Substrate level phosphorylation occurs during which step of Kreb's cycle?

Options

A. Succinyl - CoA `to` Sucinic acid
B. Isocitric acid `to` Oxalosuccinic acid
C. Oxalosuccinic acid `to alpha`- Ketoglutaric acid
D. Malic acid `to` OAA

Answer

**Step-by-Step Solution:**

1. **Understanding Substrate Level Phosphorylation**:
- Substrate level phosphorylation is a process where a phosphate group is directly transferred from a substrate to ADP to form ATP. This occurs during specific reactions in metabolic pathways.

2. **Identifying the Krebs Cycle**:
- The Krebs cycle, also known as the citric acid cycle, consists of several steps where acetyl CoA is oxidized to produce energy carriers like NADH, FADH2, and ATP (or GTP).

3. **Analyzing the Given Options**:
- The options provided in the question are:
- a) Succinyl CoA to succinic acid
- b) Isocitric acid to oxalosuccinic acid
- c) Oxalosuccinic acid to alpha-ketoglutaric acid
- d) Oxaloacetic acid to citrate

4. **Focusing on the Relevant Step**:
- Among the options, the step where succinyl CoA is converted to succinic acid is crucial. This reaction is catalyzed by the enzyme succinyl CoA synthetase.

5. **Understanding the Reaction**:
- During the conversion of succinyl CoA to succinate, energy is released, which is used to phosphorylate ADP to ATP (or GTP in animals). This is the key characteristic of substrate level phosphorylation.

6. **Conclusion**:
- Therefore, substrate level phosphorylation occurs during the conversion of succinyl CoA to succinic acid.

**Final Answer**: Substrate level phosphorylation occurs during the step from succinyl CoA to succinic acid in the Krebs cycle.

---