Pyruvate oxidation a. Where does pyruvate oxidation occur? b. What is the purpose of pyruvate oxidation? c. Show the equation of this reaction including all substrates and all products. d. What is the total energy and CO2 output of pyruvate oxidation: Per glucose? Per pyruvate? NADH + H FADH2 ATP CO2
The Correct Answer and Explanation is:
Here are the correct answers for the worksheet, followed by a detailed explanation.
3. Pyruvate oxidation
a. Where does pyruvate oxidation occur?
Mitochondrial matrix (in eukaryotes)
b. What is the purpose of pyruvate oxidation?
To convert the 3-carbon pyruvate from glycolysis into a 2-carbon acetyl group attached to Coenzyme A (Acetyl-CoA), which can then enter the Krebs cycle. This process also captures high-energy electrons in the form of NADH.
c. Show the equation of this reaction including all substrates and all products.
Pyruvate + NAD⁺ + Coenzyme A → Acetyl-CoA + NADH + H⁺ + CO₂
d. What is the total energy and CO₂ output of pyruvate oxidation:
| NADH + H⁺ | FADH₂ | ATP | CO₂ | |
| Per glucose? | 2 | 0 | 0 | 2 |
| Per pyruvate? | 1 | 0 | 0 | 1 |
Explanation
Pyruvate oxidation is a critical metabolic process that serves as the bridge between glycolysis and the citric acid cycle (also known as the Krebs cycle) in aerobic respiration. Following glycolysis, which occurs in the cytoplasm, the two pyruvate molecules produced from one glucose molecule are actively transported into the mitochondrial matrix, the site of pyruvate oxidation.
The primary purpose of this step is to chemically prepare pyruvate for entry into the citric acid cycle. The three-carbon pyruvate molecule is too large and chemically unsuitable to enter the cycle directly. Therefore, it undergoes a significant transformation catalyzed by a large multi-enzyme complex called the pyruvate dehydrogenase complex. This transformation involves three main events for each pyruvate molecule:
- Decarboxylation: A carboxyl group (-COO⁻) is removed from pyruvate and released as a molecule of carbon dioxide (CO₂). This is the first stage in cellular respiration where CO₂ is produced as a waste product.
- Oxidation: The remaining two-carbon fragment is oxidized, meaning it loses electrons. These high-energy electrons are transferred to the electron carrier molecule NAD⁺, reducing it to NADH. A proton (H⁺) is also released into the solution. The NADH produced is a crucial energy-carrying molecule that will later donate its electrons to the electron transport chain to generate a significant amount of ATP.
- Formation of Acetyl-CoA: The oxidized two-carbon group, known as an acetyl group, is then attached to Coenzyme A (CoA), a molecule derived from a B vitamin. This forms the final product, acetyl-CoA. Acetyl-CoA acts as the fuel for the citric acid cycle, delivering its acetyl group to be further oxidized.
Since one molecule of glucose yields two molecules of pyruvate, the entire process of pyruvate oxidation occurs twice per glucose. Consequently, the total output from one starting glucose molecule is two molecules of Acetyl-CoA, two molecules of NADH, and two molecules of CO₂. No ATP or FADH₂ is generated during this intermediate step.
