In which of the following reaction substrate-level phosphorylation occurs during respiration ?

Options

A. Citric acid `rarr alpha` - ketoglutaric acid
B. Malic acid `rarr` oxaloacetic acid
C. `alpha ` - ketoglutaric acid `rarr` Succinyl - CoA
D. Succinyl - CoA `rarr` Succinic acid

Answer

To determine in which reaction substrate-level phosphorylation occurs during respiration, we need to analyze the given reactions related to the TCA (tricarboxylic acid) cycle.

### Step-by-Step Solution:

1. **Understanding Substrate-Level Phosphorylation**:
- Substrate-level phosphorylation is a metabolic reaction that results in the formation of ATP or GTP by the direct transfer of a phosphate group to ADP or GDP from a high-energy substrate.

2. **Identifying the Reactions**:
- We have four reactions to consider:
1. Citric acid to α-ketoglutaric acid
2. Malic acid to oxaloacetic acid
3. α-ketoglutaric acid to succinyl-CoA
4. Succinyl-CoA to succinic acid

3. **Analyzing Each Reaction**:
- **Citric acid to α-ketoglutaric acid**: This reaction involves oxidative decarboxylation and does not produce ATP.
- **Malic acid to oxaloacetic acid**: This reaction also does not involve ATP production.
- **α-ketoglutaric acid to succinyl-CoA**: This is another oxidative decarboxylation reaction and does not produce ATP.
- **Succinyl-CoA to succinic acid**: This reaction is where substrate-level phosphorylation occurs. Here, the high-energy thioester bond in succinyl-CoA is used to generate ATP (or GTP) directly.

4. **Conclusion**:
- The correct answer is the fourth reaction: **Succinyl-CoA to succinic acid**, where substrate-level phosphorylation takes place.

### Final Answer:
The reaction in which substrate-level phosphorylation occurs during respiration is: **Succinyl-CoA to succinic acid**.