Consider the following compounds:-
1. Phosphoenol pyruvate-
2. Malic acid-
3. Pyruvic acid-
4. Oxaloacetic acid-
What is the correct sequence of these intermediate compounds as they form in `C_(4)` dicarboxylic acid pathway flowing the PEP-carboxylation reaction?

Options

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

Answer

To determine the correct sequence of the intermediate compounds in the C4 dicarboxylic acid pathway following the PEP-carboxylation reaction, we can break down the process step by step:

### Step-by-Step Solution:

1. **Identify the Starting Compound**:
The process begins with **Phosphoenolpyruvate (PEP)**, which is a 3-carbon compound. In the C4 pathway, PEP reacts with carbon dioxide (CO2) to form a 4-carbon compound.

2. **Formation of Oxaloacetic Acid**:
The first reaction involves the enzyme PEP carboxylase, which catalyzes the reaction between PEP and CO2 to produce **Oxaloacetic Acid (OAA)**. This is the first intermediate formed in the pathway.

3. **Conversion to Malic Acid**:
Oxaloacetic Acid can then be converted into **Malic Acid** (another 4-carbon compound) through a reduction reaction. This is the second intermediate formed.

4. **Transport to Bundle Sheath Cells**:
The Malic Acid is transported to the bundle sheath cells, where it undergoes decarboxylation.

5. **Formation of Pyruvic Acid**:
In the bundle sheath cells, Malic Acid is converted into **Pyruvic Acid** (a 3-carbon compound) by releasing CO2. This is the third intermediate formed.

6. **Regeneration of Phosphoenolpyruvate**:
Finally, Pyruvic Acid can be transported back to the mesophyll cells, where it is converted back into **Phosphoenolpyruvate (PEP)**, completing the cycle. This is the fourth intermediate formed.

### Correct Sequence:
Based on the steps above, the correct sequence of the intermediate compounds as they form in the C4 dicarboxylic acid pathway following the PEP-carboxylation reaction is:
1. **Oxaloacetic Acid (OAA)**
2. **Malic Acid**
3. **Pyruvic Acid**
4. **Phosphoenolpyruvate (PEP)**

Thus, the correct order is: **4 (Oxaloacetic Acid) → 2 (Malic Acid) → 3 (Pyruvic Acid) → 1 (Phosphoenolpyruvate)**.

### Final Answer:
The correct sequence is **4 → 2 → 3 → 1**.

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