Question

Compare the energy cost, in ATP equivalents, of synthesizing stearate from mitochondrial acetyl-CoA to the energy recovered by degrading stearate to acetyl-CoA.

Short answer

The cost of energy is 56 ATP for stearate synthesis as against 30 ATP recovered from stearate degradation. Thus, only half of the energy used to synthesise stearate is returned when stearate is broken down into acetyl-CoA.

Step-by-step solution

Production of nine acetyl-CoA molecules

Stearateis a $C18$ saturated fatty acid that undergoes eight rounds of oxidation to produce nine acetyl-CoA molecules. In order to generate enough ATP, acetyl-CoA is subsequently employed in the citric acid cycle.

 Step 2: Transfer of nine acetyl-CoA molecules

9 ATP is required to transfer$9$ acetyl-CoA molecules from mitochondria to the cytosol during stearate production.

 Step 3: Usage of energy for stearate synthesis

7 rounds of stearate synthesis use $7\mathbf{ATP}$ (these are used in the acetyl-CoA carboxylase reaction) and 14NADH.

$1NA\text{DH=2.5ATP}$

Therefore,

,$\begin{array}{c}14\text{\hspace{0.17em}NADPH}=14\times 2.5\\ =\mathbf{35}\mathbf{ATP}\end{array}$

.

The final stage for stearation

The final stage, elongation of palmitate $\left(\text{Cl}6\right)$ to stearate $(n+2)$, necessitates a total of 1NADH and 1NADPH [(equal to $(2.5+2.5)\text{\hspace{0.17em}ATP}=5\mathbf{AT}P$)].

Thus, the total cost of energy would be $(9+7+35+5)ATP=56\mathbf{ATP}$

Usage of energy for the degradation of stearate

In the acyl-CoA synthetase reaction, the oxidation of stearate to $9$ acetyl-CoA molecules uses 2ATP while generating 8 FADH₂ [equal to $(8\times 1.5)\text{\hspace{0.17em}ATP}=12\text{\hspace{0.17em}ATP})$as ${\text{1FADH}}_2=1.5ATP$] and $8\text{\hspace{0.17em}NADH}$[equivalent to$(8\times 2.5)\text{\hspace{0.17em}ATP}=20\text{\hspace{0.17em}ATP})$ ] in$8$ rounds of $\beta$ oxidation.

As a result,$(12+20-2)\mathbf{ATP}=30ATP$ would be the energy yield.

Hence, the energy cost for stearate synthesis is 56ATP and for its degradation.