Question

Consider the complete oxidation of arachidic acid, ${\mathrm{CH}}_3-{\left({\mathrm{CH}}_2\right)}_{18}-\mathrm{COOH},\mathrm{a}{\mathrm{C}}_{20}$ fatty acid. 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 arachidic acid?

Short answer

The complete oxidation of arachidic acid produces 10 acetyl CoA units, requires 9 cycles of $\beta$-oxidation, and yields 136 ATP.

Step-by-step solution

- Count the Carbon Atoms

Arachidic acid has the formula ${\text{CH}}_3-({\text{CH}}_2{)}_{18}-\text{COOH}$, which consists of 20 carbon atoms.

- Determine Acetyl CoA Units Produced

Each unit of acetyl CoA produced from fatty acid oxidation consists of 2 carbon atoms. Therefore, the total number of acetyl CoA units produced from a ${\text{C}}_{20}$ fatty acid is $\frac{20}{2}=10$.

- Calculate the Number of $\beta$-Oxidation Cycles

Since each cycle of $\beta$-oxidation shortens the acyl chain by 2 carbons and produces 1 acetyl CoA, the number of cycles needed is equal to the number of acetyl CoA units minus 1. Therefore, $\text{Number of cycles}=10-1=9$.

- Compute Total ATP Yield

Each acetyl CoA entering the citric acid cycle generates 10 ATPs. There are also FADH2 and NADH produced during $\beta$-oxidation. Specifically, each of the 9 cycles produces 1 FADH2 (1.5 ATP each) and 1 NADH (2.5 ATP each). Thus, the total ATP from acetyl CoA is \10 \times 10 = 100 \, from FADH2 is \9 \times 1.5 = 13.5 \, and from NADH is \9 \times 2.5 = 22.5\. Adding these together, we get \100 + 13.5 + 22.5 = 136 \ ATP in total.

Key concepts

Acetyl CoA Production

Acetyl CoA production is a crucial step in the metabolism of fatty acids. When a fatty acid such as arachidic acid undergoes oxidation, it is broken down into multiple units of acetyl CoA. To understand how this works, it’s important to know that each acetyl CoA unit consists of 2 carbon atoms. Arachidic acid has 20 carbon atoms, so dividing by 2, we determine that it produces 10 acetyl CoA units. These units then enter the citric acid cycle (also known as the Krebs cycle), where they are further oxidized to generate ATP, the energy currency of cells.

Beta Oxidation Cycles

Beta oxidation is the process by which fatty acids are broken down in the mitochondria to generate acetyl CoA. For arachidic acid, with its 20 carbon atoms, several cycles of beta oxidation are required. Each cycle shortens the fatty acid chain by 2 carbon atoms and produces 1 acetyl CoA.
To determine the number of beta oxidation cycles needed, subtract 1 from the number of acetyl CoA units produced. In this case, 10 acetyl CoA units minus 1 equals 9 beta oxidation cycles.
Each cycle also produces FADH2 and NADH, which are key molecules in the production of ATP during cellular respiration.

ATP Yield in Metabolism

ATP yield in metabolism is a measure of how much energy is extracted from substances like fatty acids during their oxidation. For arachidic acid, the total ATP yield includes contributions from both the citric acid cycle and the electron transport chain. Each acetyl CoA entering the citric acid cycle produces 10 ATPs. With 10 acetyl CoA units, this totals 100 ATPs.
Additionally, the 9 beta oxidation cycles contribute to the ATP pool with each cycle generating 1 FADH2 (equivalent to 1.5 ATPs) and 1 NADH (equivalent to 2.5 ATPs). This adds an extra 13.5 ATPs from FADH2 and 22.5 ATPs from NADH.
Adding these all together: 100 from acetyl CoA plus 13.5 from FADH2 plus 22.5 from NADH equals 136 ATPs in total. This impressive yield highlights the efficiency of fatty acid oxidation in producing energy!