Study aid designed to help biology students memorize the steps, intermediates, and enzymes of the Citric Acid Cycle.

The image provided is a diagram of the Citric Acid Cycle (also known as the Krebs Cycle or TCA Cycle). The task is to fill in the reactants, products, intermediates, and enzymes involved in each step of the cycle. Additionally, you are asked to mark the rate-limiting enzyme with a star (*).

Solution Explanation

The Citric Acid Cycle is a central metabolic pathway that occurs in the mitochondria of eukaryotic cells. It plays a crucial role in cellular respiration by generating energy in the form of ATP, NADH, and FADH₂. Below is a detailed explanation of each step in the cycle, along with the reactants, products, intermediates, and enzymes involved.

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1. Formation of Citrate

- Reactants: Acetyl-CoA (2 carbons), Oxaloacetate (4 carbons)
- Products: Citrate (6 carbons)
- Enzyme: Citrate synthase
- Note: This is the first step of the cycle.

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2. Isomerization to Isocitrate

- Reactants: Citrate (6 carbons)
- Products: Isocitrate (6 carbons)
- Enzyme: Aconitase
- Note: This step involves the conversion of citrate to isocitrate through an intermediate called cis-aconitate.

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3. Oxidative Decarboxylation to α-Ketoglutarate

- Reactants: Isocitrate (6 carbons)
- Products: α-Ketoglutarate (5 carbons), CO₂, NADH + H⁺
- Enzyme: Isocitrate dehydrogenase
- Note: This is a key oxidative decarboxylation step where NAD⁺ is reduced to NADH.

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4. Oxidative Decarboxylation to Succinyl-CoA

- Reactants: α-Ketoglutarate (5 carbons)
- Products: Succinyl-CoA (4 carbons), CO₂, NADH + H⁺
- Enzyme: α-Ketoglutarate dehydrogenase complex
- Note: Similar to Step 3, this is another oxidative decarboxylation step involving NAD⁺ reduction.

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5. Conversion of Succinyl-CoA to Succinate

- Reactants: Succinyl-CoA (4 carbons)
- Products: Succinate (4 carbons), CoASH, GTP (or ATP)
- Enzyme: Succinyl-CoA synthetase
- Note: Energy is conserved as GTP (or ATP) is produced.

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6. Dehydrogenation of Succinate to Fumarate

- Reactants: Succinate (4 carbons)
- Products: Fumarate (4 carbons), FADH₂
- Enzyme: Succinate dehydrogenase
- Note: FAD is reduced to FADH₂.

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7. Hydration of Fumarate to Malate

- Reactants: Fumarate (4 carbons)
- Products: Malate (4 carbons)
- Enzyme: Fumarase
- Note: This is a reversible hydration reaction.

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8. Oxidation of Malate to Oxaloacetate

- Reactants: Malate (4 carbons)
- Products: Oxaloacetate (4 carbons), NADH + H⁺
- Enzyme: Malate dehydrogenase
- Note: NAD⁺ is reduced to NADH.

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Rate-Limiting Enzyme

The rate-limiting enzyme in the Citric Acid Cycle is isocitrate dehydrogenase. This enzyme catalyzes the conversion of isocitrate to α-ketoglutarate, which is a slow step due to its thermodynamic constraints. Mark this enzyme with a star (*).

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Final Diagram Fill-In

Here is how the diagram should be filled in:

1. Acetyl-CoA (2C) → Citrate (6C)
- Enzyme: Citrate synthase

2. Citrate (6C) → Isocitrate (6C)
- Enzyme: Aconitase

3. Isocitrate (6C) → α-Ketoglutarate (5C), CO₂, NADH + H⁺
- Enzyme: *Isocitrate dehydrogenase* (Mark with a star)

4. α-Ketoglutarate (5C) → Succinyl-CoA (4C), CO₂, NADH + H⁺
- Enzyme: α-Ketoglutarate dehydrogenase complex

5. Succinyl-CoA (4C) → Succinate (4C), CoASH, GTP (or ATP)
- Enzyme: Succinyl-CoA synthetase

6. Succinate (4C) → Fumarate (4C), FADH₂
- Enzyme: Succinate dehydrogenase

7. Fumarate (4C) → Malate (4C)
- Enzyme: Fumarase

8. Malate (4C) → Oxaloacetate (4C), NADH + H⁺
- Enzyme: Malate dehydrogenase

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Final Answer

\boxed{
\text{See the detailed steps above for filling in the diagram. Mark isocitrate dehydrogenase with a star (*).}
}