The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a series of biochemical reactions critical for cellular respiration in aerobic organisms. It occurs in the mitochondria and plays a key role in energy production through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. Key stages in the Krebs cycle include carboxylation and reduction reactions, each involving specific substrates, enzymes, and coenzymes.
Key Steps Involving Carboxylation and Reduction
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Carboxylation Reaction:
- Enzyme: Citrate Synthase
- Substrates: Acetyl-CoA and Oxaloacetate (OAA)
- Product: Citrate
- Process: The cycle begins with the condensation of acetyl-CoA (2 carbons) and oxaloacetate (4 carbons) to form citrate (6 carbons). This reaction involves the addition of a two-carbon unit (from acetyl-CoA) to a four-carbon unit (oxaloacetate), a process that can be viewed as carboxylation at the molecular level, as it adds a carbon atom to the structure. Importantly, this reaction is catalyzed by the enzyme citrate synthase.
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Reduction Reactions:
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Isocitrate to α-Ketoglutarate:
- Enzyme: Isocitrate Dehydrogenase
- Substrate: Isocitrate
- Products: α-Ketoglutarate and CO₂
- Coenzyme: NAD⁺ (producing NADH)
- Process: In this step, isocitrate undergoes oxidative decarboxylation, where it loses a carbon dioxide molecule and is oxidized, thus reducing NAD⁺ to NADH.
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α-Ketoglutarate to Succinyl-CoA:
- Enzyme: α-Ketoglutarate Dehydrogenase
- Substrate: α-Ketoglutarate
- Products: Succinyl-CoA and CO₂
- Coenzyme: NAD⁺ (producing NADH)
- Process: A further oxidative decarboxylation occurs where α-ketoglutarate loses another CO₂ and is converted to succinyl-CoA, again reducing NAD⁺ to NADH.
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Succinate to Fumarate:
- Enzyme: Succinate Dehydrogenase
- Substrate: Succinate
- Products: Fumarate
- Coenzyme: FAD (producing FADH₂)
- Process: In this dehydrogenation reaction, succinate is converted to fumarate, and FAD is reduced to FADH₂.
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Fumarate to Malate:
- Enzyme: Fumarase (Fumarate Hydratase)
- Substrate: Fumarate
- Product: Malate
- Process: This is a hydration reaction where water is added to fumarate to form malate.
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Malate to Oxaloacetate:
- Enzyme: Malate Dehydrogenase
- Substrate: Malate
- Products: Oxaloacetate
- Coenzyme: NAD⁺ (producing NADH)
- Process: Finally, malate is oxidized to regenerate oxaloacetate, reducing NAD⁺ to NADH yet again.
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Summary of Reduced Coenzymes and CO₂ Production
Overall, through the Krebs cycle:
- Three NAD⁺ are reduced to NADH.
- One FAD is reduced to FADH₂.
- Two CO₂ molecules are released for each acetyl-CoA that enters the cycle.
- Energy is captured in the form of GTP or ATP in the step where succinyl-CoA is converted to succinate.
The NADH and FADH₂ produced in the Krebs cycle ultimately feed into the electron transport chain, leading to further ATP production through oxidative phosphorylation.
In summary, the Krebs cycle encapsulates essential carboxylation and reduction reactions that are integral to cellular respiration, showcasing the intricate nature of metabolic pathways and their role in energy production.
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