Model Answer
0 min readIntroduction
The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a pivotal metabolic pathway in aerobic respiration, occurring in the mitochondrial matrix of eukaryotic cells. Its primary function is the complete oxidation of acetyl-CoA, derived from carbohydrates, fats, and proteins, to carbon dioxide, generating ATP (or GTP), FADH₂, and crucially, NADH. NADH is a high-energy electron carrier that plays a vital role in the subsequent oxidative phosphorylation, where its electrons are used to drive ATP synthesis, thus making the Krebs cycle an indispensable part of cellular energy production.
Reactions Producing NADH During the Krebs Cycle
The Krebs cycle involves a series of redox, dehydration, hydration, and decarboxylation reactions. For each molecule of acetyl-CoA that enters the cycle, three molecules of NADH are typically produced within the cycle itself. There is also an additional NADH produced during the preparatory step (pyruvate oxidation) that converts pyruvate to acetyl-CoA, which is often considered in conjunction with the Krebs cycle's overall NADH yield from glucose. The three specific steps within the Krebs cycle where NAD⁺ is reduced to NADH are:1. Isocitrate Dehydrogenase Reaction
This is the first oxidative decarboxylation step in the Krebs cycle. Isocitrate, a six-carbon molecule, is oxidized, and a molecule of carbon dioxide is released. During this process, NAD⁺ acts as an electron acceptor and is reduced to NADH. The product of this reaction is α-ketoglutarate, a five-carbon molecule.Isocitrate + NAD⁺ → α-Ketoglutarate + CO₂ + NADH + H⁺
2. α-Ketoglutarate Dehydrogenase Complex Reaction
Following the formation of α-ketoglutarate, it undergoes another oxidative decarboxylation. This reaction is catalyzed by the α-ketoglutarate dehydrogenase complex, which is structurally and mechanistically similar to the pyruvate dehydrogenase complex. α-Ketoglutarate is oxidized, another molecule of carbon dioxide is released, and NAD⁺ is reduced to NADH. Coenzyme A (CoA) is simultaneously incorporated, forming succinyl-CoA, a four-carbon molecule with a high-energy thioester bond.α-Ketoglutarate + NAD⁺ + CoA-SH → Succinyl-CoA + CO₂ + NADH + H⁺
3. Malate Dehydrogenase Reaction
The final reaction that produces NADH in the Krebs cycle involves the oxidation of malate to oxaloacetate. Malate, a four-carbon molecule, is oxidized, and NAD⁺ is reduced to NADH. This regeneration of oxaloacetate is crucial as it allows the cycle to continue by condensing with a new molecule of acetyl-CoA.Malate + NAD⁺ → Oxaloacetate + NADH + H⁺
These three reactions collectively contribute to the significant production of NADH, which then feeds into the electron transport chain to generate the bulk of cellular ATP.Summary of NADH Producing Reactions
The following table summarizes the reactions within the Krebs cycle that produce NADH:| Step Number | Enzyme | Substrate | Product | NADH Produced |
|---|---|---|---|---|
| 1 | Isocitrate Dehydrogenase | Isocitrate | α-Ketoglutarate | 1 |
| 2 | α-Ketoglutarate Dehydrogenase Complex | α-Ketoglutarate | Succinyl-CoA | 1 |
| 3 | Malate Dehydrogenase | Malate | Oxaloacetate | 1 |
It is important to note that an additional molecule of NADH is produced during the oxidative decarboxylation of pyruvate to acetyl-CoA, a reaction that precedes the entry of acetyl-CoA into the Krebs cycle. While not technically part of the cycle itself, it is intimately linked to glucose catabolism.
Conclusion
The production of NADH during the Krebs cycle is a critical aspect of aerobic cellular respiration, underpinning the efficient generation of energy in living organisms. Through three distinct oxidative steps catalyzed by isocitrate dehydrogenase, α-ketoglutarate dehydrogenase complex, and malate dehydrogenase, NAD⁺ is reduced to NADH. Each turn of the cycle yields three NADH molecules, which, along with the NADH from glycolysis and pyruvate oxidation, are essential for driving the electron transport chain and subsequent ATP synthesis via oxidative phosphorylation. This intricate biochemical pathway highlights the fundamental mechanisms by which cells extract and convert energy from nutrients.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.