The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a key metabolic pathway that plays a crucial role in cellular respiration. It takes place in the mitochondria and involves several reactions, including reduction and carboxylation steps. Here, we'll focus on these reactions, the substrates involved, and the enzymes that catalyze them.
Reduction Reactions in the Krebs Cycle
Reduction reactions in the Krebs cycle primarily involve the reduction of NAD⁺ and FAD to NADH and FADH₂, respectively, which are vital for energy production in the electron transport chain.
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Isocitrate to α-Ketoglutarate:
- Substrate: Isocitrate
- Enzyme: Isocitrate dehydrogenase
- Reaction: Isocitrate is oxidatively decarboxylated to α-ketoglutarate. During this process, NAD⁺ is reduced to NADH, and carbon dioxide (CO₂) is released.
- Overall Reaction:
\[ \text{Isocitrate} + \text{NAD}^+ \rightarrow \text{α-Ketoglutarate} + \text{NADH} + \text{CO}_2 \]
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α-Ketoglutarate to Succinyl-CoA:
- Substrate: α-Ketoglutarate
- Enzyme: α-Ketoglutarate dehydrogenase complex
- Reaction: α-Ketoglutarate undergoes oxidative decarboxylation to form succinyl-CoA. This reaction also reduces NAD⁺ to NADH and releases another CO₂.
- Overall Reaction: \[ \text{α-Ketoglutarate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Succinyl-CoA} + \text{NADH} + \text{CO}_2 \]
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Succinate to Fumarate:
- Substrate: Succinate
- Enzyme: Succinate dehydrogenase
- Reaction: Succinate is oxidized to fumarate, with FAD being reduced to FADH₂ in the process.
- Overall Reaction: \[ \text{Succinate} + \text{FAD} \rightarrow \text{Fumarate} + \text{FADH}_2 \]
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Malate to Oxaloacetate:
- Substrate: Malate
- Enzyme: Malate dehydrogenase
- Reaction: Malate is oxidized to oxaloacetate, and NAD⁺ is reduced to NADH.
- Overall Reaction: \[ \text{Malate} + \text{NAD}^+ \rightarrow \text{Oxaloacetate} + \text{NADH} \]
Carboxylation Reactions in the Krebs Cycle
Carboxylation reactions in the Krebs cycle involve the addition of a carboxyl group (-COOH) to a substrate.
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Citrate Formation (Condensation):
- Substrates: Acetyl-CoA and Oxaloacetate
- Enzyme: Citrate synthase
- Reaction: Acetyl-CoA condenses with oxaloacetate to form citrate. This reaction does not introduce a carboxyl group but is essential for the cycle's initiation.
- Overall Reaction: \[ \text{Acetyl-CoA} + \text{Oxaloacetate} \rightarrow \text{Citrate} + \text{CoA} \]
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Isocitrate Formation (Isomerization):
- Substrate: Citrate
- Enzyme: Aconitase
- This step is an isomerization and involves the conversion of citrate to isocitrate, which creates the substrate necessary for further carboxylation.
The Krebs cycle is a continuous cycle that regenerates oxaloacetate and involves multiple redox, carboxylation, and isomerization reactions. The net outcome includes the generation of NADH and FADH₂, which are critical for ATP production via oxidative phosphorylation in the electron transport chain.
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