Question

Lauric acid, \(\mathrm{CH}_{3}-\left(\mathrm{CH}_{2}\right)_{10}-\mathrm{COOH}\), which is found in coconut oil, is a saturated fatty acid. a. Draw the condensed structural formula of lauric acid acyl CoA. b. Indicate the \(\alpha\) and \(\beta\) carbon atoms in the fatty acyl molecule. c. How many acetyl CoA units are produced? d. How many cycles of \(\beta\) oxidation are needed? e. Account for the total ATP yield from \(\beta\) oxidation of lauric acid by completing the following calculation:

Short answer

a. \(\text{CH}_{3}-(\text{CH}_{2})_{10}-\text{CO}-\text{SCoA}\), b. \(\alpha\)-carbon: \(\text{CH}_{2}^1\), \(\beta\)-carbon: \(\text{CH}_{2}^2\), c. 6, d. 5, e. 80 ATP

Step-by-step solution

Draw Lauric Acid Acyl CoA Condensed Structural Formula

Lauric acid, \(\text{CH}_{3}-(\text{CH}_{2})_{10}-\text{COOH}\), attached to Coenzyme A, will have the structure: \(\text{CH}_{3}-(\text{CH}_{2})_{10}-\text{CO}-\text{SCoA}\).

Identify the \(\alpha\) and \(\beta\) Carbon Atoms

In the acyl-CoA molecule, the \(\alpha\)-carbon is the one adjacent to the carbonyl carbon (the first carbon after the double bond in \(\text{-CO}-\)), and the \(\beta\)-carbon is the next carbon along the chain. For lauric acid acyl CoA, they are: \(\alpha\)-carbon: \(\text{CH}_{2}^1\), \(\beta\)-carbon: \(\text{CH}_{2}^2\).

Determine Number of Acetyl CoA Units Produced

Each round of \(\beta\)-oxidation cleaves a 2-carbon unit as acetyl CoA until the entire fatty acid is converted. Lauric acid has 12 carbons. Thus, \(\frac{12}{2} = 6\) acetyl CoA units are produced.

Calculate Number of \(\beta\)-Oxidation Cycles Needed

One cycle of \(\beta\)-oxidation processes 2 carbons, producing 1 acetyl CoA, until there are no carbons left to process. Total cycles needed for lauric acid: \(6 - 1 = 5\).

Calculate Total ATP Yield from \(\beta\)-Oxidation

Each cycle of \(\beta\)-oxidation yields 1 NADH, 1 FADH2, and 1 acetyl CoA. Additionally, each acetyl CoA entering the citric acid cycle yields 3 NADH, 1 FADH2, and 1 GTP (equivalent to ATP). \- \(\beta\)-oxidation of lauric acid: 5 cycles. Total NADH: \(5 + 3 \times 6 = 23\). Total FADH2: \(5 + 1 \times 6 = 11\). Total GTP/ATP: \(6\). ATP yield:

Calculate ATP from NADH and FADH2 Oxidation

Each NADH generates about 2.5 ATP and each FADH2 generates about 1.5 ATP in the electron transport chain. Total ATP from NADH: \(23 \times 2.5 = 57.5\) ATP. Total ATP from FADH2: \(11 \times 1.5 = 16.5\). Adding GTP/ATP: \(57.5 + 16.5 + 6 = 80\) ATP.

Key concepts

Fatty Acid Oxidation

Fatty acid oxidation is a metabolic pathway that breaks down fatty acids to produce energy. It mainly occurs in the mitochondria of cells. During this process, fatty acids are converted into acetyl-CoA molecules, which then enter the citric acid cycle to generate ATP, the energy currency of the cell. The oxidation process involves several enzyme-catalyzed reactions that progressively shorten the fatty acid chain by removing two-carbon units. This is crucial for generating energy, especially during periods when carbohydrates are in short supply.

Acetyl CoA Production

Acetyl CoA production is the essential outcome of fatty acid oxidation. In this process, each two-carbon unit of the fatty acid chain yields one acetyl CoA molecule. For lauric acid, a 12-carbon fatty acid, this means a total of six acetyl CoA molecules are produced. The acetyl CoA formed enters the citric acid cycle, where further processing leads to the production of additional energy molecules. Acetyl CoA is a critical intermediary, not just in energy production but also in biosynthetic pathways like the synthesis of fatty acids and cholesterol.

Beta Oxidation Pathway

The beta oxidation pathway is a series of reactions that catabolize fatty acids, converting them into acetyl CoA. This pathway occurs in the mitochondria and involves four main steps:
1. Dehydrogenation: The fatty acyl-CoA is oxidized, forming a double bond and producing FADH2.
2. Hydration: Water is added, breaking the double bond and introducing a hydroxyl group.
3. Oxidation: The hydroxyl group is oxidized, forming a keto group and producing NADH.
4. Thiolysis: The molecule is cleaved by Coenzyme A, releasing acetyl CoA and a shortened fatty acyl-CoA.
These steps repeat until the entire fatty acid is converted into multiple acetyl CoA molecules. Lauric acid undergoes this cycle five times to yield six acetyl CoA molecules, explaining why 5 cycles are required for lauric acid oxidation.

ATP Yield Calculation

Calculating ATP yield from beta oxidation involves taking into account the energy produced at each step. For each cycle of beta oxidation, 1 NADH and 1 FADH2 are produced, equating to about 4 ATP. Additionally, each acetyl CoA, when further oxidized in the citric acid cycle, yields 3 NADH, 1 FADH2, and 1 GTP.
For lauric acid:
Each of the 5 cycles of beta oxidation produces 1 NADH and 1 FADH2:
- 5 NADH x 2.5 ATP = 12.5 ATP
- 5 FADH2 x 1.5 ATP = 7.5 ATP
Each of the 6 acetyl CoA entering the citric acid cycle produces:
- 6 x 3 NADH x 2.5 ATP = 45 ATP
- 6 x 1 FADH2 x 1.5 ATP = 9 ATP
- 6 GTP (equivalent to ATP) = 6 ATP
Combining all these contributions:
- Total ATP from NADH: 57.5 ATP
- Total ATP from FADH2: 16.5 ATP
- Adding the direct ATPs: 6 ATP
- Final tally: 80 ATP
Thus, the complete oxidation of lauric acid results in a total yield of 80 ATP molecules, providing a significant energy source for cellular processes.