Question

Mathematical Calculate the net ATP yield from linoleic acid \((18: 2\) \(\left.\Delta^{9,12}\right) .\) For this calculation, assume that the loss of an NADPH is the same as the loss of an NADH.

Short answer

The net ATP yield from linoleic acid is 115 ATP.

Step-by-step solution

Identify the number of carbons and double bonds

Linoleic acid has 18 carbons and 2 double bonds. The notation \(18:2 \Delta^{9,12}\) indicates 18 carbons and double bonds at positions 9 and 12.

Beta-oxidation cycles

Each beta-oxidation cycle shortens the fatty acid chain by 2 carbons. Therefore, for an 18-carbon fatty acid, there are \(18/2 - 1 = 8\) cycles.

ATP yield from acetyl-CoA

Each round of beta-oxidation produces 1 acetyl-CoA. This means \(18/2 = 9\) acetyl-CoA molecules. Each acetyl-CoA yields 10 ATP during the citric acid cycle, resulting in \(9 \times 10 = 90\) ATP.

ATP yield from FADH2 and NADH

Each beta-oxidation round produces 1 FADH2 and 1 NADH. With 8 cycles, this results in 8 FADH2 and 8 NADH. Each FADH2 yields 1.5 ATP and each NADH yields 2.5 ATP. Therefore: \(8 \times 1.5 + 8 \times 2.5 = 12 + 20 = 32\) ATP.

Subtract ATP cost for double bond processing

Each double bond in linoleic acid requires 1 NADPH, equivalent to 1 NADH (2.5 ATP). Since linoleic acid has 2 double bonds: \(2 \times 2.5 = 5\) ATP.

ATP cost for initial activation

Activation of the fatty acid consumes 2 ATP molecules.

Calculate net ATP yield

Total ATP from acetyl-CoA and electron carriers: \(90 + 32 = 122\) ATP. Subtract the cost for the double bond processing and initial activation: \(122 - 5 - 2 = 115\) ATP.

Key concepts

beta-oxidation

Beta-oxidation is a metabolic process where fatty acids are broken down in the mitochondria to generate acetyl-CoA. This acetyl-CoA can then enter the citric acid cycle. The process occurs in several steps:

• First, the fatty acid is activated in the cytoplasm by conjugating with Coenzyme A (CoA), consuming 2 ATP molecules.
• The fatty acid-CoA complex is transported into the mitochondria, where beta-oxidation commences.
• Each cycle of beta-oxidation removes two-carbon fragments from the fatty acid, releasing one acetyl-CoA molecule, and generating one NADH and one FADH2.
• For linoleic acid (18 carbons), this means 8 cycles of beta-oxidation, producing 8 NADH, 8 FADH2, and 9 acetyl-CoA (since the final two carbons form an additional acetyl-CoA).

This cycle is repeated until the entire fatty acid is converted into acetyl-CoA units. This efficient breakdown of fat molecules is crucial for energy production, especially during fasting or prolonged exercise.

acetyl-CoA

Acetyl-CoA is a vital molecule in metabolism. It plays a crucial role in the citric acid cycle and fatty acid synthesis. During beta-oxidation, each cycle produces one acetyl-CoA molecule by breaking down fatty acids. For linoleic acid:

• 9 acetyl-CoA molecules are generated from its 18 carbon atoms.
• Each acetyl-CoA enters the citric acid cycle and can yield about 10 ATP through subsequent reactions.

This conversion is highly efficient and serves as a pivotal energy source for aerobic organisms. Acetyl-CoA acts as a link between carbohydrate, protein, and fat metabolism.

citric acid cycle

The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. The process takes place in the mitochondria and includes several steps:

• First, acetyl-CoA combines with oxaloacetate to form citrate.
• Citrate undergoes a series of transformations, releasing two molecules of carbon dioxide and transferring electrons to NADH and FADH2.
• A single turn of the cycle generates 1 ATP (or GTP), 3 NADH, and 1 FADH2.

For each acetyl-CoA entering the cycle from the beta-oxidation of linoleic acid, approximately 10 ATP molecules are produced after including the ATP generated from NADH and FADH2 in the electron transport chain. The citric acid cycle is central to cellular energy production and connects to numerous metabolic pathways.

NADH and FADH2

NADH and FADH2 are crucial electron carriers in cellular respiration. They are produced during glycolysis, beta-oxidation, and the citric acid cycle. Their role includes:

• Transferring electrons to the electron transport chain in the inner mitochondrial membrane.
• Driving the production of ATP via oxidative phosphorylation.

For each round of beta-oxidation of linoleic acid:

• 8 NADH and 8 FADH2 are produced.
• Each NADH yields about 2.5 ATP, and each FADH2 yields about 1.5 ATP in the electron transport chain.
• Therefore, 8 NADH contributes 20 ATP and 8 FADH2 provides 12 ATP, adding up to 32 ATP from just these electron carriers.

The effective transfer of energy from NADH and FADH2 is fundamental to bioenergetics, allowing cells to perform essential functions.