Question

Consider the complete oxidation of palmitoleic acid, ${\mathrm{CH}}_3-{\left({\mathrm{CH}}_2\right)}_5-\mathrm{CH}=\mathrm{CH}-{\left({\mathrm{CH}}_2\right)}_7-\mathrm{COOH}$, which is a 16-carbon monounsaturated fatty acid found in animal and vegetable oils. a. How many acetyl CoA units are produced? b. How many cycles of $\beta$ oxidation are needed? c. How many ATPs are generated from the oxidation of palmitoleic acid?

Short answer

a. 8 acetyl CoA units; b. 7 cycles; c. 106 ATP generated.

Step-by-step solution

Calculate Total Carbon Atoms

Palmitoleic acid is given by the formula ${\text{CH}}_3-({\text{CH}}_2{)}_5-\text{CH}=\text{CH}-({\text{CH}}_2{)}_7-\text{COOH}$. It has 16 carbon atoms in total.

Determine Number of Acetyl CoA Units

Each acetyl CoA unit contains 2 carbon atoms. Since palmitoleic acid has 16 carbon atoms, the number of acetyl CoA units produced is $\frac{16}{2}=8$.

Calculate Cycles of $\beta$ Oxidation

Each cycle of $\beta$ oxidation removes 2 carbons from the fatty acid chain. To fully break down palmitoleic acid (16 carbons), 7 cycles of $\beta$ oxidation are needed: $16/2-1=7$ cycles.

Calculate ATP from Acetyl CoA

Each acetyl CoA generated in the TCA (Krebs) cycle produces 10 ATP. With 8 acetyl CoA molecules, $(8\times 10=80)$ ATP are generated.

Calculate ATP from FADH2 and NADH

Each cycle of $\beta$ oxidation generates 1 FADH2 and 1 NADH, each producing 1.5 and 2.5 ATP respectively. Thus, 7 cycles yield: $7\times 1.5+7\times 2.5=10.5+17.5=28$ ATP.

Compute Total ATP Generated

Total ATP produced is the sum of ATPs from acetyl CoA and ATPs from FADH2 and NADH: $80+28=108$.

Account for Initial Activation Energy

The activation of palmitoleic acid into fatty acyl-CoA requires 2 ATP. Deduct these from the total: $108-2=106$ ATP.

Key concepts

acetyl CoA

Acetyl CoA is a crucial molecule in metabolism and acts as a key player in the oxidation of fatty acids. It is formed during the breakdown of carbohydrates, fats, and proteins. In the context of fatty acid metabolism, specifically with palmitoleic acid, each acetyl CoA unit contains two carbon atoms. Since palmitoleic acid is a 16-carbon fatty acid, it will yield 8 acetyl CoA units. This process is vitally important because acetyl CoA enters the Citric Acid Cycle (also known as the Krebs Cycle or TCA Cycle), where it plays a critical role in generating energy in the form of ATP. Every acetyl CoA molecule can produce significant amounts of ATP when fully oxidized.

β oxidation

β oxidation is the metabolic process that involves the successive degradation of fatty acids. Each cycle of β oxidation shortens the fatty acid chain by two carbon atoms, producing acetyl CoA, NADH, and FADH2. For palmitoleic acid, a 16-carbon monounsaturated fatty acid, 7 cycles of β oxidation are required to completely break it down since each cycle removes 2 carbon atoms, and mathematically: $[16/2]-1=7$ cycles. This process is essential not only for reducing the fatty acid but also for generating reducing equivalents that go into ATP production via the electron transport chain.

ATP production

ATP production in the context of fatty acid oxidation involves multiple stages. Each acetyl CoA generated in the TCA cycle produces 10 ATP. With 8 acetyl CoA units from palmitoleic acid, this amounts to 80 ATP. Moreover, each cycle of β oxidation yields 1 FADH2 and 1 NADH. These molecules enter the electron transport chain where each FADH2 produces 1.5 ATP and each NADH produces 2.5 ATP. For 7 cycles, this gives: 7 FADH2 x 1.5 ATP = 10.5 ATP and 7 NADH x 2.5 ATP = 17.5 ATP, totalling 28 ATP. Adding this to the ATP from acetyl CoA yields 108 ATP. After accounting for the 2 ATP required for the initial activation of palmitoleic acid into acyl-CoA, the net ATP production is 106 ATP.

fatty acid metabolism

Fatty acid metabolism involves the oxidation of fatty acids to release energy. This can be broken down into several stages: activation, transport to the mitochondria, β oxidation, and finally, ATP production via the TCA cycle and the electron transport chain. For palmitoleic acid, the 16-carbon structure undergoes these steps, starting from activation that requires 2 ATP to form fatty acyl-CoA. Next, it is transported into the mitochondria by the carnitine shuttle. Here, β oxidation takes place, repeatedly shortening the fatty acid by two carbons per cycle and generating acetyl CoA, NADH, and FADH2. The resulting acetyl CoA enters the TCA cycle, producing ATP directly and generating even more NADH and FADH2 for indirect ATP production via the electron transport chain. Ultimately, the metabolic pathway of palmitoleic acid oxidation is highly efficient, illustrating the significant energy yield achievable from fatty acids.