Consider the following intermediates formed during Krebs cycle
1. `alpha`-ketoglutarate 2. lsocitrate
3. Succinate 4. Malate
5. Fumerates
The correct sequences in which the above intermediates are formed is -

Options

A. 1, 2, 4, 3, 5
B. 2, 1, 5, 4, 3
C. 3, 1, 2, 5, 4
D. 2,1,3, 5,4

Answer

To determine the correct sequence in which the intermediates are formed during the Krebs cycle, we can follow the steps outlined below:

### Step-by-Step Solution:

1. **Understand the Krebs Cycle**: The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. It occurs after glycolysis and involves several intermediates.

2. **Identify the Starting Point**: The cycle begins with the combination of acetyl-CoA (derived from pyruvate) and oxaloacetate, forming citrate. However, oxaloacetate is not listed in the options provided.

3. **List the Given Intermediates**: The intermediates provided in the question are:
- Alpha-ketoglutarate
- Isocitrate
- Succinate
- Malate
- Fumarate

4. **Determine the Order of Formation**:
- **Citrate Formation**: After the initial reaction, citrate is formed (not listed).
- **Isocitrate Formation**: Citrate is then converted to isocitrate. So, isocitrate is the first intermediate we will consider.
- **Alpha-Ketoglutarate Formation**: Isocitrate is then converted to alpha-ketoglutarate.
- **Succinate Formation**: Alpha-ketoglutarate is converted to succinyl-CoA, which is then converted to succinate.
- **Fumarate Formation**: Succinate is converted to fumarate.
- **Malate Formation**: Fumarate is then converted to malate.

5. **Final Sequence**: Based on the above conversions, the correct sequence of intermediates formed is:
- Isocitrate (2)
- Alpha-ketoglutarate (1)
- Succinate (3)
- Fumarate (5)
- Malate (4)

Thus, the correct order of intermediates is **2, 1, 3, 5, 4**.

### Final Answer:
The correct sequence in which the intermediates are formed during the Krebs cycle is **2, 1, 3, 5, 4**.

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