ ATP}$.

Step 3: Final Answer:
The complete oxidation of an 18-carbon fatty acid yields a net of 146 ATPs.