Consider the following
1. Succinic dehydrogenase
2. Aconitase
3. `alpha`-ketoglutarate dehydrogenase
4. lsocitric dehydrogenase
What is the correct order in which the above enzymes catalyze the reaction in Kreb's cycle?

Options

A. 1-2-3-4
B. 2 - 4 - 1 - 3
C. 3 - 2 - 4 - 1
D. 2-4-3-1

Answer

To determine the correct order of the enzymes involved in the Krebs cycle from the given list, we need to understand the sequence of reactions that occur in this metabolic pathway. Here’s the step-by-step solution:

### Step-by-Step Solution:

1. **Identify the Enzymes and Their Functions**:
- **Aconitase**: Converts citrate to isocitrate.
- **Isocitrate Dehydrogenase**: Converts isocitrate to alpha-ketoglutarate.
- **Alpha-Ketoglutarate Dehydrogenase**: Converts alpha-ketoglutarate to succinyl-CoA.
- **Succinic Dehydrogenase**: Converts succinate to fumarate.

2. **Establish the Sequence of Reactions**:
- The Krebs cycle begins with the conversion of acetyl-CoA and oxaloacetate to citrate, followed by the conversion of citrate to isocitrate (catalyzed by aconitase).
- Next, isocitrate is converted to alpha-ketoglutarate by isocitrate dehydrogenase.
- Alpha-ketoglutarate is then converted to succinyl-CoA by alpha-ketoglutarate dehydrogenase.
- Finally, succinyl-CoA is converted to succinate by succinic dehydrogenase.

3. **Order the Enzymes**:
- Based on the sequence of reactions:
1. Aconitase (1st reaction)
2. Isocitrate Dehydrogenase (2nd reaction)
3. Alpha-Ketoglutarate Dehydrogenase (3rd reaction)
4. Succinic Dehydrogenase (4th reaction)

4. **Final Order**:
- The correct order of the enzymes is: **Aconitase (2), Isocitrate Dehydrogenase (4), Alpha-Ketoglutarate Dehydrogenase (3), Succinic Dehydrogenase (1)**.
- Therefore, the correct sequence is **2, 4, 3, 1**.

### Conclusion:
The correct order in which the enzymes catalyze the reactions in the Krebs cycle is **2, 4, 3, 1**.

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