Which of the following reaction involved substrate - level phosphorylation ?

Options

A. 2 - phoshoglycerate to 2 - phosphoenolpyruvate
B. Fructose to Fructose - 6 - phosphate
C. Phosphoenolpyruvic acid to pyruvic acid
D. Fructose 1, 6 -bisphosphate to PGAL and DHAP

Answer

To determine which reaction involves substrate-level phosphorylation, we need to understand what substrate-level phosphorylation is. It refers to the direct synthesis of ATP from ADP and a phosphorylated intermediate during a metabolic reaction.

### Step-by-Step Solution:

1. **Understanding Substrate-Level Phosphorylation**:
- Substrate-level phosphorylation occurs when a phosphate group is directly transferred from a substrate to ADP, forming ATP. This typically happens in glycolysis and the citric acid cycle.

2. **Analyzing the Reactions**:
- **Reaction 1**: 2-phosphoglycerate to 2-phosphoenolpyruvate
- This is a dehydration reaction and does not involve the formation of ATP. Hence, it does not represent substrate-level phosphorylation.

- **Reaction 2**: Fructose to fructose 6-phosphate
- This reaction consumes ATP (it is part of the preparatory phase of glycolysis) and does not produce ATP. Therefore, it is not substrate-level phosphorylation.

- **Reaction 3**: Phosphoenolpyruvic acid (PEP) to pyruvic acid
- This reaction results in the synthesis of ATP from ADP. Here, a phosphate group from PEP is transferred to ADP, producing ATP. This is the definition of substrate-level phosphorylation.

- **Reaction 4**: Fructose 1,6-bisphosphate to phosphoglyceraldehyde
- This is a splitting reaction that does not produce ATP. Therefore, it is not substrate-level phosphorylation.

3. **Identifying the Correct Answer**:
- Based on the analysis, the only reaction that involves substrate-level phosphorylation is the conversion of phosphoenolpyruvic acid to pyruvic acid (Reaction 3).

### Conclusion:
The correct answer is **Reaction 3: Phosphoenolpyruvic acid to pyruvic acid**.

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