The link reaction and the Krebs cycle are vital stages in cell respiration that function to convert pyruvate into acetyl CoA and subsequently oxidise it. This occurs in the mitochondria and leads to the production of key energy carriers.
The Link Reaction
Conversion of Pyruvate to Acetyl CoA
- Location: The matrix of the mitochondria.
- Enzyme Catalysis: Pyruvate dehydrogenase complex catalyses this reaction.
- Decarboxylation: Pyruvate (a 3-carbon compound) loses a molecule of CO2.
- Formation of Acetyl CoA: The two-carbon compound is then bound to Coenzyme A (CoA), forming acetyl CoA.
Details of the Reaction
- Oxidation of Pyruvate: Pyruvate is oxidised, transferring electrons to NAD+, forming NADH.
- Release of CO2: One carbon atom is removed from pyruvate as CO2.
- Combination with CoA: Coenzyme A, derived from vitamin B5, attaches to the two-carbon molecule, forming acetyl CoA.
Importance of the Link Reaction
- Transition Step: It acts as a bridge between glycolysis (anaerobic) and the Krebs cycle (aerobic).
- Energy Transfer: Generates NADH, which plays a key role in the electron transport chain.
The Krebs Cycle
Overview and Location
- Location: Mitochondrial matrix.
- Continuous Process: Functions in a cyclical manner.
Phases of the Krebs Cycle
- 1. Condensation Phase:
- Acetyl CoA and Oxaloacetate: They combine to form a 6-carbon compound, citrate.
- Enzyme: Citrate synthase.
- Significance: Sets the cycle in motion.
- 2. Isomerisation Phase:
- Citrate to Isocitrate: Citrate is rearranged.
- Enzyme: Aconitase.
- 3. Oxidation and Decarboxylation Phases:
- Isocitrate to Alpha-ketoglutarate: Oxidised and decarboxylated.
- Enzyme: Isocitrate dehydrogenase.
- Alpha-ketoglutarate to Succinyl CoA: Further oxidation and decarboxylation.
- Enzyme: Alpha-ketoglutarate dehydrogenase complex.
- Significance: Produces NADH, which carries energy to the electron transport chain.
- 4. Substrate-level Phosphorylation Phase:
- Succinyl CoA to Succinate: Direct ATP or GTP production.
- Enzyme: Succinyl CoA synthetase.
- 5. Oxidation Phase:
- Succinate to Fumarate: Oxidation to form a double bond.
- Enzyme: Succinate dehydrogenase.
- Significance: Produces FADH2, another electron carrier.
- 6. Hydration Phase:
- Fumarate to Malate: Addition of water.
- Enzyme: Fumarase.
- 7. Final Oxidation Phase:
- Malate to Oxaloacetate: Oxidation.
- Enzyme: Malate dehydrogenase.
- Significance: Regenerates oxaloacetate, completing the cycle.
Significance of the Krebs Cycle
- Energy Production: Generates vital energy carriers (NADH and FADH2).
- Carbon Dioxide: Two molecules are produced as a waste product, which are exhaled.
- Anabolic Precursors: The cycle intermediates can be used to synthesise various biomolecules.
FAQ
The link reaction converts pyruvate, produced in glycolysis, into acetyl CoA, which enters the Krebs cycle. This connection is essential as it ensures a seamless transition from the initial breakdown of glucose in the cytoplasm to its further oxidation in the mitochondria. Without the link reaction, the aerobic respiration pathway would be disjointed, affecting ATP production.
In the Krebs cycle, FAD and NAD+ act as electron acceptors. They are reduced to FADH2 and NADH by accepting electrons and protons from the oxidation of intermediates in the cycle. These reduced electron carriers then transport electrons to the electron transport chain, facilitating ATP synthesis. The regeneration of FAD and NAD+ ensures the continuity of the cycle.
The Krebs cycle is considered a cycle because its starting compound, oxaloacetate, is regenerated at the end of each turn. It combines with acetyl CoA to form citrate, progressing through a series of reactions and returning to oxaloacetate. This cyclical nature allows for continuous oxidation of acetyl groups as long as substrates are available.
No, the Krebs cycle cannot operate under anaerobic conditions. It relies on the reduced electron carriers NADH and FADH2 transporting electrons to the electron transport chain, where oxygen is the final electron acceptor. Without oxygen, the electron transport chain would cease to function, leading to a lack of NAD+ and FAD, which are essential for the Krebs cycle to continue.
Coenzyme A is vital in the link reaction because it binds with the two-carbon compound created after pyruvate is decarboxylated and oxidised. This binding forms acetyl CoA, which can enter the Krebs cycle. Without Coenzyme A, the transition from the link reaction to the Krebs cycle would be halted, preventing further oxidative breakdown of glucose.
Practice Questions
The link reaction occurs in the matrix of the mitochondria and is vital in connecting glycolysis with the Krebs cycle. Pyruvate, which is formed from glycolysis in the cytoplasm, enters the mitochondria. Here, it undergoes decarboxylation, where a molecule of CO2 is released. Simultaneously, it is oxidised, and the electrons are transferred to NAD+ to form NADH. The remaining two-carbon compound binds with Coenzyme A to form acetyl CoA, which is the substrate for the Krebs cycle. This ensures the continuity of aerobic respiration.
The Krebs cycle, occurring in the mitochondrial matrix, plays a critical role in the aerobic breakdown of glucose. It oxidises acetyl groups from acetyl CoA into CO2 while reducing NAD+ and FAD to NADH and FADH2, respectively. These carriers transport electrons to the electron transport chain. An example phase is the conversion of isocitrate to alpha-ketoglutarate, catalysed by isocitrate dehydrogenase. During this phase, isocitrate is oxidised and decarboxylated, releasing CO2 and reducing NAD+ to NADH. This process contributes to the energy production within the cell, emphasising the cycle's central role in metabolism.
